Dc Machines Pune University MCQs

 This set of DC Machines Multiple Choice Questions & Answers  focuses on “Construction”.

1. Where is field winding mounted in a DC machine?

a) Stator

b) Rotor

c) Absent

d) Anywhere on stator or rotor

Answer: a

Explanation: The field winding  is mounted on salient-poles on the stator and the armature winding  is wound in slots on a cylindrical rotor. In AC machines field winding is mounted on rotor.

2. What are the materials used for brushes in dc machines?

a) Iron

b) Carbon

c) Aluminum

d) Steel

Answer: b

Explanation: On some extent carbon brush can act as a self-lubricating brush. On moment, polishes the commutator segments. Damage to the commutators is less when copper brushes are used on occurrence of sparkover.

3. Function of yoke is to provide the return path for magnetic flux.

a) True

b) false

Answer: a

Explanation: The function of yoke is that it protects the entire machine from dust and dirt. It also provides mechanical support for the magnetic poles. It acts as the return path for the magnetic flux.

4. The angle  made by brushes with axes of adjoining filed poles is ______________

a) 450

b) 1800

c) 900

d) 300

Answer: c

Explanation: Brushes in a DC machine are normally placed electrically in the interpolar regions and therefore make an angle of 900 electrical with the axes of adjoining filed poles.

5. In a DC machine, rectification process is carried out in order to get unidirectional output . This rectification process is carried out by ______________

a) Half wave rectifier

b) Full wave rectifier

c) Mechanical rectification

d) Centre tapped rectifier

Answer: c

Explanation: In a DC machine electronic rectification is not used. Instead they use mechanical rectification with the help of commutator-brush assembly.

6.Which of the following part is used in construction of DC machine but not in AC machine?

a) Armature Winding

b) Field winding

c) Commutator

d) Shaft

Answer: c

Explanation: Commutator is used in mechanical rectification process, to convert induced AC to output DC. In AC machine, we don’t need rectification process.

7. In a DC machine fractional pitch winding is used to _______________

a) To improve cooling

b) To reduce sparking

c) To reduce copper losses

d) To increase generated EMF

Answer: b

Explanation: Due to poor performance of brush, poor undercutting of commutator, incorrect spring pressure sparking at brush faces happen. To overcome this sparking fractional pitch winding is used.

8. In normal dc machines operating at full-load conditions, the most powerful electromagnet is _________

a) Field winding

b) Interpole Winding

c) Interpole and compensating winding together

d) Armature winding

Answer: a

Explanation: Electromagnet is more powerful when its MMF is high. At full-load condition, field winding contains maximum ampere turns, hence it is most powerful electromagnet in a DC machine.

9. If a DC motor is connected to AC supply what will happen then?

a) Not run

b) Burn

c) Run at normal speed

d) Run at extremely low speed

Answer: b

Explanation: If a DC motor is connected to AC supply, an alternating current pass through the brushes and commutator to the armature winding, while it passes through the commutator it is converted into DC so that the group of conductors under successive field poles carry current in same direction. So, the flux per pole will remain constant and not vary. There will be production of heat due to flow of eddy current in field winding and the motor will be burned.

10. The armature of DC motor is laminated to ____________

a) To reduce mass

b) To reduce hysteresis loss

c) To reduce eddy current loss

d) To reduce inductance

Answer: c

Explanation: The armature is built up in a cylindrical or drum shape high grade silicon steel in form of lamination. By using laminations, the circular path of eddy currents is terminated. Hence heating and ultimately damage to the armature can be reduced by lamination.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Applications of DC Machines”.

1. Why are the DC motors preferred for traction applications?

a) Torque and speed are inversely proportional to armature current

b) Torque is proportional to armature current

c) Torque is proportional to square root of armature current

d) The speed is inversely proportional to the torque and the torque is proportional to square of armature current

Answer: d

Explanation: DC motors are used for traction as, according to the characteristics of DC motors speed is inversely proportional to torque and square of armature current as well, if linear magnetization is concerned. Thus, DC motors are perfectly suitable for traction.

2. Which of the following load application normally needs starting torque more than the rated torque?

a) Blowers

b) Conveyors

c) Air compressors

d) Centrifugal pumps

Answer: b

Explanation: Conveyors need high starting torque initially, and constant torque later. Thus, DC series motor is used in conveyors as it provides very high starting torque, which is practically 5 times the rated torque.

3. Which of the following motors can be used to drive the rotary compressor?

a) DC shunt motor

b) DC series motor

c) Universal motor

d) Synchronous motor

Answer: d

Explanation: Rotary compressor generally demand constant speed operation throughout the load. Sometimes, DC machines are not able to produce constant speed throughout the process hence, synchronous machine is used.

4. Which DC motor is used with flywheel for intermittent light and heavy loads?

a) Series motor

b) Shunt motor

c) Cumulatively compounded motor

d) Differentially compounded motor

Answer: c

Explanation: Cumulative compound DC motor is used with flywheel carrying peaks and so to smooth out the load on the motor as well as to reduce peaks on power system. Without flywheel the motor construction will be much larger.

5. Separately excited DC generators are still used in _________________

a) Thermal power plants

b) Ward Leonard speed control system

c) Hydro power plant

d) In all fields

Answer: b

Explanation: Separately excited DC generators are still used in wide output voltage control like in Ward Leonard speed control. In all power plants today, generally AC generators are used due to low cost and less maintenance required.

6. In world today, around 25% of the motors are manufactured are DC motors.

a) True

b) False

Answer: a

Explanation: For a dc machine, of course, the main attraction lies in its flexibility, versatility and ease of control. This explains why in spite of its rather heavy initial investment it still retains its charm in strong competitive industrial applications.

7. Maximum torque in a DC machine is limited by ______________

a) Commutation

b) Heating

c) Losses other than heating

d) Stability

Answer: a

Explanation: While for all other motors maximum torque is restricted to certain value as various losses in other motors lead to heating of the core materials. In DC machines for maximum torque commutation time will obviously decrease and beyond some point commutation process can’t be fastened.

8. Which of the following motor can replace DC series motor?

a) DC shunt motor

b) Cumulative compound motor

c) Wound-rotor induction motor

d) Synchronous motor

Answer: c

Explanation: DE series motor’s closest rival is the wound-rotor induction motor with a rotor resistance control. But ultimately the availability and economics of a dc power is the deciding factor rather than the motor characteristic.

9. Which motor has almost replaced DC shunt motor from its applications?

a) Wound-rotor induction motor

b) Differential compound motor

c) Air motor

d) Squirrel caged induction motor

Answer: d

Explanation: Owing to the relative simplicity, cheapness and ruggedness of the squirrel cage induction motor, the shunt motor is less preferred for constant-speed drives except at low speeds. At high or medium speed applications we use induction motor, mostly squirrel caged.

10. DC shunt motor is still used instead of synchronous motor in _____________

a) High speed applications

b) Low speed applications

c) Medium and high-speed applications

d) Everywhere

Answer: b

Explanation: At low speeds, DC shunt motors are comparable with synchronous motors. The outstanding feature of a DC shunt motor however is its superb wide range flexible speed control above and below the base speed using solid-state controlled rectifiers.

11. Which type motors are preferred for lathes?

a) DC shunt motors

b) Squirrel Cage induction motor

c) Synchronous motor

d) DC Shunt Motors or Squirrel cage induction motor

Answer: d

Explanation: Lathes machines requires uniform torque which is provided with squirrel cage induction or DC shunt motors. Hence, they are preferred for lathes. DC shunt motor and induction motor of squirrel cage type follow same shunt characteristics.

12. When an electric train is moving down a hill, the DC motor will operate as ____________

a) DC series motor

b) DC series generator

c) DC shunt motor

d) DC shunt generator

Answer: b

Explanation: Normally in electric traction purposes DC series motors are employed. At above condition the back emf is greater than supply voltage hence, it will operate as series generator which will provide energy back to the supply.

13. Nowadays, DC motor is widely used only in ____________

a) Pumping sets

b) Air compressors

c) Electric traction

d) Machine shops

Answer: c

Explanation: DC machine is used in tractions, because its characteristics suit perfectly with application. Also, its smooth braking capacity and ability to transfer from one mode to other is very good.

14. Which motor is preferred in highly explosive atmosphere?

a) Air motor

b) Shunt motor

c) Series motor

d) Battery operated motor

Answer: a

Explanation: A pneumatic motor  is a motor which does mechanical work by expanding compressed air. Air motors generally convert the compressed air energy to mechanical work through either linear or rotary motion, and are preferred in highly explosive atmosphere.

15. For which of the following operations a DC motor is preferred over an AC motor?

a) Low speed operation

b) High speed operation

c) Variable speed operation

d) Fixed speed operation

Answer: c

Explanation: Due to the various speed control techniques available and with the help of electrical brakes various speed levels can be obtained in DC motor. While in AC motor for same purpose we require frequency change which requires another complex circuitry.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Brushes and Commutator Assembly – 1”.

1.The armature in DC machines is always placed on rotor because _______

a) Otherwise commutation will not be possible

b) Otherwise there will not be any induced emf

c) Otherwise current will not flow

d) Can be placed anywhere

Answer: a

Explanation: A DC machine is a heteropolar structure with stationary poles and the rotating armature. The armature winding of a DC machine is placed on the rotor to improve commutation i.e. to convert the alternating voltage produced in the winding into direct voltage at the brushes.

2. In a DC machine, rectification provided with commutator is ___________

a) Half wave rectification

b) Full wave rectification

c) Semi controlled rectification

d) Uncontrolled rectification

Answer: b

Explanation: In any electromagnetic machine the voltage generated is always alternating one as per Faraday’s law. For a DC machine the output must be unidirectional. This is carried out by a commutator. Hence, commutator provides full wave rectification.

3. Commutator performs rectification so that output of the machine is bi-directional.

a) True

b) False

Answer: b

Explanation: Commutator and brush assembly of the DC machine performs the mechanical rectification process so induced AC is converted into DC . Commutation process provides full wave rectification.

4. Which of the following method is used to connect commutator segments to armature conductors?

a) Brazing

b) Normal wires

c) Insulation pads

d) Copper lugs

Answer: d

Explanation: Commutator in connected to the armature using lugs. Generally, they are made with copper. They are tightly bolted to the armature in order to prevent the centrifugal forces from causing the segments to fly away.

5. In D.C. generators, rapid brush ware causes due to _________

a) Severe sparking

b) Rough commutator surface

c) Imperfect contact

d) Severe sparking, rough commutator surface or imperfect contact

Answer: d

Explanation: Brushes are the parts in a DC machine which are always in contact with rotating and stationary parts. Thus, imperfect contact, rough surfaces, sparking all these may reduce the life of brushes.

6. What are the number of the brushes in the lap winding?

a) Double the number of poles

b) Same as the number of poles

c) Half the number of poles

d) Two

Answer: b

Explanation: In a lap winding, the number of parallel paths, A, is always equal to the number of poles, P, and also to the number of brushes. In wave windings, the number of parallel paths, a, is always two , and there may be two or more brush positions.

7. When Copper brushes are used in DC machine?

a) Where low voltage and high currents are involved

b) Where high voltage and small currents are involved

c) Cannot be determined

d) Where low voltage and low currents are involved

Answer: a

Explanation: Due to various limitations, copper brushes are used in low voltage applications. For various other voltage ratings, different carbon-graphite proportions are used in manufacturing of brush materials.

8. In DC generators, current is fed up to the external circuit from armature through _______

a) Commutator

b) Direct wire

c) Slip rings

d) Cannot be determined

Answer: a

Explanation: In any rotary machine current is induced in Sine wave format, according to Faraday’s law. For DC machine commutator provides mechanical rectification so that output is in the unidirectional format.

9. What are the number of the brushes in the wave winding?

a) Same or more than the number of poles

b) Same or less than the number of poles

c) Exactly half the number of poles

d) Depends on other parameters

Answer: a

Explanation: In wave windings, the number of parallel paths, A, is always two , and there may be two or more brush positions. When two adjacent commutator bars make contact with a particular brush, p/2 coils are shorted by the brush in the wave winding.

10. Which conductors are in point of contact with brushes?

a) Lie under south pole

b) Lie under north pole

c) Lie in inter polar region

d) Are farthest from the poles

Answer: c

Explanation: Because of the diamond shape of coils, the brushes which are physically opposite the pole centres are electrically connected to coil-sides lying close to the interpolar region. Thus, electrically the brushes are displaced 90° elect. From the axes of the main poles.

11. In case of DC machine winding, number of commutator segments is equal to ______

a) Number of armature coils

b) Number of armature coil sides

c) Number of armature conductors

d) Number of armature turns

Answer: a

Explanation: Armature current is induced in a DC machine, which is fed up to the external circuit. Thus, it needs to be unidirectional. So, for converting bidirectional current to unidirectional commutators which are equal in number of armature coils need to be used.

12. How total number of brushes in a commutator are determined in a given DC machine?

a) Speed of armature

b) Type of winding

c) Voltage

d) Amount of current to be collected

Answer: d

Explanation: Brushes are in contact with rotating part and stationary part. Thus, if more amount of current is to be carried, it requires more number of brushes. Hence brush number depends directly on the amount of current that needs to be collected and fed up in or out.

This set of DC Machines Interview Questions and Answers focuses on “Brushes and Commutator Assembly – 2”.

1. In a DC generator the ripples in the direct emf generated can be reduced by________

a) Using conductor of annealed copper

b) Using commutator with large number of segments

c) Using carbon brushes of superior quality

d) Using equalizer rings

Answer: c

Explanation: Brushes carry current to/from rotating parts from/to stationary part. Ripples can be avoided if brushes are maintained. Else, brushes will have some voltage drop in it and we’ll not get simple repeating part in emf.

2. The drop in the voltage for which of the following types of brush can be expected to be least?

a) Graphite brushes

b) Carbon brushes

c) Metal graphite brushes

d) Cannot be determined

Answer: c

Explanation: Metal graphite brushes are ideal for a variety of applications because of their low resistivity. Thus, drop will be less in metal graphite brushes. Metal graphite brushes are used on commutators of plating generators where low voltage and high brush current densities are encountered.

3. What is the requirement of the good commutation?

a) Brushes should be of proper grade, size and material

b) Brushes should smoothly run in the holders

c) Smooth, concentric commutator properly undercut

d) Brushes should be of proper grade, size, material, run smoothly in the holders and concentric commutator properly undercut

Answer: d

Explanation: Brushes are in contact with commutator. So, for good commutation brushes must be of superior quality so that brushes will give/receive appropriate current to and from commutator. Also, the contact between brushes and commutator must be smooth for proper commutation process.

4. How to avoid grooves in the commutation of DC machine with the help of brush?

a) Brushes of opposite polarity should track each other

b) Brushes of same polarity should track each other

c) Brush position has no effect on the commutator grooving

d) Brushes should not track each other

Answer: a

Explanation: Brushes are located such that they are displaced 900 electrically from the axes of main poles. The two positive and two negative brushes are respectively connected in parallel for feeding the external circuit.

5. Reason behind the rapid wear of brushes is __________

a) Abrasion from dust

b) Excessive spring pressure

c) Rough commutator bars

d) Abrasion from dust, excessive spring pressure and rough commutator bars

Answer: d

Explanation: Brushes undergo various forces due to their location in a DC machine, they are in contact with rotating and stationary part of the machine. Hence, rough contact between commutator and brushes, inappropriate pressure on brush to rotating part may affect quality of commutation process.

6. For both lap and wave windings, what is the number of commutator bars equal to?

a) Slots

b) Armature conductors

c) Winding elements

d) Poles

Answer: c

Explanation:

7. Spacing between the brushes for a 4-pole machine in terms of commutator segments for 12 conductor segments is _____

a) 48

b) 3

c) 2

d) 6

Answer: b

Explanation: The spacing between adjacent brushes in terms of the commutator segment is ratio of number of commutator segments with poles for a given DC machine.

C/P= 12/4= 3.

It may also be noted that C/P need not necessarily be an integer.

8. Spacing between the brushes for a 4-pole machine in terms of commutator segments is equal to 6. What will be the number of armature slots?

a) 48

b) 3

c) 24

d) 6

Answer: c

Explanation: The spacing between adjacent brushes in terms of the commutator segment which is also equal to armature slots is ratio of number of commutator segments with poles for a given DC machine.

C= P*Spacing= 4*6= 24.

9. What is the range of the brush friction coefficients for medium category?

a) 0.40 and above

b) 0.22 to 0.40

c) 0.11 to 0.22

d) 0.08 to 0.11

Answer: b

Explanation: Brush friction is influenced by many variables including brush temperature, spring force, current, atmospheric conditions, mechanical conditions, ring or commutator materials, surface films, speed and other factors. Brush friction is of medium category when, coefficient of friction lies in between 0.22 to 0.44.

10. Specific resistance for a brush is given by _________

a) R =  / 

b) R =  / 

c) R =  * 

d) Doesn’t depend on E, W, T, I, L.

Answer: a

Explanation: Specific resistance is measured in the length direction of the brush, since resistance in the direction of width or thickness may be considerably different. For, E = voltage drop over length L, I = amps of current passed through the sample, W = width of sample, T = thickness on sample, L = that portion of the length, over which the voltage drop E is measured, R is calculated by R =  / .

11. How many poles to be used in DC machine if brushes are placed 4 commutator segments apart for 16 commutator segments?

a) 8

b) 12

c) 2

d) 4

Answer: d

Explanation: Number of poles can be found by dividing the total commutator segments to spacing between brushes. Hence, number of poles = 16 commutator segments/ 4 commutator spacing= 4 poles.

12. DC generators are disconnected/connected from/to the busbars only under the floating condition because ____________

a) To avoid sudden loading of the prime mover

b) To avoid burning of all switch contacts

c) To avoid mechanical jerk to the shaft

d) To avoid sudden loading of the prime mover, burning of all switch contacts and avoid mechanical jerking to the shaft

Answer: d

Explanation: Brushes are the medium between rotating and non-rotating part of the DC machine. If sudden change in connections are done, whole machine undergoes change in all electrical quantities, which may damage machine. Thus, machines are connected and disconnected only at floating condition.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “PMDC Motors”.

1. In PMDC motors we use magnetic material with ___________

a) Low residual flux

b) Low coercivity

c) High residual flux

d) All magnetic materials

Answer: c

Explanation: In market magnetic materials with various properties are available, but for DC motor application we need magnetic materials with high residual flux in order to get high no load voltage. Along with residual flux, coercivity of material should be high.

2. In PMDC magnetic material is placed instead of _____________

a) Armature

b) Rotor

c) Stator

d) Can be placed anywhere

Answer: c

Explanation: In PMDC motors we keep magnetic material or magnetic poles in the place of field winding of original DC machine. That is we are changing stator windings with two permanent magnetic poles bounded on casing.

3. PMDC motors are _________________

a) Smaller than corresponding rated field wound motor

b) Larger than corresponding rated field wound motor

c) Almost same size that of corresponding rated field wound motor

d) Can be smaller or greater than corresponding rated field wound motor

Answer: a

Explanation: PMDC motors are always of smaller size compare to field wound motors of same rating. Here, field winding is replaced by small block of magnet so that power output remains same. PMDC motors are costly compare to DC motor of same rating.

4. PMDC motor offers ___________

a) Cumulative compound type characteristic

b) Differential compound type characteristic

c) Series characteristic

d) Shunt characteristic

Answer: d

Explanation: The stator is made of permanent magnet in the PMDC motor hence, there is no need for separate field excitation for stator. So, PMDC acts as a shunt motor with fixed field. This can be seen from the construction also.

5. Which speed control method is employed in PMDC?

a) Field control method

b) Armature control method

c) Armature Field control method

d) Cannot be determined

Answer: b

Explanation: Unlike in DC motors, field is provided by permanent magnets in PMDC. This implies that field provided in PMDC motor is permanent and can’t be altered unless whole construction is modified. Thus, armature voltage control method is employed here.

6. What is the air gap flux density for PMDC motor?

a) B g = µ 0 (t m /l g ) H m

b) B g = – µ 0 H m

c) B g = – µ 0 (t m /l g ) H m

d) B g = – µ 0 (lg/t m ) H m

Answer: c

Explanation: Since, flux crosses the air gap length lg two times a thickness of permanent magnet two times tm. We get, 2*l g * H g + 2*t m H m = 0 and we know that air gap flux density is equal to µ times the magnetic field intensity.

7. Which of the following is most favoured choice for PMDC motor?

a) Neodymium-iron-boron

b) Iron-nickel

c) Nickel-neodymium

d) Cannot be given

Answer: a

Explanation: Obvious choice of PMDC motor is neodymium-iron-boron which has high coercivity and high retentivity. Its characteristic is almost a straight line. Hence, neodymium-iron-boron combination is preferred over the other materials.

8. Speed-torque characteristic of PMDC motors is _______________

a) Starting from zero increasing continuously

b) Starting from some positive value and remaining constant

c) Starting from some positive value and decreasing exponentially

d) Starting from some positive value and decreasing on straight line

Answer: d

Explanation: The material chosen for PMDC motor is such that speed torque characteristic of a given motor remains straight line for wider range of torque, for wider range of armature voltage. The straight line is such that it makes positive intercepts on both axes.

9. PMDC produces high torque on ______________

a) At low speeds

b) Only at maximum speed

c) Torque is constant

d) Cannot say

Answer: a

Explanation: PMDC produces high torque even at very low speeds. In DC shunt motor torque produced at very low speeds is lower than the torque produced at the same speeds in PMDC motor.

10. PMDC gives better speed regulation than DC shunt motor.

a) True

b) False

Answer: a

Explanation: PMDC motor exhibits better speed regulation and efficiency than dc shunt motor. The main problem of dc shunt motor is going to run-away when the field terminals are opened. But in PMDC there is no run-away problem, so it gives practical benefit to the industry applications.

11. If higher terminal voltage is applied to PMDC what will happen?

a) Speed at 0 torque will be reduced

b) Torque at 0 speed will be reduced

c) Torque at 0 speed will be increased

d) Torque will remain constant

Answer: c

Explanation: The straight-line characteristic of PMDC is such that it produces positive intercepts on both axes. If terminal voltage is increased both of these intercepts increases, slope remaining constant. Thus, speed and torque at initial conditions both will increase.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Armature Winding and Commutator”.

1. Armature winding is mounted on a __________

a) Stator

b) Rotor

c) Can be mounted anywhere on stator or rotor

d) Not required

Answer: b

Explanation: A DC machine is a heteropolar structure with stationary poles and the rotating armature. Field coil is mounted on stator. In an AC machine armature winding is located at rotor while field coil is located at stator. An ‘armature’ is a moving part as it a ‘rotor’.

2. In a DC machine, how coil-side emf varies towards the outer side of poles?

a) Decreases

b) Remains same

c) Increases

d) First increases the decreases

Answer: a

Explanation: Coil side current pattern is the same as the emf pattern. Only difference is that while the coil-side emf reduces towards the outer side of poles, the current remains the same in all the coil-sides except for alterations from pole to pole, while the coil side current reverses, the current exchanged with external circuit must be unidirectional.

3. Commutator performs rectification so that output of the machine is unidirectional.

a) True

b) False

Answer: a

Explanation: Since it is a DC machine the generated output must be DC. Any electrical machine works on induced emf concept which is AC in nature. Commutator and brush assembly of the DC machine performs the mechanical rectification process so; induced AC is converted into DC.

4. What is the difference of DC voltages in the adjoining Brushes?

a) Depends on the Shaft speed

b) Zero

c) Non-zero

d) Depends on the various other parameters

Answer: b

Explanation: Brushes are located electrically in the magnetically neutral region. Due to their location adjoining brushes are at constant DC voltage and the coil in series between the constitute one parallel path.

5. What is the effect of armature coils at points where brushes are located?

a) Induces positive emf

b) Induces negative emf

c) Induces zero emf

d) Depends on the speed of rotor

Answer: c

Explanation: Brushes are at magnetically neutral region hence, induced emf due to armature coils at brushes will be equal to zero. As in the magnetically neutral region change in flux will be equal to the zero, emf will not be induced .

6. As the armature rotates, the number of coils in series tapped by the brush pairs_________

a) Remains same

b) Increases

c) Decreases

d) Depends on rotor speed and direction of torque

Answer: a

Explanation: Brushes are located electrically in magnetically neutral region. Adjoining brush pairs are at constant DC voltage and the number of coils tapped by brush pairs also remain constant. Their disposition relative to the poles is the same.

7. Coil span for 4-pole, 12-slot armature winding is_______

a) 24

b) 48

c) 8

d) 3

Answer: d

Explanation: Coil span is defined as a ratio of number of slots in the armature winding which are also equal to the number of commutator segments to the number of poles. Here, Slots in the armature winding= 12, Number of poles= 4.

Y CS = 12/4= 3.

8. What is the nature of the coils when Y CS value is non-integral?

a) Long-pitched

b) Medium-pitched

c) Short-pitched

d) Can’t be determined by Y CS value

Answer: c

Explanation: Coil-side voltages around the coil are additive most of the time . Thus Y CS = Nearest lower integer, which means that for non-integral S/P, the coils are short-pitched.

9. For a 2-pole DC machine with coil span equal to 6, what are the number of commutator segments?

a) 3

b) 12

c) 4

d) 8

Answer: b

Explanation: Coil span is defined as a ratio of number of slots in the armature winding which are also equal to the number of commutator segments to the number of poles. Here, Number of poles= 2, Slots in the armature winding= S= P* Y CS . Here, Y CS = 6, P=2. Thus S=12. C=S=Number of commutator segments.

10. When coil sides are pole pitch apart, the DC armature winding is called as ___________

a) Multiplex

b) Fractional-pitch

c) Full-pitch

d) Pole-pitch

Answer: c

Explanation: Pole pitch is called as center to center distance between two adjacent poles. When measured in electrical degrees one pole itch is equal to 1800. Coil span is simply a peripheral distance between two sides of a coil. If the coil span is equal to the pole pitch, then the armature winding is said to be full pitched coil.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Lap Winding”.

1. Resultant pitch in the lap winding is__________

a) Depends on Y b value

b) Depends on Y f value

c) Depends on Y b and Y f value

d) Always equal to 2

Answer: d

Explanation: In a lap winding the “finish” of one coil is connected to “start” of the adjoining coil. The coil side displacement of the front-end connection is called the front-pitch. The coil side displacement of the back-end connection is called the back-pitch. Resultant-pitch is equal to difference between Y b and Y f which is equal to 2, irrespective of Y b and Y f value.

2. What is the condition of retrogressive winding?

a) Y b > Y f

b) Y b < Y f

c) Y b = Y f

d) No condition in bterms of Y b and Y f

Answer: b

Explanation: The coil side displacement of the front-end connection is called the front-pitch. The coil side displacement of the back-end connection is called the back-pitch. The direction in which the winding progresses depends upon which is more, Y b or Y f . For retrogressive winding Y b < Y f .

3. What is the value of Y b for a lap winding with a 4-pole, 12-slot armature with two coil sides/slot. Assume single-turns coils.

a) 3

b) 5

c) 7

d) 9

Answer: c

Explanation: Coil span is defined as a ratio of number of slots in the armature winding which are also equal to the number of commutator segments to the number of poles. Here, Slots in the armature winding= 12, Number of poles= 4.

Y CS = 12/4= 3.

Y b = 2 YCS +1= 7.

4. What is the value of Yf for a lap winding with a 4-pole, 12-slot armature with two coil sides/slot? 

a) 3

b) 5

c) 7

d) 9

Answer: b

Explanation: Coil span is defined as a ratio of number of slots in the armature winding which are also equal to the number of commutator segments to the number of poles. Here, Slots in the armature winding= 12, Number of poles= 4.

Y CS = 12/4= 3.

Y b = 2Y CS +1= 7

Y f =Y b – 2= 7-2= 5.

5. Equalizer rings are needed in lap winding.

a) False

b) True

Answer: b

Explanation: Each parallel path in lap winding is under the influence of one pair of poles, so if a machine consists of multiple pairs of poles then dissimilarities occurs, due to which unequal voltages may be induced in the paths and a circulating current may flow. In wave winding each path is under the influence of all poles, so voltages are induced in each path causing no such dissimilarities like lap winding. Equalizers in lap windings are used to remove this dissimilarity, they’re not needed in wave winding.

6. What is the symmetry requirement of lap winding?

a) 2C/P= 0

b) 2C/P= integer

c) 2C/P= non-integer

d) Can’t express mathematically

Answer: b

Explanation: To avoid no-load circulating currents and certain consequential commutation problems, all the parallel paths must be identical so as to have the same number of coil-sides. Symmetry thus requires ratio of 2C/P is equal to the integer. Also, US/P equal to integer represents the same.

7. What is the relation between number of parallel paths and number of poles?

a) A = P

b) A < P

c) A > P

d) No relation exists

Answer: a

Explanation: Complex winding can be divided into different parallel paths lying under different pole pairs. It is, therefore, concluded that the number if parallel paths is equal to the number of poles. In wave winding number of parallel paths is equal to 2.

8. Current flowing through the armature conductors Ic is related to total current Ia by_______

a) Ic = A Ia

b) Ic = Ia/A

c) Ic = A 2 Ia

d) Ic = A/Ia

Answer: b

Explanation: Two positive and two negative brushes are respectively connected in parallel for feeding the external circuit. As per the ring diagram Ia splits into the number of poles equally. Poles = Parallel paths. Thus, Ic = Ia /A.

9. Value of commutator pitch in lap winding is_____

a) +2

b) +1 or -1

c) -2

d) Different for different parameters

Answer: b

Explanation: Two ends of coil are connected across the adjacent commutator segments. Depending on the type of winding that is, retrogressive or progressive, we have two values for commutator pitch.

For progressive winding, commutator pitch = +1. For retrogressive winding, commutator pitch = -1.

10. What is the value of Yf for a lap winding with a 4-pole, 12 commutator segments, with two coil sides/slot.?

.

a) 9

b) 3

c) 11

d) 5

Answer: a

Explanation: Coil span is defined as a ratio of number of slots in the armature winding which are also equal to the number of commutator segments to the number of poles. Here, Slots in the armature winding= Number of commutator segments= 12, Number of poles= 4.

Y CS = 12/4= 3.

Y b = 2Y CS +1= 7

Y f =Y b + 2= 7+2= 9.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Wave Winding”.

1. What will be the value of “Y f + Y b ” for a wave winding?

a) Equal to Y c

b) Half of the Y c value

c) Double of the Y c value

d) Four times Y c value

Answer: c

Explanation: In the wave winding, as the number of coil-sides is double the number of segments, the top coil-side of the second coil will be numbered as (1+2*Y c ). After numbering other coil sides,

1 + 2*Y c – Y f = 1+ Y b

So Y f + Y b = 2Y c .

2. For a progressive wave winding Y c = ______

a) 2C/P

b) 2/P

c) 2/P

d) 2C/

Answer: b

Explanation: Starting at segment 1 and after going through P/2 coils or Yc  segments, the winding should end in segment 2 for progressive winding or segment  for retrogressive winding. That is mathematically,

Y c  = 

Y c = 2/P

3. Number of parallel paths in wave winding are ______

a) Equal to P

b) Equal to P/2

c) 2

d) Depends on other parameters

Answer: c

Explanation: In wave winding all coils are divided into 2 groups- all coils carrying clockwise current are series connected and so are all coils with counter-clockwise current- and these 2 groups are in parallel because the winding is closed. Thus, a wave winding has 2 parallel paths irrespective of number of poles.

4. What is the spacing between the brushes for a wave winding when a machine is 6-pole DC armature with 16 slots having 2-coil sides per slot and single-turn coils.

a) 4 segments

b) 8 segments

c) 16 segments

d) 12 segments

Answer: b

Explanation: Only 2 brushes are required in this case as the number of poles in wave winding is equal to 2. So, spacing between the brushes is equal to total number of segments i.e. total slots divided by 2. Spacing between brushes = C/A = 16/2 = 8 segments.

5. What is the relation between conductor current and armature current in wave winding?

a) Ic = Ia

b) Ic = 2Ia

c) Ic = 4Ia

d) Ic = Ia/2

Answer: d

Explanation: the number of parallel paths in the in a wave winding is equal to 2. So, armature current will get divided equally into total number of conductors/paths. Conductor current in a wave wounded machine is half of the Ia.

6. For a conductor current equal to 4mA, Current carried by a particular brush in a 2-pole machine will be _____

a) 16mA

b) 8mA

c) 2mA

d) 10mA

Answer: b

Explanation: Conductor current in a wave wounded machine is half of the Ia. So, Ia= 8mA. All positive and all negative brushes are respectively connected in parallel to feed the external circuit. Thus, IBRUSH = Ia /. Solving we get Brush current = 8mA.

7. Equalizer rings are needed in the wave winding.

a) True

b) False

Answer: b

Explanation: The armature coil forms 2 parallel paths under the influence of all pole-pairs so that the effect of the magnetic circuit asymmetry is equally present in both the parallel paths resulting in equal parallel-path voltages. Thus, equalizer rings are not needed in wave winding.

8. For a wave winding when a machine is 6-pole DC armature with 16 slots having 2-coil sides per slot and single-turn coil, Yf value is ____

a) 5

b) 3

c) 2

d) 7

Answer: a

Explanation: Y cs = 16/6 = 2 slots 

Y b = 2*2+1 = 5

Y c = 2/6 = 5 segments

Y f = 2Y c – Yb = 5.

9. Wave winding machines are used in ______ currents applications.

a) High

b) Moderate

c) Low

d) Can be used anywhere

Answer: c

Explanation: Lap winding machine has the advantage of large number of parallel paths and lower conductor current and is therefore used in low voltage and high current applications. Wave winding has fixed number of parallel paths so, wave wounded machine is used in low currents application.

10. For a wave wounded machine number of brushes for small, large machines respectively is ________ _________

a) 2, 2

b) 4, 2

c) 2, P

d) Both values depend on the given conditions

Answer: c

Explanation: For a small wave wounded machine number of parallel paths are 2, thus 2 brushes are used. For a large machine total number of brushes is equal to the total number of poles. The spacing between adjacent brushes is C/P commutator segments.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “EMF and Torque Production – 1”.

1. In which mode machine is operating, given that conductor current is in the same direction of conductor emf?

a) Motoring

b) Generating

c) Can’t be determined using directions

d) In both modes for different cycles

Answer: b

Explanation: If the conductor current is in the same direction of conductor emf then machine outputs electrical power and absorbs mechanical power. So, when mechanical power is absorbed machine is said to be in a generating mode. When conductor emf and conductor current are in opposite directions then machine is said to be in a motoring mode.

2. Nature of the flux density wave in the air gap is__________ 

a) Flat topped with quarter wave symmetry

b) Point topped with quarter wave symmetry

c) Flat topped with half wave symmetry

d) Point topped with half wave symmetry

Answer: a

Explanation: In a DC machine magnetic structure is such that the flux density wave in the air gap is flat topped with quarter wave symmetry as long as armature current is equal to 0. For non-zero value of armature current, this quarter wave symmetry is disturbed because of armature reaction.

3. In a DC machine, average energy stored in the magnetic field remains constant independent of the armature rotation.

a) True

b) False

Answer: True

Explanation: In a DC machine, barring the irrecoverable losses of both electric and magnetic origin, there is balance between electrical and mechanical powers of the machine; the average energy stored in the magnetic field remains constant irrespective of armature rotation.

4. Emf produced by DC machine, for zero armature current  and non-zero armature current  can be related as__________

a) E1 = E2

b) E1 > E2

c) E1 < E2

d) Can’t be determined

Answer: a

Explanation: In a DC machine flux density wave in the air gap is flat topped with quarter wave symmetry as long as armature current is equal to 0. For non-zero value of armature current, this quarter wave symmetry is disturbed because of armature reaction. Emf produced is independent of B-wave shape, thus we will get same value for both cases.

5. Average coil emf for 20 coil turns  and 40 coil turns , will have ratio E1/E2=____ .

a) 1/2

b) 2/1

c) 1/4

d) 4/1

Answer: a

Explanation: Emf generated in a DC machine is directly proportional to number of coil turns, Flux per pole, number of poles and armature speed in rad/s. Thus, ratio E1/E2= 20/40 .

6. What is the average coil emf generated in a 4-pole DC machine having flux/pole equal to 0.1 wb rotating at 1500 rpm? 

a) 19 kV

b) 1.9 kV

c) 190 V

d) 19 V

Answer: a

Explanation: Average coil emf generated= ∅ωNP/π.

E= 0.1*1500*100*4/3.14

E= 60000/3.14

E≅ 19 Kv.

7. Emf and torque produced in a DC machine are proportional to ________ and _________ respectively.

a) Armature speed and armature emf

b) Armature emf and armature speed

c) Armature current and armature emf

d) Armature speed and armature current

Answer: d

Explanation: Average coil emf generated= ∅ωNP/π. Machine torque = ka*∅*Ia. Thus, average coil emf generated can also be represented as ka*∅*ω. So, average coil emf is directly proportional to ω  and average torque is directly proportional to Ia .

8. What is the value of Np in an average coil emf equation, for 10 armature conductors with 2 parallel paths?

a) 2

b) 3

c) 2.5

d) 4

Answer: c

Explanation: In an emf equation Nc= Cp * Np. Here, Cp= coils/ parallel path. Np is defined as number of turns per parallel paths which is also called as ratio of total armature conductors to the twice of number of parallel paths. Np= 10/= 10/4= 2.5.

9.What is the torque equation in terms of B, Ic, l, Zr ?

a) Bav*Ic*l*Zr

b) Bav*Ic*l/Zr

c) Bav*Ic*Zr/l

d) Can’t be expressed

Answer: a

Explanation: Avg. conductor force f= Bav*l*Ic. Here, Bav= Average flux density over pole, l= acyive conductor length. Thus, torque T= Z*f = Bav*l*Ia*Z. This torque is constant because both the flux density wave and current distribution is fixed in space at all times.

T developed= Bav*Ic*l*Zr .

10. What is the value of pole pitch  for mean air gap radius= 0.5mm and P=4?

a) 0.785* 10 -6

b) 0.785* 10 -3

c) 0.785* 10 -2

d) 0.785* 10 -4

Answer: b

Explanation: Pole pitch is called as center to center distance between two adjacent poles. When measured in electrical degrees one pole itch is equal to 1800. Pole pitch can be calculated as ratio of 2πr/P.

Pole pitch= 2*3.14* 0.5* 10 -3 / 4= 0.785* 10 -3 m.

This set of DC Machines Questions and Answers for Freshers focuses on “EMF and Torque Production – 2”.

1. A 4-pole Dc wound machine is lap wound with 400 conductors. The pole shore is 20 cm long and average flux density over one-pole pitch is 0.4 T, the armature diameter being 30 cm. What is the value of flux/pole?

a) 0.188 Wb

b) 18.88 Wb

c) 0.0188 Wb

d) 1.888 Wb

Answer: c

Explanation: Flux is defined as flux density for a given surface area. Here, Surface area can be calculated and multiplied with B to give the value of flux. Flux= 2πr*l*B. now, for calculating flux per pole, divide it by P=4. So, Flux per pole after substituting all values is equal to 0.0188 Wb.

2. A 4-pole Dc wound machine is lap wound with 400 conductors. The pole shore is 20 cm long and average flux density over one-pole pitch is 0.4 T, the armature diameter being 30 cm. What is the value of induced emf?

a) 188 V

b) 276 V

c) 94 V

d) 188 mV

Answer: a

Explanation: Induced emf in a DC machine is equal to,

dc-machines-questions-answers-freshers-q2

3. Coil torque for 20 kA armature current  and 40 mA armature current , will have ratio T1/T2=____ .

a) 1/2

b) 2/1

c) 1/4

d) 4/1

Answer: a

Explanation: Torque produced in a DC machine is directly proportional to number of coil turns, Flux per pole, number of poles and armature current. Thus, ratio T1/T2= 20/40 .

4. If the no load speed of DC motor is 1300 rpm and full load speed is 1100 rpm, then its voltage regulation is ____________

a. 12.56%

b. 18.18 %

c. 17.39%

d. 18.39%

Answer: b

Explanation: For A DC machine when all other parameters are fixed average coil emf generated is proportional directly to the speed of the dc motor. Voltage regulation is defined as ratio of difference of no load voltage and full load voltage to the full load voltage. VR= *100%.

5. If the average coil emf of a DC motor is doubled and flux is halved  then its shaft speed will become ___________

a. Twice of the original speed

b. Square of the original speed

c. Four times of the original speed

d. Half of the original speed

Answer: c

Explanation: Induced emf in a DC machine is equal to,

dc-machines-questions-answers-emf-torque-production-2-q5

From the emf equation we get speed of the shaft i.e. n α E/Z, when all other parameters are kept constant. So, when E is doubled n becomes twice the original, halving flux on reduced emf will quadrupled the speed of a DC machine.

6. A 4-pole wave wound DC motor drawing an armature current of 20 A has provided with 360 armature conductors. If the flux per pole is 0.015 Wb then the torque developed by the armature of motor is _______

a. 10.23 N-m

b. 34.37 N-m

c. 17.17 N-m

d. 19.08 N-m

Answer: b

Explanation: DC Machine torque equation: T = ka*∅*Ia. Here, ka= ZP/, Z= total armature conductors, P= No. of poles, A= No. of parallel paths. For a wave winding A=2. So, substituting all the values in the torque equation we get torque equal to 34.37 N-m.

7. In a DC machine, what is the torque induced beyond the pole shoes?

a) 0

b) 2/π *∅*i

c) π *∅*i/2

d) Can’t be calculated

Answer: a

Explanation: In a construction of a DC machine poles are located in magnetically neutral region. The magnetic field at the pole terminals in a DC machine will be equal to 0. Thus, cross product with the current flowing through the armature yields zero.

8. For a constant emf, if field current is reduced then the speed of the DC motor will_____

a) Remains same

b) Increases

c) Decreases

d) Can’t say

Answer: b

Explanation: When the field current is reduced, the field produced by the field winding also reduces. Thus, the term Φ from the emf equation also decreases. For all other parameters kept constant speed of the DC machine is inversely proportional to field. Hence, speed of DC motor will increase.

9. For an ideal DC machine, which phenomenon will reduce the terminal voltage?

a) Armature reaction

b) Commutation

c) Armature ohmic losses

d) All will contribute in reducing the terminal voltage

Answer: b

Explanation: In an ideal case, Commutation does not reduce the terminal voltage of a dc machine. In a non-ideal case, commutation takes place improperly at desired timings, thus losses contribute to the terminal voltage reduction. Armature reaction, ohmic losses due to winding resistance contribute to the losses in the terminal voltage.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Electromagnetic Power and Circuit Models”.

1. Product of torque and mechanical angular velocity ω is_____

a) Ea/ω

b) Ea*Ia

c) ω/Ea

d) Can’t tell

Answer: b

Explanation: According to statement of energy conversion, electrical and mechanical power of the machine must balance in a machine. Ea*Ia is referred to as electromagnetic power. Thus, torque = / ω.

2. Condition for linear magnetization is______

a) φ α I f

b) φ α Ia

c) φ α 1/I f

d) φ α 1/Ia

Answer: a

Explanation: For a linear magnestization to take place, flux produced by the current flowing through particaluar flux producing coil must vay in direct proportion. Here, φ is produced due to field winding which carried current I f .

3. A 4-pole Dc wound machine is lap wound with 400 conductors. The pole shore is 20 cm long and average flux density over one-pole pitch is 0.4 T, the armature diameter being 30 cm. Here, motor is drawing 25 A current at 1500 rpm.

What is the available torque  at shaft?

a) 29.9

b) 59.8

c) 14.95

d) 44.85

Answer: a

Explanation: Gross mechanical power developed is equal to Ea*Ia. Ea*Ia is referred to as electromagnetic power. Thus, torque = / ω. Substituting all the values we get torque = 29.9 Nm.

4.When Ea>Vt machine is said to be operating in which of the following mode?

a) Depends on the Shaft speed

b) Generating

c) Motoring

d) Inducing

Answer: b

Explanation: The machine operates in generating mode  when Ia is in the direction of induced emf Ea. For the given armature circuit Vt = armature terminal voltage= Ea – Ia * Ra; which implies Vt < Ea.

5. When Ea<Vt machine is said to be operating in which of the following mode?

a) Depends on the Shaft speed

b) Generating

c) Motoring

d) Inducing

Answer: c

Explanation: The machine operates in motoring mode  when Ia is in the direction opposite of induced emf Ea. For the given armature circuit Vt = armature terminal voltage= Ea + Ia * Ra; which implies Vt> Ea.

6. Simple equation of DC machine operating in generating mode, with non-zero Ra value is________

a) Vt = Ea – Ia/Ra

b) Vt = Ea + Ia*Ra

c) Vt = – Ea + Ia*Ra

d) Vt = Ea – Ia*Ra

Answer: d

Explanation: The machine operates in generating mode  when Ia is in the direction of induced emf Ea, also Ea>Vt. For the given armature circuit, with non-zero value of Ra, Vt= armature terminal voltage= Ea- Ia*Ra.

7. Simple equation of DC machine operating in motoring mode, with non-zero Ra value is________

a) Vt = Ea – Ia/Ra

b) Vt = Ea + Ia*Ra

c) Vt = – Ea + Ia*Ra

d) Vt = Ea – Ia*Ra

Answer: b

Explanation: The machine operates in motoring mode  when Ia is in the direction opposite of induced emf Ea, also Ea<Vt. For the given armature circuit, with non-zero value of Ra, Vt= armature terminal voltage= Ea+ Ia*Ra.

8. When torque of the electromagnetic origin is in the opposite direction of rotation of armature, machine is said to be operating in which of the following mode?

a) Depends on other parameters

b) Generating

c) Motoring

d) Inducing

Answer: b

Explanation: In a generating mode, torque of the electromagnetic origin is in the opposite direction of rotation of armature, implying mechanical power is absorbed and a prime mover is needed to run the machine. Ea and Ia are in the same directions.

9. When torque of the electromagnetic origin is in the direction of rotation of armature, machine is said to be operating in which of the following mode?

a) Depends on other parameters

b) Generating

c) Motoring

d) Inducing

Answer: c

Explanation: In a motoring mode, torque of the electromagnetic origin is in the direction of rotation of armature, implying electrical power is absorbed and a prime mover is not needed to run the machine. Ea and Ia are in the opposite directions.

10. What is the armature current for a DC motor if terminal voltage is 255V and open circuit voltage is equal to 250V, where armature resistance is 0.05Ω?

a) 10A

b) 100A

c) 1KA

d) 1A

Answer: b

Explanation: Open circuit voltage means the voltage at armature current equal to 0, i.e. Vt=Ea= 250V. Actual Vt at loaded condition is 255V. For motoring mode, Vt – Ea = Ia*Ra. By substituting all the given values, we get Ia= 100A.

11. For a DC generator feeding 100kW power into 230V mains, having armature resistance and field resistance equal to 0.08Ω and 115Ω resp. The value of armature current is_____

a) 436.8 A

b) 434.8 a

c) 432.8 A

d) Data insufficient

Answer: a

Explanation: Field current is equal to 230/115 = 2A, by Ohm’s law. When running as a generator line current IL= 100k/230= 434.8A. Since power is supplied to 230V mains, Ia=Il+If = 434.8+2 =436.8A.

For motoring mode Ia= IL- If.

12. What is the armature current for a DC generator if terminal voltage is observed to be 245V and open circuit voltage is equal to 250V, where armature resistance is 0.05Ω?

a) 10A

b) 100A

c) 1KA

d) 1A

Answer: b

Explanation: Open circuit voltage means the voltage at armature current equal to 0, i.e. Vt=Ea= 250V. Actual Vt at loaded condition is 245V. For motoring mode, Ea – Vt= Ia*Ra. By substituting all the given values, we get Ia= 100A.

13. For a DC motor taking 10kW power from 230V mains, having armature resistance and field resistance equal to 0.08Ω and 115Ω resp. The value of armature current is_____

a) 45.68 A

b) 43.48 a

c) 41.48 A

d) Data insufficient

Answer: c

Explanation: Field current is equal to 230/115 = 2A, by Ohm’s law. When running as a motor line current IL= 10k/230= 43.48A. Since power is taken from 230V mains, Ia=IL- If = 43.48-2 =41.48A. For generating mode Ia= IL+ If.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Armature Reaction -1”.

1. In a DC machine, the form of armature mmf waveform is __________

a) Triangular

b) Sinusoidal

c) Saw tooth

d) Rectangular

Answer: a

Explanation: All the conductors on the armature periphery between adjacent brushes carry currents  in one direction and the current distribution alternates along the periphery. Because of commutator action, armature current distribution is in the steps of UNcIc. Thus, mmf waveform can be generalized by joining peak points to get triangular wave.

2. In a DC machine, the direct axis is __________

a) Axes of main poles

b) Axes perpendicular to axes of main poles

c) Not determined from the poles position

d) Can be drawn anywhere

Answer: a

Explanation: Direct axes is simply defined as the line passing through the axes of main poles. Maximum flux passes through this line. It’s also called as Direct Axis. Direct axes is always perpendicular to the geometrical neutral axis of machine.

3. In a DC machine, the form of flux density distribution  waveform is __________

a) Triangular

b) Sinusoidal

c) Saw tooth

d) Trapezoidal

Answer: d

Explanation: Flux density waveform is symmetrical and square wave with distortion at the zero points, causing wave to be in trapezoidal shape. The wave is flat topped, which get distributed due to armature mmf distribution, giving rise to the resultant flux distribution wave.

4. Due to the effect of armature reaction in DC machine, the flux per pole and generated voltage ______ and ________ respectively.

a) Increases, decreases

b) Decreases, decreases

c) Decreases, increases

d) Increases, increases

Answer: b

Explanation: The nature of armature reaction in a dc machine is cross-magnetizing with its axis  along the q-axis . It causes no change in flux/pole if the iron is unsaturated but causes reduction in flux/pole  in presence of iron saturation.

5. Armature reaction of an unsaturated DC machine is  ________

a) Cross-magnetizing

b) Demagnetizing

c) Magnetizing

d) Cannot be determined

Answer: a

Explanation: Initially at unsaturated condition in a DC machine armature reaction lies along the q-axis. It will cause no change in flux/pole if iron is unsaturated. Now, when iron gets saturated axis gets shifted which will cause reduction in flux/pole.

6. What is the effect of demagnetizing component of armature reaction?

a) Reduces generator emf

b) Increases armature speed

c) Reduces interpole flux density

d) Results in sparking trouble

Answer: a

Explanation: When the armature of a dc machine carries current, the distributed armature winding produces its own mmf  known as armature reaction. The demagnetizing component acts in the opposite direction, reducing flux/pole in a machine, which will ultimately reduce generator emf.

7. What is the reason behind short circuit in armature?

a) Insulation failure between two commutator bars

b) Insulation failure between two turns of a coil

c) Two or more turns of the same coil getting grounded

d) Insulation failure between two commutator bars, two turns of a coil or the same coil getting grounded

Answer: d

Explanation: Armature short circuit may occur due to contact of two commutator bars or due to contact in of two coil turns as commutators are connected to respective coil sides. If two or more turns of coil are grounded then they have common end which again leads to short circuit.

8. What will happen at poles due to armature reaction in DC generator?

a) Demagnetisation of leading pole tip and magnetisation of trailing pole tip

b) Demagnetisation of trailing pole tip and magnetisation of leading pole tip

c) Demagnetising at the centre of all poles

d) Magnetising at the centre of all poles

Answer: a

Explanation: Leading pole tip  and trailing pole tip  are the two edges of the pole, they depend upon the direction of motion of the armature . While performing a motion the armature first saws an edge of the pole, that edge is called leading pole tip. Thus, at leading pole tip there will be demagnetization.

9. In DC generator, how armature reaction is produced?

a) Its field current

b) Armature conductors

c) Field pole winding

d) Load current in armature

Answer: d

Explanation: When the armature of a dc machine carries current i.e. load current in armature, the distributed armature winding produces its own mmf  known as armature reaction. The tothe field ampere-turns (AT f ) and armature ampere-turns (AT a ).

10. In a DC generator, the effect of armature reaction on the main pole flux is to _________

a) Reduce it

b) Distort it

c) Reverse it

d) Reduce and distort it

Answer: d

Explanation: When non-zero load current is passed through the armature winding, the distributed armature winding produces its own mmf known as armature reaction. According to its nature cross-magnetizing and demagnetizing, it will distort or reduce the main flux distribution.

11. In a DC machine brushes are normally located along GNA.

a) True

b) False

Answer: a

Explanation: Brushes are generally located at 900 to direct axis. The axis 900 to the direct axis is called as quadrature axis . Generally, q-axis is along the geometric neutral axis  of machine. The brushes in a DC machine are normally located along the q-axis.

12. Armature reaction at 90 0 to the main field is called as ____________

a) Demagnetizing mmf

b) Cross-magnetizing mmf

c) Anti-magnetizing mmf

d) Magnetizing mmf

Answer: c

Explanation: The armature reaction flux strengthens each main pole at one end and weakens it at the other end . Armature reaction with axis at 90° to the main field axis is known as cross-magnetizing mmf.

13. Increase in flux density at one end of the pole is less than the decrease at the other end. This is called as ____________

a) Demagnetizing mmf

b) Cross-magnetizing mmf

c) Anti-magnetizing mmf

d) Magnetizing mmf

Answer: a

Explanation: If the main pole excitation is such that iron is in the saturated region of magnetization , the increase in flux density at one end of the poles caused by armature reaction is less than the decrease at the other end, so that there is a net reduction in the flux/pole, a demagnetizing effect.

14. Peak flux density in terms of total flux density is given by _________

a) AT a  = AT a  /P

b) AT a  = AT a  *P

c) AT a  = AT a  /P*P

d) AT a  = AT a  *P 2

Answer: a

Explanation: Peak flux density is defined as ratio of total flux density given to the number of poles for a given DC machine. Peak flux density in terms of total flux density is given by AT a  = AT a  /P.

This set of DC Machines Interview Questions and Answers for freshers focuses on “Armature Reaction – 2”.

1. Flux density in the interpolar region drops down because of ______

a) AT a 

b) Large air gap

c) Absence of magnetic poles

d) Depends on other parameters

Answer: b

Explanation: The exact way to find the flux density owing to the simultaneous action of field and armature ampere-turns is to find the resultant ampere-turn distribution AT resultant  = AT f  + AT a . The flux density of AT a  which, because of large air-gap in the interpolar region, has a strong dip along the q-axis even though AT a  is oriented along it.

2. Resultant ampere-turn distribution of a DC machine is given by _________

a) AT resultant  = AT f  – AT a 

b) AT resultant  = – AT f  + AT a 

c) AT resultant  = -AT f  – AT a 

d) AT resultant  = AT f  + AT a 

Answer: d

Explanation: The exact way to find the flux density owing to the simultaneous action of field and armature ampere-turns is to find the resultant ampere-turn distribution AT resultant  = AT f  + AT a , where ∅ is the electrical space angle.

3. Which axis undergo shifting as a result of armature reaction?

a) GNA

b) MNA

c) Both GNA and MNA

d) Remains fixed

Answer: b

Explanation: Apart from distortion of the resultant flux density wave, its MNA also gets shifted from its GNA by a small angle α so that the brushes placed in GNA are no longer in MNA as is the case in the absence of armature current.

4. Armature reaction in a machine is demagnetizing due to _________

a) Machine is designed with iron which is slightly saturated

b) Machine is designed with iron which is unsaturated

c) Depends on the application where machine is being is used

d) Can’t tell

Answer: a

Explanation: The armature reaction in a DC machine is cross-magnetizing causing distortion in the flux density wave shape and a slight shift in MNA. It also causes demagnetization because a machine is normally designed with iron slightly saturated.

5. Which of the following are effects of armature reaction?

a) Increase in iron losses

b) Commutation problems

c) Possibility of commutator sparking

d) Increase in iron losses, commutation problems and commutator sparking

Answer: d

Explanation: Armature reaction in a DC machine is a result of distortion of main field flux distribution by armature current, which produces its own mmf called armature mmf. Directly or indirectly armature reaction is the problem occurring in DC machine as it causes various effects, which reduce machine efficiency.

6. A 250 kW, 400 V, 6-pole dc generator has 720 lap wound conductors. Armature current is ____

a) 625A

b) 6.25A

c) 62.5A

d) 0.625A

Answer: a

Explanation: Armature current multiplied by the armature voltage is called as rating of a DC generator. Thus, 250 kW is the given rating while 400 V is the armature voltage. So, armature current is equal to 250*1000/400 = 625A.

7. What is the total ampere conductors/pole  if 600 lap wound conductors carry 120A current through conductors ?

a) 18000

b) 9000

c) 4500

d) 13500

Answer: a

Explanation: Ampere-conductors/pole =ZIc/P= Zla/AP. Ampere conductors per pole is calculated by multiplying total no. of conductors with the current carried by them divided by the total no. of poles.

Ampere-conductors/pole = 600*120/4 =18000.

8. What is the total ampere turns/pole  if 600 lap wound conductors carry 120A current through conductors ?

a) 18000

b) 9000

c) 4500

d) 13500

Answer: b

Explanation: Ampere-conductors/pole =ZIc/P= Zla/AP. Ampere turns per pole is calculated by multiplying total no. of conductors with the current carried by them divided by the twice the total no. of poles.

Ampere-turns/pole = 600*120/8 =18000/2= 9000 .

9. If total ampere turns per pole is equal to 6000 A-turns, peak ampere turns for a 4-pole machine is _____

a) 24000

b) 3000

c) 1500

d) 4500

Answer: c

Explanation: Peak flux density in terms of total flux density is given by ATa  = ATa  /P. Thus, for a 4-pole machine, ATa = 6000 and P=4. Thus, Peak flux density is equal to 6000/4= 1500.

10. What is the total ampere turns per pole for 720 lap wounded conductors with carrying armature current equal to 625A in a 6-pole machine?

a) 6252 AT/pole

b) 625.2 AT/pole

c) 62.52 AT/pole

d) 8252 AT/pole

Answer: a

Explanation: For a given machine number of parallel paths is equal to 6. So, conductor current will be equal to armature current divide by no. of parallel paths i.e. 625/6. Conductor current = 104.2 A. Total armature ampere-turns, ATa = ½= 6252 AT/pole.

11. For 6252 AT/Poles, if brush shift is of 2.50 mech. Degrees, what will be the demagnetizing ampere-turns per pole for a 6-pole DC machine?

a) 521

b) 5731

c) 5231

d) 571

Answer: a

Explanation: From given mech. Degrees shift we need to find electrical degrees shift. Electrical shift= mechanical shift*. Thus, electrical shift is equal to 7.50. Demagnetizing ampere-turns is given by 6250* = 521 AT/Pole.

12. For 6252 AT/Poles, if brush shift is of 2.50 mech. Degrees, what will be the cross-magnetizing ampere-turns per pole for a 6-pole DC machine?

a) 521

b) 5731

c) 5231

d) 571

Answer: b

Explanation: For calculations, from given mech. Degrees shift we need to find electrical degrees shift. Electrical shift= mechanical shift*. Thus, electrical shift is equal to 7.50. Cross-magnetizing ampere-turns is given by 6250* = 5731 AT/Pole.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Ways to Reduce Effects of Armature Reaction – 1”.

1. The armature reaction in a DC machine causes distortion in the main field flux. Effect of armature reaction can be reduced by_________

a) Increasing the length of air gap

b) Decreasing the length of air gap

c) Increasing the number of poles

d) Decreasing the number of poles

Answer: b

Explanation: Decreasing the air gap is simple to say but hard to achieve, due to various other limitations. But it is one of the way to reduce dropping down of armature mmf flux density distribution, which further reduces the distortion of resultant flux density.

2. In order to neutralize armature mmf perfectly under the pole shoe, the ampere-conductors of compensating winding must be  _______

a) Not equal to the total armature ampere conductors under the pole shoe

b) Equal to the total armature ampere conductors under the pole shoe

c) Half of the total armature ampere conductors under the pole shoe

d) Twice of the total armature ampere conductors under the pole shoe

Answer: b

Explanation: The number of ampere-turns required for compensating winding is AT cw /pole = AT a  * = [IaZ/]*. Here, pole arc= pole pitch, is given. Thus ampere-conductors required are equal to the total armature ampere conductors under the pole shoe.

3. What is the ampere turns per pole for compensating winding in DC machines?

a)  * armature ampere turns per pole

b)  * armature ampere turns per pole

c)  * total ampere conductors per pole

d) Cannot be determined

Answer: a

Explanation: The number of ampere-turns required for this purpose is ATcw /pole = ATa  * = [IaZ/]*. The compensating winding neutralizes the armature mmf directly under the pole while in the interpolar region, there is incomplete neutralization.

4. Where are no equalizer rings connected?

a) Only wave winding

b) Only lap winding

c) Both wave winding and lap winding

d) Cannot be determined

Answer: a

Explanation: The armature coils forming each of the two parallel paths are under the influence of all pole-pairs so that the effect of the magnetic circuit asymmetry is equally present in both the parallel paths resulting in equal parallel-path voltages. Thus, equalizer rings are not needed in a wave winding.

5. Inequality in brush arm currents caused due to different emf induced in different parallel paths may give rise to copper losses. These effects can be avoided by using _________

a) Compensating windings

b) Interpoles

c) Equalizer rings

d) Cannot be determined

Answer: c

Explanation: The armature coil in lap windings have equal to or more than 2 parallel paths. Under the influence of pole pairs there may arise asymmetry in various paths resulting in different parallel path voltages. So, equalizer rings are connected in order to achieve symmetry again.

6. Which of the following method will completely neutralize the armature reaction in a DC machine?

a) Only compensating winding

b) Only interpoles

c) Compensating winding and interpoles

d) Cannot be determined

Answer: c

Explanation: To neutralize the cross magnetizing effect of armature reaction, a compensating winding is used. The compensating windings consist of a series of coils embedded in slots of the pole faces. Interpoles are designed to overcome the effects of the armature reactance and the self-induction of the machine.

7. What is the exact location of compensating winding?

a) Across armature

b) In series with armature

c) Across armature or in series

d) Cannot be determined

Answer: b

Explanation: By adding a compensating winding in the pole face plate which carries armature current in the opposite direction of current in the adjacent armature windings, the position of the flux at the pole face plate can be restored to the position it would have with zero armature current.

8. Exact purpose of compensating winding in a DC generator is _________

a) Mainly to reduce the eddy currents by providing local short-circuits

b) To provide path for the circulation of cooling air

c) To neutralise the cross-magnetising effect of the armature reaction

d) Cannot be determined

Answer: c

Explanation: Armature reaction can be reduced with the help of compensating winding. To neutralize the cross-magnetizing effect of armature reaction in a DC machine, a compensating winding is used. The compensating windings consist of a series of coils embedded in slots of the pole faces.

9. Function of interpole flux is_________

a) Neutralise the commutating self-induced emf

b) Neutralise the armature reaction flux

c) Neutralise both the armature reaction flux as well as commutating emf induced in the coil

d) Perform none of the above functions

Answer: c

Explanation: Interpoles are designed to overcome the effects of the armature reactance and the self-induction of the machine i.e. to neutralise both the armature reaction flux as well as commutating emf induced in the coil.

10. Why dummy coils are connected in DC machine?

a) To reduce eddy current losses

b) To enhance flux density

c) To amplify voltage

d) To provide mechanical balance for the rotor

Answer: d

Explanation: Dummy coils are connected in armature winding of type wave winding. In a lap winding Y c = +/-1 irrespective of the number of armature coils so that coils can always be chosen to completely fill all the slots . In a wave winding the number of coils must fulfil the condition C =P/2 Y c +- 1 while at the same time C must also be governed by C =1/2 US.

11. The most likely cause of sparking at the brushes in a DC machine is /are ____________

a) Open coil in the armature

b) Defective interpoles

c) Incorrect brush spring pressure

d) Open coil in armature, defective interpoles and incorrect brush spring pressure

Answer: d

Explanation: Brushes are the point of contacts between rotating and non-rotating parts. So, if point of contact is rough then brushes will face damage and will lead to sparking in them. Defective interpoles will contribute in irregular commutation ultimately it will lead to sparking.

12. Each of the following statements regarding interpoles is true except______________________

a) They are small yoke-fixed poles spaced in between the main poles

b) They are connected in parallel with the armature so that they carry part of the armature current

c) Their polarity, in the case of generators is the same as that of the main pole ahead

d) They automatically neutralize not only reactance voltage but cross-magnetisation as well

Answer: b

Explanation: Interpoles are located in interpolar regions hence, they are called as interpoles. They are small narrow poles which speed up the commutation process .

This set of DC Machines Questions and Answers for Experienced people focuses on “Ways to Reduce Effects of Armature Reaction – 2”.

1. Axis undergo shifting as a result of armature reaction, can be balanced by ______

a) Increase in armature current

b) Decrease in armature current

c) Introducing interpoles

d) Removing interpoles

Answer: c

Explanation: Apart from distortion of the resultant flux density wave, its MNA also gets shifted from its GNA by a small angle α so that the brushes placed in GNA are no longer in MNA as is the case in the absence of armature current, due to armature reaction. This effect is countered by the interpoles placed in GNA.

2. The choice of average coil voltage determines the minimum number of commutator segments for its design.

a) True

b) False

Answer: a

Explanation: The maximum allowable voltage between adjacent segments is 30–40 V, limiting the average voltage between them to much less than this figure. The choice of the average coil voltage determines the minimum number of commutator segments for its design, to avoid any flashover and ultimately short circuit.

3. Compensating winding will provide incomplete neutralization ____________

a) Under pole region

b) In interpolar region

c) Everywhere

d) Complete neutralization

Answer: b

Explanation: The compensating winding neutralizes the armature mmf directly under the pole while in the interpolar region, there is incomplete neutralization. Further, the effect of the resultant armature mmf in interpolar region is rendered insignificant because of large interpolar gap.

4. Cross-magnetizing effect of armature reaction can be reduced by __________

a) Removing saturation in teeth and pole-shoe

b) Making smooth pole shoes

c) Introducing saturation in teeth and pole-shoe

d) Cannot be determined

Answer: c

Explanation: The cross-magnetizing effect of the armature reaction can be reduced by making the main field ampere-turns larger compared to the armature ampere-turns such that the main field mmf exerts predominant control over the air-gap flux. This is achieved by introducing saturation in the teeth and pole-shoe.

5. Cross-magnetizing effect of armature reaction can be reduced by __________

a) Removing saturation in teeth and pole-shoe

b) Making smooth pole shoes

c) Chamfering the pole shoes

d) Cannot be determined

Answer: c

Explanation: By chamfering the pole-shoes which increases the air-gap at the pole tips. This method increases the reluctance to the path of main flux in a DC machine but its influence on the cross-flux is much greater. This is because the cross flux has to cross the air-gap twice.

6. To counter the effect of shift in MNA due to armature reaction, which of the following component can be shifted?

a) Poles

b) Commutator

c) Brushes

d) Cannot be determined

Answer: c

Explanation: To counter the effect of shift in MNA due to armature reaction, the brushes could be shifted. A small brush shift in appropriate direction, in the direction of rotation for generator and in opposite direction for motor, also helps in commutation.

7. Calculate the number of conductors on each pole piece required in a compensating winding for a 6-pole lap-wound dc armature containing 286 conductors. The compensating winding carries full armature current. Assume ratio of pole arc/ pole pitch = 0.7.

a) 6

b) 8

c) 9

d) 7

Answer: a

Explanation: The number of ampere-turns required for compensating winding is AT cw /pole = AT a  * = [IaZ/] * .

N cw /pole =  *  = [286 / ] 0.7 = 2.78.

Compensating conductors/pole = 2 * 2.78 = 6 .

8. A compensating winding with ampere-turns greater than peak ampere turns is required in order to neutralize the effect of armature reaction because _____________

a) Pole arc = Pole pitch

b) Pole arc > Pole pitch

c) Pole arc < Pole pitch

d) Can’t be determined using pole arc, pole pitch

Answer: b

Explanation: The number of ampere-turns required for compensating winding in a DC machine is AT cw /pole = AT a  * = [IaZ/] * . Thus, if compensating winding ampere turns are more then, pole arc is definitely greater than pole pitch.

9. If pole arc is less than pole pitch, a compensating winding will have ampere-turns _________ .

a) Less

b) Equal

c) More

d) Can’t be specified

Answer: a

Explanation: The ampere-turns required for compensating winding in a DC machine is AT cw /pole = AT a  * = [IaZ/] * . Thus, if compensating winding ampere turns are less then, pole arc is smaller than pole pitch and vice-versa.

10. What is the pole arc/pitch ratio, if 360 AT compensating winding is used where 1960AT is peak value?

a) 0.7

b) 0.8

c) 0.9

d) 0.6

Answer: d

Explanation: The ampere-turns required for compensating winding in a DC machine is AT cw /pole = AT a  *. If compensating winding of 360AT is used the, 360/1960 will give ratio of pole pitch /pole arc, equal to 0.6.

11. Only drawback of compensating winding is _______

a) Cost

b) Unavailability of material

c) Construction

d) Not a single drawback

Answer: a

Explanation: Compensating winding is the best method in order to prevent the effect of armature reaction and its consequences. Only problem is compensating winding is expensive, but it is must to use them in machines with heavy overloads occur.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Commutation Process -1”.

1. Commutation is delayed due to __________

a) Leakage reactance

b) Effect of armature reaction

c) Leakage reactance and armature reaction

d) Because of other factors

Answer: c

Explanation: The leakage inductance L c of the coil undergoing commutation has induced in it reactance voltage L c (di c /d t ) which opposes the change in current thereby delaying commutation. The effect of armature reaction causes shift in MNA, delaying the whole commutation process ultimately.

2. Why brushes shifting method is not employed in practical commutation?

a) Expensive

b) Construction problems

c) Causes demagnetization

d) Used practically

Answer: c

Explanation: Brushes are located at GNA’s, a small voltage is induced in the commutating coil. It opposes current commutation as the commutating coil is cutting the flux which has the same sign as that of the pole being left behind. It could be partially remedied by shifting the brushes towards MNA but that causes direct demagnetization and is therefore not employed in practice.

3. Which are the methods for getting an ideal commutation?

a) Resistance commutation

b) Voltage commutation

c) Current commutation

d) Resistance and voltage commutation

Answer: d

Explanation: Adding resistance between commutator segments and brushes, thus, reducing L/R and consequently getting faster commutation is one of the method. In voltage commutation we, introduce narrow poles called as interpoles to fasten the process.

4. In resistance commutation method we add resistance between __________________

a) Brushes and external circuit

b) Commutator and armature winding

c) At field winding

d) Commutator and brush

Answer: d

Explanation: High contact resistance between commutator segments and brushes, achieved by using carbon brushes, adds resistance to the circuit of the commutating coil thereby reducing the time constant  of the current transient (i c ), helping it to change faster in the desired direction.

5. What is the effect on time constant of transient current, as a result of resistance commutation?

a) Remains same

b) Decreases

c) Increases

d) Increases then decreases

Answer: d

Explanation: We add high contact resistance between commutator segments and brushes thus, it adds resistance to the circuit of the commutating coil thereby reducing the time constant  of the current transient (i c ), helping it to change faster in the desired direction.

6. Which voltage is neutralized in voltage commutation process?

a) Armature

b) Reactance

c) Field

d) Cannot be determined

Answer: b

Explanation: To speed up the commutation process, the reactance voltage must be neutralized by injecting a suitable polarity dynamical  voltage into the commutating coil. In order that this injection is restricted to commutating coils, narrow interpoles are provided in the interpolar region.

7. Interpoles are excited with ____________

a) Armature current

b) Field current

c) Separate supply

d) Mains current

Answer: a

Explanation: These interpoles apply a local correction to the air-gap flux density wave such that a pip of appropriate flux density exists over the commutating coil to induce in it a voltage of the same sign as that of coil current after commutation. Hence, they are excited with armature current.

8. Interpoles are excited by keeping them in ____________ with armature.

a) Series

b) Parallel

c) Anywhere

d) Not kept with armature

Answer: a

Explanation: In voltage commutation method we use interpoles to speed up the commutation process. For neutralization of reactance voltage at all loads, the interpoles must be excited by armature current by connecting them in series with armature.

9. Polarity of interpoles is one pole ahead in the direction of armature rotation in _____________

a) Motor

b) Generator

c) Always ahead

d) Always behind

Answer: b

Explanation: polarity of an interpole is that of the main pole ahead in the direction of armature rotation for the generating mode and that of the main pole left behind with respect to the direction of rotation for motoring mode.

10. What is the size of interpolar air gap compare to main pole air gap?

a) Small

b) Same

c) More

d) Can be of any size

Answer: c

Explanation: The interpolar air-gap is kept larger than that of the main pole so that their magnetic circuit is linear resulting in cancellation of the reactance voltage  at all loads. Large air-gap results in greater amount of leakage flux which is accommodated by tapering the interpoles with a wider base.

11. Formula for cancellation of reactance voltage on an average basis using interpoles ________

a) 2[B i l i v a ] N c = L c (di c /dt) = L c (2l c /t c )

b) [B i l i v a ] N c = L c (di c /dt) = L c (2l c /t c )

c) 2[B i l i v a ] N c = L c (di c /dt) = L c (l c /t c )

d) Cannot be determined

Answer: a

Explanation: 2[B i l i v a ] N c = L c (di c /dt) = L c (2l c /tc).

With Bi determined by above equation, the ampere-turns needed to cancel the armature reaction ampere-turns and then to create the necessary flux density are given by, AT i = AT a +(B i /µ 0 )lg i

12. For a given 4-pole machine, carrying armature current 56.82 A, with 846 conductors. The mean flux density in the air gap under the interpoles is 0.5 Wb/m 2 on full load and radial gap length is 0.3 cm. Ampere-turns required for an interpole is ____________

a) 3198

b) 2099

c) 4198

d) 6297

Answer: c

Explanation: Required ampere-turns are given by AT i = AT a  +(B i /µ 0 )lg i . Substituting the

AT i =[/ ] +(0.5/4π*10 -7 )* 0.3*10 -2 =4198. Turns can be found by dividing with armature current.

13. A 440 V, 4-pole, 25 kW, dc generator has a wave-connected armature winding with 846 conductors. The mean flux density in the air-gap under the interpoles is 0.5 Wb/m 2 on full load and the radial gap length is 0.3 cm. Number of turns required on each interpole is _________

a) 74

b) 84

c) 64

d) 54

Answer: a

Explanation: Corresponding value of ampere-turns for an interpole

AT i = AT a  +(B i /µ 0 ) lg i .

= I a * Z/2*A*P + (B i /µ 0 ) lg i .

Assuming I a = I line I a = 25*10 3 /440= 56.82 A.

AT i = [/ ] +(0.5/4π*10 -7 )* 0.3*10 -2 =4198.

N i = 4198/56.82= 74.

This set of DC Machines Interview Questions and Answers for Experienced people focuses on “Commutation Process – 2”.

1. To achieve spark less commutation brushes of a DC generator are placed ________

a) just ahead of magnetic neutral axis

b) in magnetic neutral axis

c) just behind the magnetic neutral axis

d) can be placed anywhere

Answer: a

Explanation: Brushes collect the current due to the induced emf in the armature coils. When a brush is at any particular commutator segment, it shorts out that particular coil and draws current from the rest of the coils and fed to the commutator. To achieve all positive outcomes, we place them just ahead of MNS.

2. If in the DC machine, the reversal of current in the coil is faster than ideal or linear commutation then the commutation is said to be __________ commutation.

a) Retarded

b) Curvilinear

c) Accelerated

d) Under

Answer: c

Explanation: Speed of the commutation is dependable on change in an induced current direction. When reversal of current in coil is faster then, obviously change of coils are taking place at faster rate. Thus, commutation is said to be accelerated commutation.

3. For a DC machine, in a commutator ________

a) Copper is harder than mica

b) Mica and copper are equally hard

c) Mica is harder than copper

d) Cannot be determined

Answer: c

Explanation: Due to its mechanical strength and insulating properties mica is a satisfactory material. However, mica is much harder than the copper segments, so during manufacturing it requires to under-cut the mica by sawing slots between the adjacent segments of the commutator.

4. The insulating material used between the commutator segments is normally _______

a) Graphite

b) Paper

c) Air gap

d) Mica

Answer: d

Explanations: Each conducting segment of the commutator is insulated from adjacent segments. Mica is a good electric insulator and good thermal conductor as well. Its applications in electric fields are derived from its unique mechanical properties. Thus, it allows mica to be ductile enough for its appropriate space of application.

5. Why interpoles are tapered in a DC machine?

a) Simpler design

b) Reduction in the weight

c) Increase in acceleration of commutation

d) Cannot be determined

Answer: c

Explanation: To speed up the commutation process, the reactance voltage must be neutralized by injecting a suitable polarity dynamical  voltage into the commutating coil. In order that this injection is restricted to commutating coils, narrow interpoles are provided in the interpolar region.

6. The main function of interpoles in a loaded DC machine is to minimize _______between the brushes and the commutator.

a) Friction

b) Sparking

c) Current

d) Wear and tear

Answer: b

Explanation: Interpoles are introduced in a DC machine in order to speed up the commutation process, sp that sparking will be minimised. As, sparking arises at end of commutation period when commutation is not completed in given time.

7. Which of the following is different component?

a) Commutating poles

b) Compoles

c) Interpoles

d) Compensating winding

Answer: d

Explanation: Interpoles are located in interpolar region in a DC machine, called as interpoles. They raise up the speed of voltage commutation. So, they are also called as commutating poles. Compensating winding though used for reducing armature reaction, performs different function compare to interpoles.

8. How many coils under an adjoining pole pairs is/are connected between adjacent commutator segments in lap winding?

a) 1/2

b) 2

c) 1

d) 1/4

Answer: c

Explanation: No. of parallel paths in a lap winding are equal to P . Thus for 1 pole pir there will be exact 1 coil, connected to adjacent commutator segments in lap winding. In wave winding there will be half coils connected between adjacent commutator segments.

9. In wave winding, P/2 coils are connected under the influence of “x” pole-pairs which connect adjacent segments. X is ____

a) P

b) P/2

c) 2P

d) P/4

Answer: b

Explanation: One coil each under an adjoining pole-pair is connected between adjacent commutator segments in a lap wound DC armature, while in a wave-wound armature the only difference is that P/2 coils under the influence of P/2 pole-pairs are connected between adjacent segments.

10. The process of current reversal takes place when the coil is passing through the interpolar region.

a) True

b) False

Answer: a

Explanation: The process of current reversal called commutation takes place when the coil is passing through the interpolar region  and during this period the coil is shorted via the commutator segments by the brush located  in the interpolar region.

11. How many coil sets undergo commutation simultaneously in a wave winding?

a) 1

b) 2

c) 4

d) 3

Answer: b

Explanation: Commutation takes place simultaneously for P coils in a lap-wound machine  and two coil sets of P/2 coils each in a wave-wound machine .

12. Which coil is shorted in commutation process?

a) Coil under north pole

b) Coil under south pole

c) Coil lying in an interpolar region

d) Cannot be determined

Answer: c

Explanation: During the commutation period, the coil is shorted via the commutator segments by the brush. These brushes are located in interpolar regions electrically, and magnetically in neutral region. Thus, interpolar coil gets shorted.

13. Ideal commutation can be shown graphically by _____________

a) Straight line passing through origin

b) Straight line not passing through origin but with +ve slope

c) Straight line not passing through origin but with -ve slope

d) Curve increasing towards +ve t axis

Answer: c

Explanation: Ideal Commutation  is that in which the current of the commutating coils changes linearly from + Ic to – Ic in the commutation period. Thus, it will form a straight line with -ve slope.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Methods of Excitation”.

1. Which winding has large number of turns?

a) Shunt field

b) Series field

c) Both have same number of turns

d) Depends on requirement

Answer: a

Explanation: The shunt field winding is provided with a large number  of turns of thin wire and is excited from a voltage source. The series field winding has a few turns of thick wire and is excited from armature current by placing it in series with armature.

2. Which winding contains wire with higher thickness?

a) Shunt field

b) Series field

c) Both have same number of turns

d) Depends on requirement

Answer: b

Explanation: Series field winding is used when wire is thick. Thus, by making minimum number of turns it can be used to excite a DC machine. For a given field current, control of this field is achieved by means of a diverter, a low resistance connected in parallel to series winding.

3. Which winding have higher resistance?

a) Shunt field

b) Series field

c) Both have same number of turns

d) Depends on requirement

Answer: a

Explanation: The shunt field winding is provided with a large number  of turns of thin wire and is excited from a voltage source. The field winding, therefore, has a high resistance and carries a small amount of current. It is usually excited in parallel with armature circuit.

4. Which is more practical method used in control of series field?

a) Diverter

b) Tappings

c) Switch relay

d) Cannot be determined

Answer: b

Explanation: For a given field current, control of series field is achieved by means of a diverter, a low resistance connected in parallel to series winding. A more practical way of a series field control is changing the number of turns of the winding by suitable tappings which are brought out for control purpose.

5. In compound excitation, which winding/s is/are excited?

a) Shunt

b) Series

c) Both

d) Cannot be determined

Answer: c

Explanation: In compound excitation, both series and shunt windings are connected to the armature. For compound excitation both of these windings are excited. Two types are cumulative excitation and differential excitation.

6. In which type of excitation air gap flux increases with armature current?

a) Differential compound

b) Cumulative compound

c) Differential and Cumulative

d) Cannot be determined

Answer: b

Explanation: In compound excitation both shunt and series field are excited. If the two field aid each other , the excited is called cumulative compound. The shunt field is much stronger than the series field. The air gap flux increases with armature current.

7. In differential compound excited machine, what is the variation in air gap flux per pole with respect to armature current?

a) Increases

b) Decreases

c) Remains constant

d) Always varies

Answer: b

Explanation: If the two fields oppose each other, the excitation is called differential compound. The air gap flux/pole decreases with armature current. The series field is so designed that the increase or decrease in flux/pole is to a limited extent.

8. Which winding in compound excitation is responsible for change in air gap flux per pole?

a) Series

b) Parallel

c) Interconnected

d) No coil is responsible

Answer: a

Explanation: In a compound excited machine, both series and parallel windings are connected with armature circuit. The series winding is specially designed for increasing or decreasing flux per pole in DC machine up to certain extent.

9. How short shunt and long shunt compound winding is selected?

a) Mechanical considerations

b) Switch reversal

c) Performance

d) Mechanical considerations and switch reversal

Answer: d

Explanation: In long shunt compound, the shunt field is connected across terminals. In short shunt compound, the shunt field is connected directly across the armature. There is no significant difference in machine performance for the two types of connections. The choice between them depends upon mechanical consideration or the reversing switches.

10. If a DC compound machine connected as a motor is about to use as a generator, we reverse the series field connections.

a) True

b) False

Answer: a

Explanation: If a dc compound machine connected as a generator is run as a motor, the series field connections must be reversed as the armature current reverses. The motoring action as cumulative/ differential would then be preserved . This equally applies vice versa – motor to generator.

11. For a long-shunt compound motoring, which of the following equation is correct?

a) V t = E a + I a (R a + R se )

b) V t = E a – I a (R a + R se )

c) V t = E a + I a (R a – R se )

d) V t = -E a + I a (R a + R se )

Answer: a

Explanation: For a long shunt compound motor Rse is connected in series with armature, while in short shunt compound motors Rse is connected in series with terminal voltage. I L is supplied through the terminals which split into I f and I a .

12. V t = E a – I a (R a + R se ) is the equation for _______________

a) Short shunt compound motoring mode

b) Short shunt compound generating mode

c) long shunt compound motoring mode

d) Cannot be determined

Answer: b

Explanation: In a DC machine, for a long shunt compound motor Rse is connected in series with armature, while in short shunt compound motors Rse is connected in series with terminal voltage. IL is supplied to the terminals which is equal to I a – I f .

13. How shunt field is controlled?

a) Diverter resistor in parallel

b) Tapped field winding

c) Series regulating resistance

d) Other methods

Answer: c

Explanation:

Control of Excitation: 1) Shunt field: by a series regulating resistance.2) Series field: For small armature by a diverter resistance connected in parallel with series field. For large armature by tapped field winding so the winding turns can be changed.

14. The generator is called flat compounded if _____________

a) The series field ampere turns are such as to produce the same voltage at rated load as at no load

b) The series field turns are such as that the no load voltage is smaller than the rated load voltage

c) The rated voltage is less than the no load voltage

d) Cannot be determined

Answer: a

Explanation: According to the operating characteristics of a DC compound generator, if series field mmf produces same voltage at rated load as that of no load then it is called as flat compounded generator.

This set of DC Machines Assessment Questions and Answers focuses on “No Load with On Load Characteristics of DC Generator”.

1. Which of the following is not the operating characteristics of Dc generator?

a) No-load characteristics

b) Load characteristics

c) External characteristics

d) Internal characteristics

Answer: d

Explanation: The relationship between various parameters has to be presented graphically because of the magnetic saturation effect. Four characteristics of importance are the following: 1) No load characteristics 2) Load characteristics 3) External characteristics 4) Armature characteristics.

2. Characteristics drawn at Ia = 0 is also called as ____________

a) Magnetization characteristics

b) Non-magnetization characteristics

c) Anti-magnetization characteristics

d) Cannot be determined

Answer: a

Explanation: With I a = 0  at constant n, it is the presentation of Vt  vs If. This is the most important characteristic as it reveals the nature of the magnetization of the machine. It is easy to determine as the generator is on no load and so only low rated prime mover will serve the purpose. It is commonly called the open–circuit/magnetization characteristic.

3. Open circuit characteristics  is generally drawn across __________

a) E a vs I f , I a =constant 

b) E a vs I f , I a =0

c) E a vs I f , I a =constant

d) E a vs I f , I a =constant 

Answer: b

Explanation: Open circuit characteristics is also called as no-load characteristics or magnetization characteristics. No load clearly states that armature current will equal to 0. Thus, OCC is drawn at E a vs If, I a =0.

4. Characteristics of a DC generator drawn across Vt vs If at rated armature current and constant speed, is called as ____________

a) Load characteristics

b) No-load characteristics

c) External characteristics

d) Armature characteristics

Answer: a

Explanation: Since we have I a value which is equal to rated i.e. non-zero, it is indeed not a no-load characteristic. Axes given are V t and I f , hence it is not an armature characteristic. Thus, it’s called as load characteristic or magnetization characteristic on load.

5. In an OCC at If =0, graph starts from origin.

a) True

b) False

Answer: b

Explanation: As the machine would have been previously subjected to magnetization, a small residual voltage would be present with field unexcited. As will be seen practically, this is necessary for generator to self-excite. So, graph will start from just above the origin on Voc axis.

6. While conducting OCC, in order to avoid hysteresis loop, in which direction If should be increased?

a) -ve direction

b) +ve direction

c) In any direction

d) In both direction there exists hysteresis loop

Answer: b

Explanation: In conducting the OCC test, If must be raised gradually only in the forward direction otherwise the curve would exhibit local hysteresis loops. In OCC at If =0 there exists small residual voltage shown by non-zero Voc.

7. Air gap line is drawn at iron _________

a) Saturated

b) Unsaturated

c) Moderately saturated

d) Variable saturation

Answer: b

Explanation: The extension of the liner portion of the magnetization curve, is known as the air-gap line as it represents mainly the magnetic behaviour of the machine’s air-gap, the iron being unsaturated in this region consumes negligible ampere-turns; in any case the effect of iron is also linear here.

8. If suppose OCC is conducted at speed n1, where n1< nrated, OCC will lie ____________

a) Above OCC at nrated

b) On OCC at nrated

c) Below OCC at nrated

d) Can’t comment by only speed information

Answer: c

Explanation: For a less speed than the rated one, residual voltage appearing at terminal call Voc will also be less than Voc at rated value, it will vary in parallel manner but will never intersect OCC at rated speed.

9. E a can be determined using __________

a) No-load characteristics

b) Load characteristics

c) Cannot be determined

d) Above OCC

Answer: a

Explanation: Under load conditions Ea cannot be determined from the OCC for If in the saturation region because of the demagnetizing effect of armature reaction. We must therefore determine experimentally the equivalent demagnetizing ampere-turns ATd due to armature reaction under actual load conditions.

10. If load characteristics are drawn on OCC itself, we get curve ________

a) Above OCC

b) On OCC

c) Below OCC

d) Intersecting OCC

Answer: c

Explanation: Since on load operation of a DC machine, we’ll get terminal voltage less than the terminal voltage obtained in OCC, graph will start from below OCC. On load, the effect of armature reaction will draw load characteristics parallel to the OCC below it, causing no intersection.

11. Load characteristics drawn at Ra =0 and Ra not equal to 0, will lie _____

a) Above

b) On

c) Below

d) Intersecting

Answer: c

Explanation: Load characteristics with at Ra =0 will lie below the load characteristics drawn at Ra not equal to 0. To the load characteristic we add IaRa drop to get Ea induced emf with load. Thus, it will lie above.

12. OCC is drawn at two different speeds both less than rated speed. OCC drawn at speed N1 lies below OCC drawn at speed N2. Which of the following relation is correct?

a) N2 = N1

b) N2 < N1

c) N2 >> N1

d) Can’t comment

Answer: c

Explanation: As a speed on which OCC is taken decreases, the residual voltage appearing on Voc axis also decrease and OCC starts from below, compare to first one. Thus, N1 is less comparatively, as its OCC lies below than the OCC drawn at other speed.

13. Why No-load or load characteristics are also called as magnetization characteristics?

a) E a α I f

b) E a α φ

c) I f α φ

d) Cannot be determined

Answer: b

Explanation: As the generated voltage in the armature in the case of DC generator is proportional to terminal voltage, which also proportional to magnetic flux, as seen by residual voltage appearing at 0 field current. No-load and load characteristics are called as magnetization curves.

This set of DC Machines Problems focuses on “External and Armature Characteristics of DC Generator”.

1. Armature characteristic is _____________

a) No-load characteristic

b) Load characteristic

c) OCC Current

d) Cannot be determined

Answer: b

Explanation: It is the presentation of I a vs I f with V t held constant  and generator run at constant n and load varied. It reveals the armature reaction effect on the flux/pole. It is also called regulation characteristic.

2. External characteristics is drawn with constant term/s ____________

a) Field current

b) Speed

c) Both Field current and Speed

d) Load

Answer: c

Explanation: External characteristic is a load characteristic. In this, variation of terminal voltage across armature current is drawn keeping speed and field current constant. Load is generally kept variable.

3.Armature characteristic is drawn across ________

a) I a vs I f

b) I a vs V t

c) I f vs V t

d) E a vs I a

Answer: a

Explanation: Armature characteristic is drawn across I a vs I f . This characteristic is on load characteristic, drawn when terminal voltage and speed of the generator is kept constant. Armature characteristic is regulation characteristic.

4. what is the first step in drawing the armature characteristic?

a) I f is made 0

b) Field current is adjusted to give Voc equal to rated value

c) Switch connecting armature circuit with external circuit is made open

d) Cannot be determined

Answer: c

Explanation: While drawing armature characteristic, switch of armature circuit is made open so that armature current is set to 0. Then, field current is adjusted to give V oc equal to rated value. Switch its then closed.

5. In armature characteristic, nature of If at lower values of armature current is ______

a) Very large

b) Almost constant

c) Decreasing rapidly

d) Increasing then decreasing

Answer: b

Explanation: It is seen from the characteristic that at low values of I a , the increase in I f is very small to provide for increasing Ia *Ra drop. At large values of Ia there is a sharp increase in If to compensate for voltage drop caused by armature reaction.

6. At some point If On OCC, the value of voltage obtained which is more than rated voltage is _______

a) I a *R a + I f *R f

b) I a *R a

c) V d + I a *R a

d) Can’t tell

Answer: c

Explanation: The difference in the observed value and rated value of voltage can be obtained from the y-axis that is, from V oc . This difference is equal to V d + I a *R a . Thus, by subtracting voltage drop in armature V d can be found.

7. Which of the following is the correct formula for cumulative compound generator?

a) N se = N f (ΔI a /I f )

b) N se = N f (ΔI f /I a )

c) N se = 2N f (ΔI a /I f )

d) N se = N f (ΔI a /I f )/2

Answer: b

Explanation: According to proportionality constant obtained by analysis of OCC, various value of armature current can be obtained. From ΔI f values we can compute number of turns in series of armature in cumulative compound machine.

8. External Characteristic is a plot of _____________

a) Generated emf and load current

b) Terminal voltage and load current

c) Generated voltage and field current

d) Armature current and field current

Answer: b

Explanation: The load characteristic of a dc generator at a particular speed is the relationship between its terminal voltage and load current  and is also termed as the external characteristic. The internal characteristic is the plot between the generated emf and load current.

9. External characteristics for all separately DC generators  can be summarized as ____________

a) Parabola opening in +x direction

b) Parabola opening in -x direction

c) Parabola opening in +y direction

d) Parabola opening in -y direction

Answer: b

Explanation: All parabolas for separately excited DC generator, shunt generator, compound generator are parabolas which are opening in -x directions, only centres of all parabolas lie at different places depending on the type of generator.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Characteristics of Self Excited Generator”.

1. What is Self-excitation in DC shunt generator?

a) Field winding is connected in series of armature

b) Field winding is connected in parallel of armature

c) Field winding is not connected to the armature

d) Field Winding is not excited

Answer: b

Explanation: Rather than arranging a separate dc source for excitation purposes, practical generators are always excited from their own armature terminals, this method of excitation being known as self-excitation. A self-excited generator with such connection known as a shunt generator.

2. For a self-excited DC shunt generator I a = _____________

a) I L – I f

b) I L + I L

c) – I L – I L

d) – I L + I L

Answer: b

Explanation: For a self-excited DC shunt generator field winding is connected in parallel with armature winding so that, armature current splits into load current and field current. Load current will be given out as the machine is working as a generator.

3. For self-excited DC generator, field circuit curve in the OCC will be __________

a) Straight line not passing through the origin with +ve slope

b) Straight line passing through the origin

c) Straight line not passing through the origin with -ve slope

d) Increasing curve

Answer: b

Explanation: Field is connected such that this current increases the field mmf and therefore the induced emf, the machine will continuously build up. For the field circuit V = I f *R f , which is a straight-line relationship, called the Rf -line, in V-I f plot.

4. Intersection of Rf line and magnetization curve will give _____________

a) No load terminal voltage

b) Field current at no load

c) Both a and b

d) Not specific data

Answer: c

Explanation: When OCC is plotted against terminal voltage and field current where, field resistance line is also drawn, an intersection point will arise when OCC almost becomes constant due to saturation effect. So, corresponding value field current and no-load voltage can be observed from x and y axis resp.

5. What will happen if field resistance is decreased?

a) No load voltage will decrease

b) No load voltage will increase

c) No change in no load voltage

d) Field current will decrease

Answer: b

Explanation: When field resistance is decreased, it will increase field current by Ohm’s law. Thus, field resistance line will lie below the older line of field resistance. Eventually, it’ll intersect OCC afterwards causing increase in no-load voltage.

6. No-load voltage can’t be defined where _____________

a) Field resistance line lies below the magnetization linear line

b) Field resistance line intersects the magnetization linear line

c) Field resistance line coincides the magnetization linear line

d) Defined everywhere

Answer: c

Explanation: We get no-load voltage when magnetization curve intersects with field resistance line. If that intersection is not occurring, we’ll not get no load voltage. Thus, when both lines coincide we get undefined value of no-load voltage.

7. Machine does not excite to an appreciable value when ___________

a) Field resistance is less than critical resistance

b) Field resistance is more than critical resistance

c) In both cases a and b

d) Depends on other parameters

Answer: b

Explanation: The no-load voltage is undefined for a field resistance whose line coincides with the linear portion of the magnetization curve. With field resistance even slightly more than this value, the machine does not excite to any appreciable value.

8. No-load voltage observed at two different field resistance values, is V1 < V2. What will be the relation in field resistance values?

a) Rf 1 < Rf 2

b) Rf 1 = Rf 1

c) Rf 1 > Rf 1

d) Can’t determine from no-load voltage

Answer: c

Explanation: When field resistance value increases the line of field resistance shifts towards more +ve slope, proving that field current decreases. This line will now intersect OCC at lower regions indicating lower values of terminal voltage.

9. No-load voltage observed at two different field resistance values, is V1 < V2. What will be the relation in speed if field resistance is kept same for both cases?

a) N 1 < N 2

b) N 1 = N 1

c) N 1 > N 1

d) Can’t determine from no-load voltage

Answer: a

Explanation: For Field resistance kept constant, field resistance line will intersect OCC at different points as speed variation may shift OCC up or down. When speed is increased OCC shifts up indicating that no-load voltage will also rise up.

10. DC generator will not self-excite in __________

a) Positive feedback

b) Negative feedback

c) Zero Feedback

d) Doesn’t depend on feedback

Answer: b

Explanation: The field connection to the armature is such that the induced emf due to the residual magnetism tends to destroy the residual magnetism, is called negative feedback. In the absence of residual magnetism machine will fail to self-excite.

11. Which of the following is not a reason, behind failing of self-exciting machine?

a) Residual magnetism is absent

b) Negative feedback

c) Field resistance value is more than critical resistance value

d) Armature resistance is non-zero

Answer: d

Explanation: In case a and b, machine will tend to destroy the residual magnetism present in a core, which will not excite machine. When field resistance value is more than critical value as seen from OCC, machine will not excite to appropriate value.

12. By changing the polarity of the armature of failed self-exciting machine, machine can be started.

a) True

b) False

Answer: a

Explanation: By changing the polarity of field connections to the armature negative feedback can be turned into positive feedback, thus it will help to build up the voltage and machine will act good in self-excitation mode.

13. Which of the following will be useful to generate positive feedback?

a) Increasing the field resistance

b) Increasing the armature resistance

c) Decreasing the armature resistance

d) Changing the shaft direction

Answer: d

Explanation: Negative feedback can be remedied simply by reversing the field connection to the armature or reversing the direction of rotation. For large dc generators with permanent connections and a fixed direction of rotation, the problem is overcome by temporarily exciting the field from a battery source.

This set of DC Machines test focuses on “Characteristics of Separately Excited Generators”.

1. Characteristics of separately excited DC generator are drawn by keeping _____

a) Field current and speed both constant

b) Field current and speed both variable

c) Field current constant and speed variable

d) Field current variable and speed constant

Answer: b

Explanation: The operation considered here assumes that the armature is driven at constant speed  and the field excitation  is adjusted to give rated voltage at no-load and is then held constant at this value throughout the operation considered.

2. What is the reason behind dropping down of E a with load?

a) Field resistance

b) Load resistance

c) Internal factors

d) Armature reaction

Answer: d

Explanation: In spite of fixed excitation, E a drops off with load owing to the demagnetizing effect of the armature reaction. As the voltage drop is caused by magnetic saturation effect, it increases with load non-linearity.

3. External characteristic differ from an internal characteristic in separately excited DC generator by _________

a) I a *R a

b) I f *R a

c) I L *R L

d) I f *R f

Answer: a

Explanation: Internal characteristic is drawn across armature generated voltage, it doesn’t account the presence of armature resistance. External characteristic takes into account the presence of armature resistance as it is drawn across terminal voltage.

4. The variation in terminal voltage of DC shunt generator with respect to variation in separately excited DC generator is ___________

a) Much rapid

b) Much slower

c) Remains constant

d) Can’t say

Answer: a

Explanation: The terminal voltage drops off much more rapidly with load in a shunt generator than in a separately-excited generator because of fall in field current with terminal voltage. The external characteristic is a double-valued curve with a certain IL .

5. Which of the following characteristic lies above of all others?

a) Differential compound

b) Under compound

c) Level compound

d) Over compound

Answer: d

Explanation: All the graphs when drawn across voltage and load current, start from the same point but with increase in values of load current all machines show different elevation according to the values of series field resisitor.

6. Why differential compound generator is not used in practice?

a) High cost

b) High maintenance

c) High drop down in voltage

d) Difficult construction

Answer: c

Explanation: At a given value of load current, differential compound machine gives lowest voltage output. As the load current increases drop down in terminal voltage of DC differential compound generator is maximum.

7.How compounding level in a compound machine is adjusted?

a) By adding variable resistance in series with series field resistance

b) By adding variable resistance in parallel with series field resistance

c) By adding fixed resistance in parallel with series field resistance

d) By adding fixed resistance in series with series field resistance

Answer: b

Explanation: When fixed resistance is added in parallel with series field resistance we get only one other compounding level. So, by adding variable resistance in parallel with series field we can get various other compounding levels.

8. Which of the following have different external characteristic than other?

a) Self excited DC shunt generator

b) Separately excited generator

c) Compound DC generator

d) Series DC generator

Answer: d

Explanation: Unlike Self excited DC shunt generator, separately excited generator, compound DC generator the voltage at zero load current for DC series motor doesn’t start from some positive non-zero value, instead it starts from origin.

9. External characteristic differ from internal characteristic in DC series motor by ______

a) I a *(R a + R SE )

b) I a *R a

c) I a *R SE

d) If *R SE

Answer: a

Explanation: In series DC generator series resistor R SE is added with armature circuit, thus total drop in terminal voltage from generated armature voltage is equal to I a *(R a + R SE ). While drawing internal characteristics presence of armature resistance and series resistance is not taken into account.

10. For series DC generator, internal/external characteristic start from ____________

a) Positive non-zero voltage

b) Zero voltage

c) Negative non-zero voltage

d) Can start from anywhere

Answer: b

Explanation: For all other DC generators other than series DC generators, the external and internal characteristic as well, start from non-zero positive value of voltage. While in series DC generator both internal and external characteristic start from origin.

11. For a DC series generator what is the condition for self-excitation?

a) (R a +R se ) > RC

b) (R SE +R se +R L ) < R C

c) (R SE +R se +R L ) > R C

d) (R se +R L ) > R C

Answer: c

Explanation: Summation of armature resistance, series field resistance, load resistance must be less than the critical resistance. As seen in self-excited DC generator characteristics if this value is greater than critical resistance, voltage build-up will not be possible.

12. For a given DC series generator with critical resistance equal to 100 Ω, armature resistance is equal to 50 Ω, and series field resistance is equal to 20 Ω, is connected across load of 50 Ω. What will be the load voltage?

a) 20 kV

b) 0 V

c) 2 kV

d) Data insufficient

Answer: b

Explanation: Here, DC series motor fails to excite as addition of armature resistance, load resistance and field resistance is greater than the critical resistance of the machine by 20 Ω. Thus, machine fails to self-excite, as a result we’ll get zero terminal voltage.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Graphical Representation of External Characteristics”.

1. No load point of DC generator is __________

a) Intersection of OCC and Rf line

b) Point on the Y axis at rated field current

c) Point on the X axis at rated terminal voltage

d) Can’t find through graphical interpretation

Answer: a

Explanation: Intersection of OCC with field resistance line gives the no-load point. All the value so obtained from x and y axis respectively gives the terminal voltage and field current of a DC generator at no-load.

2. Rf line intersects with OCC in ________

a) 1 st quadrant

b) 2 nd quadrant

c) 3 rd quadrant

d) 4 th quadrant

Answer: a

Explanation: Rf line is a straight line passing through the origin and having constant slope, so rising always in a positive direction. OCC starts from some positive value on Y axis and increases till maximum point, afterwards it starts becoming constant, where generally it intersects with Rf line.

3. How armature resistance effect is shown graphically?

a) By adding I a R a product horizontally with R f line

b) By subtracting I a R a product vertically with R f line

c) By adding I a R a product vertically with R f line

d) By subtracting I a R a product horizontally with R f line

Answer: c

Explanation: For representing the voltage drop in armature resistance, we add product I a R a vertically with R f line at minimum 2 points and draw line parallel to R f to get line with V + I a R a = constant.

4. For determining IaRa maximum _____________

a) Distance between R f line and v+ I a R a line is taken

b) Distance between OCC and v+ I a R a line is taken

c) Maximum Distance between OCC and v+ I a R a line is taken

d) Can’t calculated graphically

Answer: c

Explanation: OCC when starts from some residual voltage on y axis, goes on increasing till some maximum value and then starts reducing slightly. The bulk is formed where maximum distance between OCC and v+ I a R a line is taken, to get effective frop in armature.

5. How demagnetization effect of armature reaction is shown graphically?

a) By shifting origin towards +ve y axis

b) By shifting origin towards +ve x axis

c) By shifting origin towards -ve y axis

d) By shifting origin towards -ve x axis

Answer: d

Explanation: The demagnetization caused by armature reaction can be quantified by equivalent field current Ifd which can be taken as proportional to the armature current Ia. So, by shifting the origin towards -ve x axis by Ifd we can show armature reaction graphically.

6. External characteristics of DC shunt motor lies in ______________

a) 1 st quadrant

b) 2 nd quadrant

c) 1 st and 2 nd quadrant

d) 4 th quadrant

Answer: c

Explanation: When external characteristics are plotted on graphs, and if effect of armature reaction is not considered graph lies in 1st quadrant only. When effect of armature reaction is considered we shift the origin thus, characteristics lies in 2 nd quadrant as well.

7. From magnetization characteristic at I f = 7.1 A, E a = 225 V at 1000 rpm. What will be the terminal voltage at speed 950 rpm?

a) 225 V

b) 235 V

c) 214 V

d) 220 V

Answer: c

Explanation: Speed is directly proportional to the back emf of a machine. So, E a = 225 V at 1000 rpm

E a  = 225 x 950 / 1000, this will give speed of given DC machine at 950 rpm. Upon calculations we get, E a = 213.7 V= 214 V approx.

8. For a given compound DC machine, Net field current obtained from characteristic is equal to 7.5 A, where shunt field current is equal to 5 A, armature current is 505 A, demagnetizing current equal to 0.95 A and shunt field winding of 1000 turns at rated speed of 1000 rpm. What will be the series field turns?

a) 7

b) 8

c) 5

d) 9

Answer: a

Explanation: From the excitation balance equation, I f + [N se /N f ] I a – I fd =I f .

5 + 505(N se /1000) – 0.95 = If .

Calculating for N se by substituting If , we get N se = 6.8 that is 7 turns.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Parallel Operation of DC Generator”.

1. For connecting two generators in parallel, they should have __________________

a) Same current rating

b) Same ohmic rating

c) Same voltage rating

d) All ratings must be same

Answer: c

Explanation: For connecting two DC generators in parallel we equal voltage ratings. As the generator voltage is easily adjustable in a range, so the condition stated above is not a must. But, it is desirable condition.

2. While connecting two DC generators in parallel, which of the following is not a desirable condition?

a) Same voltage rating

b) Same percentage voltage regulation

c) Same percentage speed regulation of the prime movers

d) Same current rating

Answer: d

Explanation: Same voltage rating, same percentage voltage regulation, same percentage speed regulation are the desirable conditions for connecting two generators in parallel, though these conditions are not a must.

3. We connect two generators in parallel _____________

a) For large DC load

b) For small DC load

c) For any DC load

d) For any AC or DC load

Explanation: a

Answer: We connect two generators in parallel for supplying large DC load. It is desirable to use more than one generator in parallel. This arrangement provides the security that if one generator gives way, the other can feed part load.

4. Load sharing of two generators connected in parallel is determined by __________

a) Internal characteristics

b) External characteristics

c) Both internal and external characteristics

d) It doesn’t depend on load sharing

Answer: b

Explanation: Two generators are connected in parallel such that, summation of current carried by both generators is equal to load current. In such cases we need to see external characteristics on loaded condition.

5. For a parallel operation of 2 DC shunt generators, we get net external characteristics ___________

a) Starting from the same no-load point and between generator 1 and generator 2

b) Starting from the same no-load point and below generator 1 and generator 2

c) Starting from the same no-load point and above generator 1 and generator 2

d) Can’t be determined

Answer: c

Explanation: The load sharing by these generators is determined by addition of external characteristics of both of generators. Thus, at no load generators will have common point, on y- axis so parallel characteristic will start from same point and will lie above of both.

6. When two compound generators are connected in parallel, when load current in generator 1 is increased __________

a) Both generator will share same load

b) Generator 1 will start running as motor

c) Generator 2 will be overloaded

d) Generator 2 will start running as motor

Answer: d

Explanation: If load current in G1 increases, load current in G2 decreases. Series excitation and internal voltage in G1 increases and for G2, these quantities decrease. Finally, all load shifts, which may turn to run G2 as a motor. All this leads to heavy overloading of G1.

7. While running two compound motors in parallel, we connect equalizer ring between ___________

a) Two armatures

b) Two fields

c) Two load points

d) Anywhere

Answer: a

Explanation: A low-resistance equalizer connection is made directly between the two armatures before the series fields. Any emf variations of the armatures causes equalizing circulating current which do not affect the current through the series windings. Thereby the parallel operation is stabilized.

8. When two DC series motors are connected in parallel, the resultant speed is __________

a) More than the normal speed

b) Loss than the normal speed

c) Normal speed

d) Zero

Answer: a

Explanation: When two DC series motor are connected in parallel then the resultant speed is increase the normal speed. If the voltage across each motor is assumed to be V, then current through each motor in parallel connection will be I/2. So, speed α  α . Hence the resultant speed is more than normal speed.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Operating Characteristics of DC Shunt Motor -1”.

1. For machine tools, which DC motor can be used?

a) DC Series motor

b) DC Shunt motor

c) DC cumulative compound motor

d) DC differential compound motor

Answer: b

Explanation: Shunt characteristics is the speciality of DC shunt motor. The speed of the motor almost remains constant on various loads, thus it suits perfectly to the application, where speed requirement is constant like in machine tools.

2. In a DC shunt motor, speed is related to armature current as _____________

a) Directly proportional to the armature current

b) Proportional to the square of the current

c) Independent of armature current

d) Inversely proportional to the armature current

Answer: d

Explanation: When armature reaction is ignored in a DC shunt motor, flux almost remains constant but the speed of the motor decreases according to the increase armature current, increasing Ia*Ra drop. Hence, inverse proportionality.

3. In a DC shunt motor for zero armature current we get speed ______

a) Non-zero and minimum

b) Zero

c) Non-zero and maximum

d) Doesn’t depend on armature current

Answer: c

Explanation: For zero armature current we get some non-zero value, indicated by positive intercept on speed characteristics. As armature current is increased speed of DC shunt motor starts decreasing due to increase in voltage drop at armature resistance.

4. What will be the effect of opening of field of a DC shunt motor while motor is running?

a) The speed of motor will be reduced

b) The armature current will reduce

c) The motor will attain dangerously high speed

d) The motor will continue to constant speed

Answer: c

Explanation: In a DC shunt motor if supply for the field winding is cut down, the speed would dangerously increase in order to maintain the back emf of the motor. For a constant back emf, flux is inversely proportional to the speed of DC shunt motor. So, if flux drops to zero theoretically speed will tend to infinity.

5. What will be the effect of reducing load on DC shunt motor?

a) Speed will increase abruptly

b) Speed will increase in proportion to reduction in load

c) Speed will remain almost constant

d) Speed will reduce

Answer: c

Explanation: For DC shunt motor, speed-armature current characteristics is called as a shunt characteristic as speed almost remains constant. Thus, by reducing load speed will increase negligibly, thus remaining almost constant.

6. How speed of the DC shunt motor can be increased?

a) Decreasing the field current

b) Increasing the field current

c) Decreasing the load current

d) Increasing the armature current

Answer: b

Explanation: For a constant load, load current will remain constant. Decreasing armature current will help in increasing speed for DC shunt motor. Since load current is addition of armature current and field current we’ll get less armature current for more field current.

7. Practical reason behind speed of DC shunt motor is proportional to back emf only is ____________

a) Back emf is equal to armature drop

b) Flux is proportional to field current

c) Flux is proportional to armature current

d) Flux is practically constant in DC shunt motors

Answer: d

Explanation: The field winding in DC shunt motor is connected in parallel to the armature winding and the supply. If we assume that the supply voltage for motor is constant then flux also becomes constant. At the rated speed the back emf also becomes almost constant if the load is same.

8. In a DC shunt motor, what will be the armature current at maximum load?

a) Almost negligible

b) Rated full-load current

c) Less than full-load current

d) More than full-load current

Answer: d

Explanation: As the load will increase in rapid manner, speed change in DC shunt motor will be visible. As load increase, speed will decrease, though by some small value. From, current armature characteristic armature current will be more than the earlier case.

9. In which of the following motor, ratio of starting torque to full-load torque will be least?

a) DC series motors

b) DC shunt motors

c) DC compound motors

d) Synchronous motors

Answer: b

Explanation: From torque current characteristic, we get that torque at no load or starting torque is equal to zero. As load increases, speed decreases and armature current increases so as torque also increases linearly, if effect of armature reaction is neglected.

This set of DC Machines Quiz focuses on “Operating Characteristics of DC Shunt Motor – 2”.

1. As the load is increased the speed of DC shunt motor will ___________

a) Reduce slightly

b) Increase slightly

c) Increase proportionately

d) Reduce rapidly

Answer: a

Explanation: As the load is increased, speed of the DC motor will reduce slightly, the change is so negligible that in many cases it is assumed that speed of the DC motor remains constant. Hence, characteristic is also called as shunt characteristic.

2. The armature torque of the DC shunt motor is proportional to

a) Field flux only

b) Armature current only

c) Field flux and armature current

d) Field current

Answer: b

Explanation: Torque of the DC shunt motor is directly proportional to the armature current. It’s almost a straight line if effect of armature reaction is neglected. If armature reaction is taken in consideration then torque will increase but non-linearly with armature current.

3. If a DC shunt motor is working at full load and if shunt field circuit suddenly opens _____________________

a) Will make armature to take heavy current, possibly burning it

b) Will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses

c) Nothing will happen to motor

d) Motor will come to stop

Answer: a

Explanation: At no load or lower loads, there is possibility of excess speed in such cases but here, when full load is given, armature circuit will draw very high current in order to maintain back emf. So, if fuses or circuit breakers are not used then, armature circuit may burn.

4. Speed of DC shunt motor is directly proportional to___________

a) Flux

b) Back emf

c) Terminal voltage

d) Armature resistance drop

Answer: b

Explanation: The armature circuit equation for DC shunt motor is given by Ea = Vt -IaRa. But back emf is also equal to kφn. Thus, kφn= Vt -IaRa, which further gives n = / kφ. Hence, speed is directly proportional to back emf.

5. Speed torque characteristic of DC shunt motor is _______________

a) Starting from origin

b) Starting from speed axis and increasing

c) Starting from speed axis and decreasing

d) Starting from speed axis and constant

Answer: c

Explanation: Speed-torque characteristic falls as torque increases, because armature reaction causes decrease in flux whose square is in inverse proportion with speed, causing decrease in speed rapidly compare to increase in torque.

6. Correct equation of speed-torque characteristic of DC shunt motor is____________

dc-machines-questions-answers-operating-characteristics-dc-shunt-motor-2-q6

Answer: a

Explanation: Speed-torque characteristic equation is obtained by n = / kφ and Torque = k1φIa. Thus, by combining these equations and by assuming all constants equal to k and kl we get the final equation shown above.

7. For some percentage increase in the torque, which DC motor will have the least percentage increase of input current?

a) Series motor

b) Shunt motor

c) Cumulative compound motor

d) Separately exited motor

Answer: a

Explanation: The mechanical torque T is directly proportional to the product of flux per pole φ and armature current. In case of DC series motor, up to saturation point flux is proportional to field current because Ia = If. Now if small percentage increase in armature current  will occur the same percentage of torque will increase. Whereas in DC series motor the torque is proportional to square of the armature current .

8. DC Shunt Motor has a disadvantage __________

a) Constant speed

b) Less expensive

c) Not suitable for rapidly changing loads

d) Cannot be determined

Answer: c

Explanation: In DC shunt motor, due to shunt characteristics speed variation depends not only upon the controlling resistance but on the load current also. This double dependence makes it impossible to keep the speed sensibly constant on rapidly changing load, which is in contrast with characteristic.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Operating Characteristics of DC Series Motor -1”.

1. DC series motors are used _______________

a) Where load is constant

b) Where load changes frequently

c) Where constant operating speed is needed

d) High starting speed

Answer: d

Explanation: DC series motors are used in those applications where high starting speed is required at its initial phase. Since no load operation speed of DC series motor is very high, it is always operated on some non-zero load.

2. Where compensating winding is provided in case of conductively compensated DC series motor?

a) As separately wound unit

b) In series with armature winding

c) In parallel with armature winding

d) In parallel with field winding

Answer: b

Explanation: Compensating winding is always excited with armature current in a given DC machine. So, compensating winding is provided in case of conductively compensated DC series motor in series with the armature winding.

3. No-load speed of which of the following DC motor will be highest?

a) Shunt motor

b) Series motor

c) Cumulative compound motor

d) Differentiate compound motor

Answer: b

Explanation: At no load, armature current tends to zero, flux φ tends to zero, where speed is inversely proportional to the flux, speed will tend to infinity. Thus, no load speed of DC series motor is highest.

4. The direction of rotation of a DC series motor shaft can be changed by _____________

a) Interchanging supply terminals

b) Interchanging field terminals

c) Not possible

d) Cannot be determined

Answer: b

Explanation: From speed current characteristics for a DC series motor, we get that speed is inversely proportional to flux. If flux direction is reversed, same flux will be available with negative sign in vectors, thus, speed will remain same but with the other direction.

5. Which of the following DC motor can be used at conveyors?

a) Series motor

b) Shunt motor

c) Differentially compound motor

d) Cumulative compound motor

Answer: a

Explanation: A conveyor requires high torque with steady speed, which provided by DC series motor. From torque speed characteristic of a DC series motor, it is easily stated that speed of the series motor practically remains constant at high torques.

6.Which statement is correct about DC series motor?

a) Has its field winding consisting of thick wire and more turns

b) Has a poor torque

c) Can be started easily without load

d) Has its field winding consisting of thick wire and less turns

Answer: d

Explanation: When field winding is connected in series it has to be of made by thick wire. When field winding is connected in parallel it has to be of made by thin wire. So, thick wire is used with less number of turns while thin one is used with more number of turns.

7. In which of the following applications DC series motor is invariably tried?

a) Starter for a car

b) Drive for a water pump

c) Fan motor

d) Home appliances

Answer: a

Explanation: Starter of the car simply implies start of operation at maximum load. Thus, series motor can be used in this application. For higher torques, since speed remains constant car starter is one of the best application of DC series motor.

8. The speed of a DC series motor is ___________

a) Proportional to field current

b) Proportional to the square of the armature current

c) Proportional to the armature current

d) Inversely proportional to the armature current

Answer: d

Explanation: Speed of DC series motor is inversely proportional to the armature current because, as armature current increases the flux produced also increases due to the series combination. Similarly if armature current is reduced flux is reduced which will increase speed.

9. In a DC series motor, if the armature current is halved, the torque of the motor will be equal to

a) 100% of the previous value

b) 50% of the previous value

c) 25% of the previous value

d) 10% of the previous value

Answer: c

Explanation: Torque in the case of linear magnetization of DC series motor is directly proportional to square of the armature current. So, armature current is made 1/2th of the original value, then torque will be 1/4th of the original value.

10. Applications demanding large applications at starting and small torque while running use DC series motor.

a) True

b) False

Answer: a

Explanation: Large starting torque, infinite speed at no-load, low running torque for steady speed operation are called as Series characteristics. This are shown by DC series motor, so applications like cranes, traction, etc. use DC series motor.

This set of DC Machines MCQs focuses on “Operating Characteristics of DC Series Motor – 2”.

1. Which DC motor is generally preferred for cranes and hoists?

a) Series motor

b) Shunt motor

c) Cumulatively compounded motor

d) Differentially compounded motor

Answer: a

Explanation: DC series motor is used where high starting torque is required, thus in cranes and hoists DC series motor is used widely. Torque-speed characteristics of DC series motor suits perfectly to these applications.

2. Which of the following motor has the poorest speed regulation?

a) Shunt motor

b) Series motor

c) Differential compound motor

d) Cumulative compound motor

Answer: b

Explanation: If the load is applied to the any DC motor or machine, the speed of the motor decreases automatically. Thus, in order to maintain the constant speed, the difference between no load and full load speed called as speed regulation should be maintained very less. In series motor this difference is almost equal to infinity.

3. Buses, trains, trolleys, hoists, cranes make use of DC series motor because __________

a) It provides low starting torque

b) Cost efficient

c) It provides high starting torque

d) It requires minimum power compare to other motors

Answer: c

Explanation: DC series motor provides very high starting torque, and at higher torques speed of the DC series motor is also maintained to constant. Both of these properties are used in applications of DC series motor.

4. DC motor is to drive a load which is having certain minimum value for most of the time and some peak value for short duration. We will select the _______

a) Series motor

b) Shunt motor

c) Compound motor

d) Cannot be determined

Answer: a

Explanation: When load is minimum, we will get very high speed at shaft in DC series motor, but it is advisable that DC series motor should not be used on no-load application. For the application stated in question DC series motor is perfect option.

5. Which DC motor has got maximum self-relieving property?

a) Series motor

b) Shunt motor

c) Cumulatively compounded motor

d) Differentially compounded motor

Answer: a

Explanation: DC series motor develops large starting torque initially and slow down with increase in load and so get automatically relived from heavy excessive load. Thus, DC series motor is called as self-reliving machine.

6. Which motor should not be started on no-load?

a) DC Series motor

b) DC Shunt motor

c) Synchronous motor

d) Induction motor

Answer: a

Explanation: No-load speed of the DC motor is very high, it’ll create a dangerous situation and the centrifugal forces will destroy the armature and may harm the personnel. As, flux is inversely proportional to speed, speed will be infinity by calculations.

7. Which of the following is the correct equation for DC series motor ________

a) [ V t / k φ ]- [ R se +R a / k φ ] I a

b) [ V t / k φ ]- [ R a / kφ2] Ia

c) [ V t / k φ 2 ]- [R se +R a / k φ 2 ] I a

d) [ V t / k φ 2 ] + [ R a / k φ 2 ] I a

Answer: a

Explanation: Speed-torque characteristic equation is obtained by back emf = (V t -I a R a -I a R se ) and back emf= kφn. Thus, by combining these equations and by assuming all constants equal to k we get, the final equation shown above.

8. Actual speed obtained at any value of armature current comparing to speed obtained by assuming linear magnetic assumption in DC series motor is _______

a) Same

b) Higher

c) Lower

d) Cannot say

Answer: b

Explanation: From speed current characteristics of a DC series motor is easily seen that, initial value of speeds in both of the cases is same or rather infinite, but as a armature current or load current increases speed starts dropping down relatively more in the case of linear magnetization.

9. Torque of e DC series motor is __________

a) Directly proportional to armature current

b) Inversely proportional to armature current

c) Directly proportional to square of armature current

d) Inversely proportional to square of armature current

Answer: c

Explanation: Torque in a DC series motor, in the case of linear magnetization varies directly with armature current square. Practically at higher values of armature current it tends to settle at constant value.

10. Practically, at higher values of armature current torque does not follow its relationship with armature current.

a) True

b) False

Answer: a

Explanation: At higher values of armature current or load current, saturation and demagnetization due to armature reaction becomes prominent, and DC machine shows its effect on torque. Thus, torque tends to remain at constant value.

11. What will happen to a torque, if a DC series motor is accidentally connected to single phase ac supply voltage?

a) Pulsating and unidirectional

b) Steady and unidirectional

c) Oscillating

d) Cannot be determined

Answer: a

Explanation: In DC series motor, the AC currents through the field and armature winding will be made unidirectional by the function of commutator and brushes. So, torque will be unidirectional but it will be pulsating also due to AC.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Operating Characteristics of DC Compound Motor – 1”.

1. Differentially compound DC motors are used in applications requiring _______

a) High starting torque

b) Low starting torque

c) Variable speed

d) Frequent on-off cycles

Answer: b

Explanation: Compound motor shows combine effect of shunt and series field windings. Differential compound series motor gives low starting torque, examined by torque current characteristic. Hence, applications with low starting torque are called in differentially compound DC motor.

2. Which DC motor is more preferred for elevators?

a) Shunt motor

b) Series motor

c) Differential compound motor

d) Cumulative compound motor

Answer: d

Explanation: Cumulative wound DC motors give high starting torque like a series motor and reasonable good speed regulation at high speeds like a shunt dc motor. As this type of motor offers the best of both series and shunt motor, it is practically suitable for most common applications like elevators.

3. Which DC motor is used in shears and punches?

a) Shunt motor

b) Series motor

c) Cumulative compound motor

d) Differential compound motor

Answer: c

Explanation: Cumulative compound DC motor provides high starting torque which is required at instant of starting in shears and punches. It also provides good speed regulation in order to vary pressure. So, this motor suits well to shears and punches application.

4. Which DC motor has got maximum self-loading property?

a) Series motor

b) Shunt motor

c) Cumulative compound motor

d) Differential compound motor

Answer: d

Explanation: A differentially compound DC motor, flux reduces so sharply at small increase in load at higher values of load. It is advisable that motor should not be used beyond some load value, as it may damage itself by self-loading.

5. For the same H.P. rating and full load speed, which of the following motor has poor starting torque?

a) Series motor

b) Shunt motor

c) Cumulative compound motor

d) Differential compound motor

Answer: d

Explanation: In differential compound motor, series field opposes shunt field. It has poor starting torque as the resultant flux is minimized by this opposition. The flux starts decreasing with increase in load. The decrease in flux cause the starting torque to be less than any other DC motor.

6. Unlike DC series motor, DC compound motor can be started on no-load.

a) True

b) False

Answer: a

Explanation: Cumulative motor takes best from shunt motor and series motor as well. Due to shunt field flux component, DC cumulative motor can be started on no load. No-load speed of DC cumulative motor is greater than that of DC shunt motor but it’s not equal to infinity like series motor.

7. Flywheel is used with DC compound motor to reduce the peak demand by the motor, compound motor has to be _____________

a) Level compounded

b) Under compounded

c) Cumulatively compounded

d) Differentially compounded

Answer: c

Explanation: In case of cumulative compound DC motor, whenever a mechanical load is subject to fluctuations for short durations, a flywheel is generally provided for carrying peaks. Flywheel will smooth out the load on the motor and will also reduce peaks on power system.

8. What will happen if shunt field suddenly opens, in a differential compound DC motor?

a) Motor will first stop and then run in opposite direction as series motor

b) Motor will not work and come to stop

c) Motor will work as series motor and run at infinitely high speed

d) Motor will work as series motor and run at slow speed

Answer: a

Explanation: If shunt field is suddenly opened, flux due to shunt field will become 0. As, net flux is equal to shunt field flux – series field flux, the direction of flux will be reversed and thus motor will run in opposite directions.

9. What will happen if the series field of a differential compound motor is short circuited?

a) Net flux will decrease

b) Speed will increase

c) Speed will remain constant

d) Speed will decrease

Answer: d

Explanation: As the series field is short circuited, current through series field becomes zero and MMF due to series field becomes zero. Hence, net MMF in motor becomes greater than previous case, which results in total increase in the flux created and decrease in speed.

10. What will happen if the series field of a cumulative compound motor is short circuited?

a) Net flux will increase

b) Speed will increase

c) Speed will remain constant

d) Speed will decrease

Answer: b

Explanation: As the series field is short circuited now, current through series field becomes zero and MMF due to series field becomes zero. Hence, total MMF in motor becomes less than previous case, which results in total decrease in the flux created and increase in speed.

11. DC motor is to a drive a load which is almost zero for certain part of the load cycle and peak value for short duration. We will select __________

a) Series motor

b) Shunt motor

c) Compound motor

d) Any DC motors

Answer: c

Explanation: We can’t use series motor as our load is almost zero at some points. Thus, we’ll use compound motor which can work on no load also. Cumulative compound motor is provided with flywheel so that this machine can deal with peak value.

This set of DC Machines Multiple Choice Questions & Answers focuses on “Operating Characteristics of DC Compound Motor – 2”.

1. Net field current in a compound motor is given by ______________

a) If = I f + (N se /N f ) I a

b) If = I f – (N se /N f ) I a

c) If = I f + (N f /N se ) I a

d) If = I f – (N f /N se ) I a

Answer: a

Explanation: Net flux in a compound motor is determined by addition of shunt field flux and series field flux. Thus, for calculating effective flux, net field current is find out. If is the constant value given for shunt field, while If net takes the presence of series field also.

2. With increase in armature current, speed of the cumulative compound motor _________

a) Reduces slowly compare to shunt motor

b) Reduces sharply compare to shunt motor

c) Remains constant

d) Increases in proportionality with armature current

Answer: b

Explanation: As armature current increases, the back emf decreases similar to shunt motor. By the same time series field resistance carries more armature current producing more amount of flux which is inversely proportional speed.

3. Speed current characteristic of cumulative DC compound motor at larger values of armature current is similar to that of _____________

a) DC shunt motor

b) Induction motor

c) DC differential compound motor

d) DC series motor

Answer: d

Explanation: At higher values of current speed of the DC cumulative compound motor starts decreasing due to effect of series field flux, which is also seen in DC series motor. Thus series motor and DC cumulative motor follow same characteristics.

4. Differential compound motor is used in large number of applications.

a) True

b) False

Answer: b

Explanation: A differentially compound motor has flux/pole φ = . It is seen that on over-load φ reduces sharply and so the motor acts like a series motor on no load. This is why the differential compound motor is not used in practice.

5. Where speed-current characteristic of DC shunt motor lies with respect to DC cumulative compound motor ?

a) Can be represented by same curve

b) Above

c) Below

d) Cannot be determined

Answer: b

Explanation: Because of φse increasing with Ia, the speed of the compound motor falls much more sharply than the shunt motor. Therefore, the n – armature current characteristic of the compound motor lies above that of the shunt motor for I a < I a  and lies below for I a > I a .

6. Where speed-current characteristic of DC shunt motor lies with respect to DC cumulative compound motor ?

a) Can be represented by same curve

b) Above

c) Below

d) Cannot be determined

Answer: c

Explanation: At start, speed of DC cumulative motor is more than the speed of shunt motor because of series characteristic effect. Because of φ se increasing with Ia beyond full load current, the speed of the compound motor falls much more sharply than the shunt motor.

7. Only motor listed below whose speed increases with increase in armature current is ___________

a) DC series motor

b) DC cumulative compound motor

c) DC shunt motor

d) DC differential compound motor

Answer: d

Explanation: Shunt motor speed though decreases by slight value as armature current increases, it is assumed to be constant. But series motor and cumulative motor speed decreases as armature current increases. Only differential motor flux reduces with increase in armature current thus, speed increases.

8. Torque of the cumulative compound DC motor is _____________

a) Addition of Ia and Ia 2 term

b) Subtraction of Ia from Ia 2 term

c) Subtraction of Ia 2 from Ia term

d) Addition of -Ia and -Ia 2 term

Answer: a

Explanation: DC cumulative compound motor is the addition of best of series and best of shunt field motor. For shunt type torque is proportional to armature current while in series motor torque is proportional to square of armature current.

9. Torque-current characteristic of DC cumulative compound motor _____________

a) Lies above shunt motor characteristic

b) Lies below shunt motor characteristic

c) Lies above of series and shunt motor characteristic both

d) Lies between of series and shunt motor characteristic both

Answer: c

Explanation: Starting from zero armature current the characteristic of cumulative motor lies above the series characteristic and shunt characteristic, because it is the sum of series and shunt characteristic.

10. For strong series field, speed-torque characteristic of cumulative compound motor lies __________

a) Above series motor characteristic if current is less than full load current

b) Above shunt motor characteristic if current is less than full load current

c) Below series motor characteristic if current is more than full load current

d) Below series motor characteristic if current is less than full load current

Answer: b

Explanation: Due to strong series field, speed-torque characteristic lies above of shunt field characteristic but below of series field characteristic, as no-load speed is finite for cumulative compound motor, at armature current less than full load current.

11. Which of the following DC motor has negative speed regulation?

a) Shunt motors

b) Series motors

c) Differential compound motors

d) Series and differential compound motors

Answer: c

Explanation: In differential compound motors flux will reduce as load is increased because series flux opposes shunt field flux. This will increase speed upon addition of load. Therefore, it has negative speed regulation means speed will increase due to increase in load.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “DC Motor Basics -1”.

1. What will happen if DC shunt motor is connected across AC supply?

a) Will run at normal speed

b) Will not run

c) Will Run at lower speed

d) Burn due to heat produced in the field winding

Answer: d

Explanation: In case of parallel field connection, it won’t rotate at all and will start humming and will create vibrations, as a torque produced by positive and negative cycle will cancel out each other. DC motor will be heated up and it may burn.

2. What will happen if the back emf of a DC motor vanishes suddenly?

a) The motor will stop

b) The motor will continue to run

c) The armature may burn

d) The motor will run noisy

Answer: c

Explanation: If back emf vanishes suddenly, motor circuit will try to retain back emf by drawing more current from supply. If supplying unit didn’t trip down by this time, excess current in armature may heat up the armature.

3. What will happen, with the increase in speed of a DC motor?

a) Back emf increase but line current falls.

b) Back emf falls and line current increase.

c) Both back emf as well as line current increase.

d) Both back emf as well as line current fall.

Answer: a

Explanation: In case of DC motor, the speed is proportional to the back emf . So, with the increase in speed, the back emf also increases. Therefore, armature current is also decreased, in case of series motor, armature current is equal to the line or load current.

4. Which part will surely tell that given motor is DC motor and not an AC type?

a) Winding

b) Shaft

c) Commutator

d) Stator

Answer: c

Explanation: All other parts except brushes and commutator are same in AC machine when outer looks are only taken in consideration. Commutator is used only in DC machine for providing mechanical rectification and not in AC machine.

5. In DC motor, which of the following part can sustain the maximum temperature rise?

a) Field winding

b) Commutator

c) Slip rings

d) Armature winding

Answer: a

Explanation: Maximum temperature rise can be sustained by field winding, as it is not involved in rotary parts. Field winding is present away from rotary parts of the machine, so temperature rise in the machine will not produce any effect in machine rotations.

6. Direction of rotation of motor is determined by ____________

a) Faraday’s law

b) Lenz’s law

c) Coulomb’s law

d) Fleming’s left-hand rule

Answer: d

Explanation: Flemings laws can be summarized as whenever, a current carrying conductor comes under a magnetic field, there will be a force acting on the conductor and on the other hand, if a conductor is forcefully brought under a magnetic field, there will be an induced current in that conductor.

7. The current drawn by the armature of DC motor is directly proportional to ________________

a) Torque

b) Speed

c) The voltage across the terminals

d) Cannot be determined

Answer: a

Explanation: From the equation of torque generated in a DC machine, we know that in both DC motor and DC generator, current drawn is directly proportional to the torque required by the machine.

8. Which power is mentioned on a name plate of a motor?

a) Gross power

b) Power drawn in kVA

c) Power drawn in kW

d) Output power available at the shaft

Answer: d

Explanation: Name plate of the motor shows rated values i.e. rated speed, rated current, rated voltage. It also shows output power available at shaft when all other quantities are set to rated values.

9. An electric motor is having constant output power. So, motor will have a torque speed characteristic _______________________

a) Circle about the origin.

b) Straight line parallel to the speed axis.

c) Straight line through the origin.

d) Rectangular hyperbola

Answer: d

Explanation: In case of DC motor for the constant output power, Ea×Ia = T×ω. As T×ω = Constant, so the torque speed characteristic is in the form of rectangular hyperbola, which represented as xy= constant.

10. Which of the following quantity will decrease if supply voltage is increased?

a) Starting torque

b) Operating speed

c) Full-load current

d) Cannot be determined

Answer: c

Explanation: When supply voltage is increased full load current will decrease in order to keep output power constant, which will decrease torque at that moment, while starting torque will remain as it is, irrespective of any change in supply voltage.

11. In which of the following case we will get maximum power?

a) E a = 2 x supply voltage

b) E a = supply voltage

c) Supply voltage = 2 x E a

d) supply voltage = 4 x E a

Answer: c

Explanation: For a motor, from power equation it is known that,

dc-machines-questions-answers-dc-motors-basics-1-q11

12. Sometimes motor has to be de-rated.

a) True

b) False

Answer: a

Explanation: Derating refers to the operation of equipment at reduced capacity/power or speed. Derating in motors can be caused due to the following reasons- Frequency, Voltage, Ambient temperature, Altitude.

13. The armature shaft of a DC motor must be able to withstand ______________

a) Bending moment due to weight of the armature.

b) Any unbalanced magnetic pull on the armature core.

c) Twisting stains due to transmission of torque.

d) Bending moment, unbalanced magnetic pull and twisting stains

Answer: d

Explanation: The armature shaft must be able to withstand any unbalanced magnetic pull on the armature core, bending moment due to weight of the armature and commutator, twisting stains due to transmission of torque, for a good and long run application of motor.

14. In DC machines the residual magnetism is present. The order of residual magnetism is ___________

a) 2 to 3 per cent

b) 10 to 15 per cent

c) 20 to 25 per cent

d) 50 to 75 per cent

Answer: a

Explanation: In a DC machine residual magnetism is present and it plays very important role in starting of any DC machine. It’s present because of the previous application on the same motor, it also provides some no-load voltage.

15. Sparking is discouraged in a DC motor.

a) True

b) False

Answer: b

Explanation: Sparking at brushes and commutator segments lead to damage of commutators, which is the main distinguishable component in a DC machine. So, it is advisable to fasten the commutation speed and avoid sparking.

This set of DC Machines online test focuses on “DC Motor Basics – 2”.

1. Which of the following motor is used where high starting torque and wide speed range control is required?

a) All motors

b) Induction motor

c) Synchronous motor

d) DC motor

Answer: d

Explanation: DC series motor gives the maximum starting torque at starting. With the help of Ward Leonard speed control method various speeds are also available. It provides one mode to othe transmission also.

2. The armature voltage control of DC motor will provide ___________

a) Constant power drive

b) Constant voltage drive

c) Constant current drive

d) Constant torque drive

Answer: d

Explanation: By changing the armature voltage we can run motor to give various speeds at constant torque. While changing the field is also another idea to change speed but it’ll be variable torque constant power operation.

3. As there is no back emf at the instant of starting a DC motor, in order to prevent a heavy armature current from flowing though the armature circuit ___________

a) Series resistance is connected with armature

b) Parallel resistance is connected to the armature

c) armature is temporarily open circuited

d) a high value resistor is connected across the field winding

Answer: a

Explanation: To prevent large armature current from damaging the armature circuit we connect series resistance with armature and in order to avoid losses we reconnect the same resistance with field circuit once machine is started, i.e. the exact principle of starters.

4. What will happen to torque if back emf and speed of the DC motor is doubled?

a) Remain unchanged

b) Reduce to one-fourth value

c) Increase four folds

d) Be doubled

Answer: a

Explanation: Torque depends on armature current, so as long as armature current remains constant torque will not change. Speed depends directly with back emf thus, it will definitely become double when back emf is doubled.

5. At the instant of starting, when a DC motor is put on supply, it will behave like ______________

a) Highly resistive circuit

b) Low resistance circuit

c) Capacitive circuit

d) Inductive circuit

Answer: b

Explanation: At the starting when back emf is very low, machine will fetch large amount of armature current from supply acting as a low resistive circuit. In order to avoid such case, we use starters in DC machine which add some resistance to armature at the starting.

6. All others are advantages of DC motor over AC motor except _______________

a) Low cost

b) Wide speed range

c) Stability

d) High starting torque

Answer: a

Explanation: Dc machine is very useful over AC when operation is concerned but its superior speed control system itself requires 2 more machines. DC machine requires high maintenance due to commutator and brushes so it is an expensive machine.

7. If a DC motor designed for 45°C ambient temperature is to be used for 55°C ambient temperature, then the motor ___________

a) Of lower HP should be selected

b) Of higher HP should be selected

c) Can be used for 50°C ambient temperature also

d) Is to be de-rated by a factor recommended by manufacturer and select the next higher H.P. motor

Answer: d

Explanation: The ambient temperature is one of a factor in derating. If the ambient temperature given is high, the insulation may reach its maximum temperature limit quickly. Hence, the motor may have to be de-rated, and for application next motor must be used.

8. Torque developed by a DC motor depends upon _____________

a) magnetic field

b) active length of the conductor

c) current flow through the conductors

d) Current, active length, no. of conductors, magnetic field all

Answer: d

Explanation: Torque produced by a DC motor is directly proportional to an armature current flowing through the machine. While it also depends on some constant values like active length, no. od armature conductors and magnetic field which are constant for a given machine.

9. For very sensitive and wide speed control, which of the following method is preferred?

a) Armature control

b) Ward-Leonard control

c) Multiple voltage control

d) Field control

Answer: b

Explanation: In ward-Leonard method, by using big circuitry, very fine speed control over the whole range from zero to normal speed and that too in both directions can be obtained. The motor-generator set can provide speed both below and above the rated speed and in both direction, by this method.

10. Field weakening control method is used for the speed _________

a) Above rated speed

b) Below rated speed

c) Can be used anywhere

d) Can’t tell

Answer: a

Explanation: By reducing of field current, flux will also reduce, because flux is directly proportional to the field current passing through the winding. As speed is inversely proportional to flux, so this method can be used for only above rated speed.

11. Armature resistance control method is used for the speed _____________

a) Above rated speed

b) Below rated speed

c) Can be used anywhere

d) Can’t tell

Answer: b

Explanation: In armature circuit resistance method armature resistance can be only increased, by adding another resistance in series. As a result, the back emf will decrease. Speed is directly proportional to back emf so, it will also decrease. Therefore, this method is used for below rated speed.

12. Which function is performed by counter emf of a DC motor?

a) Exceeds supply voltage

b) Aids applied voltage

c) Helps in energy conversion

d) Regulates its armature voltage

Answer: c

Explanation: Back emf or counter emf makes the DC a self-regulating machine i.e. it makes the DC motor to draw as much armature current as it is just enough to develop the required load torque. Thus, it helps in energy conversion and it is always less than applied voltage.

13. The output power of any electrical  motor is taken from the ______________

a) Field

b) Coupling mounted on the shaft

c) Armature

d) Motor frame

Answer: b

Explanation: The output power of any electrical motor is always taken from the coupling mounted on the shaft because the motor converts input electrical power into output mechanical power. So, we get mechanical power at shaft coupling.

14. Why field winding of a DC series motor is provided with thick wire?

a) As it carries large load current

b) To provide large flux

c) In order to reduce eddy current to provide large flux

d) To reduce the of insulting materials

Answer: a

Explanation: In DC series motor, the cross-sectional area of wire used for field coils has to be large because the in this case field coils carry the load or armature current. As the cross-sectional area is large, the resistance is minimum and series voltage drop is also minimum.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Starting of DC Motors – 1”.

1. Why starters are required in a DC motor?

a) Back emf of these motors is zero initially

b) These motors are not self-starting

c) These motors have high starting torque

d) To restrict armature current as there is no back emf at starting

Answer: d

Explanation: At the time of starting , the induced emf of a motor is zero such that current drawn by armature, from rated voltage supply would be I a = V/R a . Since armature resistance is very low, armature current drawn is very high and will damage the machine.

2. For which motors DOL starter can be used?

a) Up to 5 H.P

b) Up to 10 H.P

c) Up to 15 H.P

d) Up to 20 H.P

Answer: a

Explanation: DOL starters are limited to the small rating motors where distribution system  can withstand high starting currents without excessive voltage dips. For a large rating motor, ranging from 5 HP to 25 HP, oil immersed DOL starters are used which provides insulation against sparking on contact points, increases the life of starter.

3. A three-point starter is used for _________

a) Shunt motors

b) Shunt as well as compound motors

c) Shunt, compound and series motors

d) Not for DC motors

Answer: b

Explanation: 3-point starters are used only for shunt and compound motors, they are not used for series motors. Three-point starter is employed where motor field current can be varied in a narrow range and so does the motor speed.

4. The starting resistance of a DC shunt motor is generally ______

a) low

b) Around 0.5 kΩ

c) Around 5 kΩ

d) Infinitely large

Answer: a

Explanation: Starting resistance of a DC shunt motor and DC compound motor is low. Well, that’s the reason why we use starters in a DC motors, in order to limit the armature current flowing through the armature and to protect machine circuitry.

5. In a manual shunt motor starter relay positions are ________________________

a) Over load relay is connected in series and no volt relay in parallel with the load

b) Over load relay is connected in parallel and no volt relay in series with the load

c) Over load relay and no volt relay are both connected in series with the load

d) Over load relay and no volt relay are both connected in parallel with the load

Answer: a

Explanation: In simple manual shunt motor starter like 3-point starter over load relay coil is kept in series with DC mains while no volt coil is kept in parallel with DC mains. Both the coils are equally important in a motor starter circuit.

6. What will happen if DC motor is used without starter?

a) Heavy sparking at brushes

b) It’ll start smoothly

c) Will not start at all

d) Depends on load

Answer: a

Explanation: It would cause intolerably heavy sparking at the brushes which may destroy the commutator and brush-gear. Sudden development of large torque will cause mechanical shock to the shaft, reducing its life. Such heavy current cannot be generally permitted to be drawn from the source of supply.

7. Motor will start quickly when used without starter.

a) True

b) False

Answer: a

Explanation: The only thing in favour of direct starting must be mentioned here. Since the torque of the motor with direct start is much higher, the motor starts much more quickly. As a consequence, the Joule input per start is much less than that with resistance start.

8. In shunt and compound motor starting the shunt field should be made on with full starting resistance in ____________

a) Series with field

b) Parallel with field

c) Series with armature

d) Parallel with armature

Answer: c

Explanation: In shunt and compound motors starting the shunt field should be switched on with full starting resistance in armature circuit. A short time delay in this position allows the field current to build up to the steady value of the inductive field transients.

9. A starter is required for a 220-V shunt motor. The maximum allowable current is 55 A and the minimum current is about 35 A. The armature resistance of the motor is 0.4 Ω. What will be the number of sections of starter resistance required?

a) 5

b) 4

c) 6

d) 8

Answer: c

Explanation: I 1 =55 A, I 2 =35 A

So, γ= Ratio of upper limit to the lower limit = 55/35 = 1.57, R 1 = 200/55= 4 Ω

Now, γ n-1 = 4/0.4 = 10. By substituting γ value, we get n = 6.

10. γ is given as 1.585. Resistance at maximum allowable current is given equal to 4 Ω, what is the 5 th step resistance?

a) 0.235

b) 0.370

c) 1.476

d) 2

Answer: a

Explanation: γ is given as 1.585. So γ -1 is equal to 0.631. R 1 is provided and it is equal to 4 Ω.

r 1 =  * 4 = 1.476 Ω

r 2 = 1.476*0.631= 0.931 Ω, similarly calculating till r 5 = 0.235 Ω.

11. Four-point starter is used when ______________

a) Motor field current is varied in narrow range

b) Motor speed is varied in small range

c) Motor field current is varied over wide range

d) Can be used anywhere

Answer: c

Explanation: Three-point starter is employed where motor field current can be varied in a narrow range and so does the motor speed while four-point starter is used when motor field current can vary over a wide range and so does the motor speed.

This set of DC Machines online quiz focuses on “Starting of DC Motors – 2”.

1. In three-point starter, as a starting handle is rotated __________

a) The resistance is added into armature circuit

b) The resistance is removed from field circuit

c) The resistance is added into field circuit

d) Resistance is neither added nor removed

Answer: c

Explanation: The starting resistance is arranged in steps between conducting raised studs. As the starting handle is rotated about its fulcrum, it moves from one stud to the next, one resistance step is cut out, and it gets added to the field circuit.

2. How much torque is ensured by resistance adding arrangement in 3-point starter?

a) Low and non-zero

b) Infinite

c) High finite

d) Zero

Answer: c

Explanation: As a starting handle is rotated one resistance step is added into field circuit. There is a short time wait at each stud for the motor to build up speed. This arrangement ensures a high average starting torque.

3. The resistance of NVC is _______

a) Small

b) Large

c) Infinite

d) Zero

Answer: a

Explanation: The resistance of no volt coil  is small. NVC resistance forms a part of field resistance when resistor rotating handle of three-point starter is moved from min. to max. position. Starting resistance is also added to the field circuit.

4. NVC will release the handle electromagnetically when ________

a) In the case of failure of field current

b) If the resistance is very high

c) At the end of each rotation

d) At the start of each rotation

Answer: a

Explanation: In case of failure of field current  NVC coil releases the handle , which goes back to the OFF position under the spring action.

5. Over-load coil performs the function when __________

a) In the case of failure of field current

b) If the resistance is very high

c) If armature current increases beyond certain value

d) At the start of each rotation

Answer: c

Explanation: The contact of this relay at armature current above a certain value  closes the NVC ends, again bringing the handle to OFF position. NVC and OL release are protections incorporated in 3-point starter.

6. In 4-point starters the resistance is added in series with NVC because __________________

a) To increase field current

b) To increase armature current

c) To limit the NVC current

d) To limit armature current

Answer: c

Explanation: To overcome the problem caused when the field current is low, NVC is connected across the two lines, one line connected to F terminal through the starter and other directly to the second line from another L terminal of the starter. To limit the NVC current a protective resistance R is connected in series with it.

7. What will be the γ value for starter taking 4 steps, where ratio of resistance at maximum allowable current to armature resistance is equal to 1.8?

a) 1.2164

b) 1.8

c) 2.2468

d) 0.8220

Answer: a

Explanation: γ is defined as the ratio of upper current limit to the lower current limit in starters of DC machine. γ n-1 = ratio of resistance at maximum allowable current to the armature resistance. Substituting values for n=4, we get γ=1.2164.

8. Maximum allowable current for a 240-V DC shunt motor is equal to 65 A. Minimum allowable current for same is equal to 40 A. What will be the value of γ?

a) 0.6153

b) 1.265

c) 1.625

d) 2.652

Answer: c

Explanation: γ is defined as the ratio of upper limit current value to the lower limit current value. So, γ will be the ratio of 65/40. From calculations, we get γ= 1.625. Note that γ is unitless quantity.

9. For a certain machine having γ = 1.8, we are using starter with 5 steps. What will be the resistance at step 3 if step 2 resistance is equal to 2 Ω?

a) 2.111

b) 1.111

c) 3.6

d) 10.8

Answer: b

Explanation: γ is defined so as to calculate step resistance from the given maximum limit of current and minimum limit of the same. This γ = r n /r n-1 . So, for calculating resistance at step 3 we’ll substitute the corresponding values in the equation, which will give step 3 resistance as 1.1111Ω.

10. Which of the following is the correct formula for calculating step resistance?

a) γ = r n /r n-1

b) γ = r n /r n+1

c) γ = r n *r n-1

d) γ = r n *r n+1

Answer: a

Explanation: For given values of armature currents upper and lower, corresponding equivalent resistances are calculated. So, by the induction formula we calculate the value of γ and the step resistance as well, where n denotes the number of steps.

11. Which of the following starter can sufficiently start the DC series motor?

a) 3-point starter

b) 4-point starter

c) 2-point starter

d) Cannot be determined

Answer: c

Explanation: 2-point starter is enough to start the DC series motor. Since armature and field winding are in series already high armature current will not flow. No requirement of 3 or 4-point starter in DC series motor.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Speed Control Using Field Control of Shunt Motor”.

1. The speed of a DC shunt motor can be increased by ______

a) Increasing the resistance in armature circuit

b) Increasing the resistance in field circuit

c) Reducing the resistance in the field circuit

d) Reducing the resistance in the armature circuit

Answer: b

Explanation: Speed of the DC motor is directly proportional to the back emf and inversely proportional to the flux produced by field. Where, flux produced is directly proportional to the current passing through the field winding .

2. What will happen if excitation of DC shunt motor is changed?

a) Torque will remain constant

b) Torque and power both will change

c) Torque will change but power will remain constant

d) Torque, power and speed, all will change

Answer: c

Explanation: The motor will accelerate the mechanical load connected during this period but no increase in the mechanical load as P load = T 1 W 1 = T 2 W 2 where W 2 >W 1 . So, at the higher speed there is less electrical torque for the same mechanical load / power.

3. If the speed of a DC shunt motor is increased, the back emf of the motor will ___________

a) Increase

b) Decrease

c) Remain same

d) Become zero

Answer: a

Explanation: From, the speed-current characteristics of DC shunt motor we know that speed of the motor is directly proportional to the back emf and inversely proportional to the flux. So, for more speed there will be more back emf generated.

4. The speed of a DC shunt motor can be made more than full load speed by __________

a) Reducing the field current

b) Decreasing the armature current

c) Increasing the armature current

d) Increasing the excitation current

Answer: a

Explanation: Speed of the DC motor obtained from speed equation is inversely proportional to flux produced by the field. So, reducing the field current flux produced by armature will decrease, and speed will increase.

5. Speed regulation of DC shunt motor is calculated by ratio of difference of full load speed and no-load speed with full load speed.

a) True

b) False

Answer: b

Explanation: Speed regulation is defined as a ratio of difference of no-load speed with full load speed with no-load speed. Here, no-load speed is more than the full load speed. Thus, we divide difference by no-load value and not by full load value.

6. Which speeds can be obtained from field control of DC shunt motor?

a) Lower than rated speeds

b) Greater than rated speeds

c) Lower and greater than rated speeds

d) Neither lower nor greater than rated speeds

Answer: b

Explanation: Speeds greater than rated speeds can be obtained by lowering the flux of shunt field motor. Field cannot be made any stronger, it can only be weakened by this method. Thus, speed lower than the rated speed can’t be obtained.

7. No load speed of the DC shunt motor is 1322 rpm while full load speed is 1182 rpm. What will be the speed regulation?

a) 12.82 %

b) 11.8 %

c) 16.6 %

d) 14.2 %

Answer: b

Explanation: Speed regulation is equal to  / . By substituting all the values, speed regulation= / 1182. Speed regulation is given by 0.118. In percentage notation SR= 11.8 %.

8. Speed regulation of a DC shunt motor is equal to 10%, at no load speed of 1400 rpm. What is the full load speed?

a) 1233 rpm

b) 1273 rpm

c) 1173 rpm

d) 1123 rpm

Answer: b

Explanation: Speed regulation is equal to 0.1 which is also equal to  divided by full load speed. Thus, by substituting all known quantities we get full load speed = 1400/1.1 = 1272.7 rpm so, speed equal to 1273 rpm.

9. Where will speed-torque characteristics will lie when armature reaction is considered?

a) Below the speed-torque characteristics when armature reaction is not considered

b) Above the speed-torque characteristics when armature reaction is not considered

c) On the speed-torque characteristics when armature reaction is not considered

d) Can be anywhere with the speed-torque characteristics when armature reaction is not considered

Answer: b

Explanation: The speed-torque characteristic which has a small linear drop due to the second term  and translates upwards as the field is weakened due to the armature reaction. The demagnetizing effect of the armature reaction causes the characteristics to somewhat bend upwards with increasing torque .

10. Working range of the speed-torque characteristic, with increasing speed will ___________

a) Reduce

b) Increase

c) Remain same

d) Cannot comment

Answer: a

Explanation: The working range of the speed-torque characteristic reduces with increasing speed in order for the armature current not to exceed the full-load value with a weakening field. Thus, armature current gives the bound limit for curve.

11. For speed x rpm, we get field current I f1 and for speed y rpm, we get the field current I f2 . If y is greater than x then, ________________

a) I f1 f2

b) I f1 >I f2

c) I f1 =I f2

d) Cannot comment on I f1 , I f2

Answer: b

Explanation: When speed-torque characteristic for different speeds is plotted on the same graph, we get the curve limited by armature currents also. For any value of field current flux through the field is directly proportional current, while flux is inversely proportional to speed.

12. 400-V dc shunt motor takes a current of 5.6 A on no-load and 68.3 A on full-load. Armature reaction weakens the field by 3%. What is the ratio of full-load speed to no-load speed? Given: Ra = 0.18 Ω, brush voltage drop= 2 V, R f = 200 Ω.

a) 1.2

b) 0.8

c) 1.4

d) 1

Answer: d

Explanation: If = 400/200= 2 A

No-load:

I a0 = 5.6 – 2 = 3.6 A

E a0 = 400 – 0.18 \ 3.6 – 2 = 397.4 V

Full-load:

Ib> a  = 68.3 – 2 = 66.3 A

E a  = 400 – 0.18 / 66.3 – 2 = 386.1 V

n /n  = [386.1/397.4] [1/0.97] = 1.

13. In which of the following method, effect of armature reaction is more?

a) Field weakening method

b) Armature resistance control

c) Same in both methods

d) Cannot be determined

Answer: a

Explanation: In field weakening method we are reducing the working flux to increase the speed, by reducing the field current. Therefore, effect of armature flux on main field flux will increase in case of field weakening method.

This set of Basic DC Machines Questions and Answers focuses on “Speed Control Using Field Control of Series Motor”.

1. Which of the following DC motor has the poorest speed control?

a) Differentially compounded motor

b) Cumulatively compounded motor

c) Shunt motor

d) Series motor

Answer: d

Explanation: DC series motor at no load condition gives infinite speed ideally. Practically it will damage all the armature circuit. Thus, as the load is reduced speed of the motor will go on increasing rapidly. So, speed control is very poor in series motor.

2. In variable speed motor ____________

a) Stronger commutating field is needed at low speed than at high speed

b) Weaker commutating field is needed at low speed than at high speed

c) Same commutating field is needed at low speed and at high speed

d) Cannot be determined

Answer: b

Explanation: According to commutating fields of machine, speed and overall performance of a machine is determined. For low speeds of motor in a variable speed motor we need weaker commutating field.

3. The speed of a motor falls from 1200 rpm at no-load to 1050 rpm at rated load. The speed regulation of the motor is ____________

a) 12.36%

b) 14.28%

c) 16.77%

d) 18.84%

Answer: b

Explanation: Speed of regulation of DC series motor is given by ratio of difference of no-load speed and full load speed with full load speed. Thus, in this case, speed regulation = /1050= 14.28%.

4. Which of the following is not the method of speed control in DC series motor?

a) Diverter

b) Tapped-field control

c) Variable resistance in series with armature

d) Series- parallel control

Answer: c

Explanation: Diverter means adding a variable resistance in parallel with field winding. Tapped field control is the method where field ampere turns are adjusted in steps by varying the number of turns included in the circuit. In series parallel method field winding is connected so that it will form two parts.

5. For speed reversal, field control method is suitable.

a) True

b) False

Answer: b

Explanation: Field control method is not suited to applications needing speed reversal; since the only way to reverse speed is to disconnect the motor from the source and reverse the field/armature polarity. The field circuit being highly inductive, it is normally the armature which is reversed.

6. For large motors what is the ratio of compensating winding is required for increasing the speed of the motor?

a) 2:1

b) 4:1

c) 6:1

d) 8:1

Answer: a

Explanation: For motors requiring a wide range of speed control, the field ampere-turns are much smaller than the armature ampere-turns at high speeds causing extreme distortion of the flux density in the air-gap. This leads to poor commutation. Compensating winding can be used to increase the speed range which can be 2 to 1 for large motors.

7. In diverter resistor field control method of DC series motor, variable resistor is added ________

a) In parallel with field

b) In series with field

c) In parallel with armature

d) In parallel with load

Answer: a

Explanation: Diverter is a small variable resistor, which is connected in parallel with field winding. By varying this resistance value, the field current and consequently the field ampere-turns can be varied and speed can be controlled.

8. Why it is advisable to use inductively wound diverter resistor?

a) To make speed control on more range

b) For long-life of machine

c) Cost efficient

d) To avoid oscillations in speed

Answer: d

Explanation: One precaution to be taken in this method in order to avoid oscillations in speed initiated by load changes is to use an inductively wound diverter resistor. Thus, speed controlled output will vary smoothly when load is changed.

9. For higher diverter resistance, speed-torque characteristic will lie ____________

a) Above speed-torque characteristic of lower resistance

b) Below speed-torque characteristic of lower resistance

c) On the speed-torque characteristic of lower resistance

d) Cannot say

Answer: b

Explanation: Speed-torque characteristic for decreasing values of diverter resistance or decreasing value of k d will lie above of the previous or lower value of diverter resistance. Thus, for higher diverter value curve will lie below the previous one.

10. In tapped field control method ____________

a) A variable resistor is connected in parallel

b) A variable resistor is connected in series

c) Field winding is made with more out pins

d) Another field winding is added with the previous one

Answer: c

Explanation: In tapped field control of the DC series motor a field winding is simply tapped at various points and many out pins taken, so that any out pin can be connected and particular part of field winding will be selected in the circuit, while other part is made open.

11. Which of the following method will not give many speed values for a DC series motor?

a) Diverter

b) Series-parallel

c) Field tapped

d) All field control methods will give many speed values

Answer: b

Explanation: In series-parallel method, field winding is equally divided in two parts which are connected in series and parallel with respect to each other. Thus, in series connection we’ll get one speed and in parallel mode we’ll get one speed. These 2 speeds are only possible.

12. In series-parallel speed control method ______________

a) Speed given by parallel connection is more

b) Speed given by series connection is more

c) Both speeds can be equal

d) Will depend on other parameters

Answer: a

Explanation: In parallel case, total ampere turns will be exactly half of the ampere turns of the series case. Since speed of any DC motor is inversely proportional to the MMF or flux or ampere turns developed, speed of the parallel control method will always large than the series one.

13. Where diverters are used?

a) In shunt motors

b) In series motors

c) In both motors

d) All other motors except shunt and series motors

Answer: b

Explanation: Diverters are used in series motors for speed control, they are not used in shunt motors. Since, shunt field winding resistance is very high, if we connect a diverter across shunt field winding, total current will flow through diverter and it will almost short circuit the shunt field winding. This will increase the motor speed to a very high value.

14. What will happen to the speed of a series motor if the temperature of armature resistance is increased?

a) Not change

b) Decreases

c) Increases

d) Cannot be determined

Answer: b

Explanation: In a DC series motor, when the resistance increases due to rise in temperature, the IR drop in the circuit will increase, which reduces the effective voltage applied to the armature, hence speed decreases due to the direct proportionality.

This set of DC Machines Questions and Answers for Aptitude test focuses on “Speed Control Using Armature Control in DC Motor”.

1. For which speed control method we get minimum efficiency?

a) Voltage control method

b) Field control method

c) Armature control method

d) Cannot be determined

Answer: c

Explanation: In armature control method, we vary armature voltage by adjusting variable resistance value. So, when we define armature efficiency, it is equal to η = [ (V t -I a R e ) I a ] / V t I a = 1-I a R e / V t . Thus, efficiency is reduced drastically for large speed reductions.

2. The speed of a DC motor can be varied by changing ______________

a) Field current

b) Applied voltage

c) Resistance in series with armature

d) Field current, applied voltage or resistance in series with armature any method will work

Answer: d

Explanation: In field control method we change the flux produced by machine with the help of field current. If applied voltage is changed, definitely speed will change. By adding resistance in series with armature voltage of the armature can be varied to change the speed.

3. For constant torque drive which of the following speed control method is preferred?

a) Field control

b) Armature voltage control

c) Shunt armature control

d) Voltage control

Answer: b

Explanation: Armature control method provides a constant-torque drive. In the shunt motor case by keeping the field current at maximum value full motor torque can be obtained at full-load armature current at all speeds.

4. When the armature of a DC motor rotates, emf induced is in machine is called as ___________

a) Self-induced emf

b) Mutually induced emf

c) Back emf

d) Cannot be determined

Answer: c

Explanation: According to terminology used in DC machines, emf induced in a machine when armature rotates is called as back emf. This back emf value is directly proportional to the speed of the motor.

5. Which of the following method is used for DC motor with 12+ HP requiring frequent start, stop, speed reversal?

a) Drum type controller is used

b) Three-point starter is used

c) Four-point starter is used

d) Cannot be determined

Answer: a

Explanation: Drum controllers are used when an operator is controlling the motor directly/manually. The drum controller is used to start, stop, reverse, and vary the speed of a motor, by its mechanism. This type of controller is used on crane motors, elevators, machine tools, and other heavy applications.

6. Flux density distribution is distorted by armature control method.

a) True

b) False

Answer: b

Explanation: Unlike field control method, the main field ampere-turns are maintained at a large value, flux density distortion caused by armature reaction is limited. Unlike field control scheme, speed reversal can be easily implemented here.

7. In rheostatic series control method of armature we add ____________

a) Variable resistor in parallel with armature

b) Variable resistor in series with armature

a) Fixed resistor in parallel with armature

b) Fixed resistor in series with armature

Answer: b

Explanation: In rheostatic series control method, we add variable resistance in series with the armature so that armature voltage can be varied, by varying the voltage drop at series resistor. By increasing value of series resistor speed can be decreased.

8. By series armature resistance method, we can get _________________________

a) Speed above rated speed

b) Speed equal to rated speed

c) Speed below rated speed

d) All speeds are possible

Answer: c

Explanation: The drawback of armature speed control method is we get only speeds which are below the rated speed. As the value of series resistance increases, back emf decreases which result in lowering of the speed.

9. Consider the armature control method, where 10 Ω resistance is connected in series with armature in first case. For second case resistance value is changed 100 Ω. Then ____________

a) Speed-torque characteristic of both will start from same point

b) For first case speed-torque characteristic will start below to the case 2

c) For first case speed-torque characteristic will start above to the case 2

d) Can’t say because other parameters are unavailable

Answer: a

Explanation: Speed-torque characteristic will start from same point because at zero torque presence of series resistance is not taken into account. Afterwards speed torque characteristic with higher series resistance value will lie below the other.

10. Speed regulation of armature series control method is __________

a) Very good

b) Zero

c) Poor

d) Cannot comment

Answer: c

Explanation: The speed regulation of the method is poor as for a fixed value of series armature resistance, the speed varies directly with load, being dependent upon the resistance voltage drop. In general, rheostatic control is economically feasible only for very small motors  or for short-time, intermittent show-downs for medium-sized motors.

11. In shunted rheostatic armature control method ______________

a) Variable resistor is added in parallel with armature

b) Variable resistor is added in series with armature

c) Variable resistor is added in parallel with armature and another variable resistor is added in series

d) Variable resistor is not added in whole circuit

Answer: c

Explanation: In shunted armature speed control method we add one variable resistance in series with armature and one variable resistance in parallel with armature, so by varying the ratio of this resistances we can get various different speeds.

12. Which of the following is correct statement?

a) Speed regulation of rheostatic armature control method is better than that of shunted armature control

a) Speed regulation of rheostatic armature control method is worse than that of shunted armature control

a) Speed regulation of rheostatic armature control method is almost equal to that of shunted armature control

a) Speed regulation of rheostatic armature control method and of shunted armature control are equally worst.

Answer: b

Explanation: Speed regulation of shunted armature control method is better than that of rheostatic armature control. External power loss in shunted armature control method is very high compare to that of rheostatic armature control method.

13. By series parallel method of armature control how many different speeds are possible?

a) 4

b) 8

c) 2

d) Infinite

Answer: c

Explanation: Here two identical motors are coupled together mechanically to a common load. Two speeds at constant torque are possible in this method—one by connecting the motors armatures in series and the other by connecting them in parallel.

14. Which method is best for series traction motors?

a) Rheostatic armature control

b) Shunted armature control

c) Series parallel control method

d) Any method can be employed

Answer: c

Explanation: Series-parallel control method is superior to the rheostatic control insofar as efficiency is concerned. However, it is limited to two speed steps. The method is commonly employed for speed control of series traction motors.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Ward Leonard Speed Control Method”.

1. Ward Leonard method is ___________

a) Armature control method

b) Field control method

c) Combination of armature control method and field control method

d) Totally different from armature and field control method

Answer: c

Explanation: Ward Leonard method is the combination of armature control method and field control method, which can also be called as voltage control method. This is the most efficient method of speed control over wide range.

2. Which of the following component is not used in Ward Leonard method?

a) AC motor

b) DC generator

c) DC motor

d) AC generator

Answer: d

Explanation: Whole unit of Ward Leonard speed control unit consists of various units like DC generator, DC motor, AC motor, exciter circuit and various pots which are used for carrying out smooth operation.

3. In Ward Leonard speed control method for lowering the speed of the motor ______________

a) Reduce armature voltage

b) Increase armature voltage

c) Increase field current

d) Decrease field current

Answer: a

Explanation: In Ward Leonard speed control method, speed can be reduced under base value by reducing armature voltage. By increasing field current speed can be reduced but this is not employed in Ward Leonard method.

4. Reducing the armature voltage will give us _______________

a) Variable torque speed control

b) Constant torque speed control

c) Variable and constant both can be achieved

d) Cannot comment on torque

Answer: b

Explanation: As seen from speed torque characteristics, reducing armature voltage will reduce the speed of the motor below base value but torque will remain same. Thus, reducing armature voltage will give constant torque speed control.

5. In Ward Leonard speed control method for increasing the speed of the motor ______________

a) Reduce armature voltage

b) Increase armature voltage

c) Increase field current

d) Decrease field current

Answer: d

Explanation: In Ward Leonard speed control method, speed can be increased above base value by weakening of the field, which can be done by lowering field current value. By increasing armature voltage speed can be increased but this is not employed in Ward Leonard method.

6. Reducing the field current will give us _______________

a) Constant torque and variable power speed control

b) Constant torque speed control

c) Variable power speed control

d) Constant power speed control

Answer: b

Explanation: As seen from speed torque characteristics, reducing field current will increase the speed of the motor above base value but power will remain same. Thus, reducing armature voltage will give constant power speed control, with variable torque.

7. Speed-power characteristic for Ward Leonard speed control method _________________

a) Will start from origin

b) Will start from some positive value on power axis

c) Will start from some positive value on speed axis

d) Depends on other parameters

Answer: a

Explanation: Speed power characteristic of DC motor is plotted when, Ward Leonard speed control method is employed. For speed equal to zero, which is less than base speed, we get constant torque but variable power operation. Thus, power will start increasing from origin.

8. Efficiency of Ward Leonard method is ____________

a) Higher than rheostatic control method but lower than shunted field control method

b) Lower than rheostatic control method

c) Higher than rheostatic control method and shunted field control method

d) Depends on load

Answer: c

Explanation: Unlike all other methods, external resistance is not added in the circuit of control system. Thus, efficiency of Ward Leonard control method is always highest at various different speeds.

9. Ward Leonard method is an ideal choice for motor which undergoes frequent starting, stopping, speed reversal.

a) True

b) False

Answer: a

Explanation: Absence of external resistance increases efficiency. Also, when the generator emf becomes less than the back emf of the motor, electrical power flows back from motor to generator, is converted to mechanical form and is returned to the mains via the driving ac motor. This aspect makes Ward Leonard method perfect for given application.

10. Starting gear used in Ward Leonard method___________

a) Is of small size

b) Is of large size

c) Size depends on application

d) Is absent

Answer: d

Explanation: No special starting gear is required in Ward Leonard method of speed control. As the induced voltage by generator is gradually raised from zero, the motor starts up smoothly. Speed reversal is smoothly carried out.

11. To get the speed of DC motor below the normal speed without wastage of electrical energy we use __________________

a) Ward Leonard control

b) Rheostatic control

c) Any of the Ward Leonard or rheostatic method can be used

d) Not possible

Answer: a

Explanation: Ward Leonard method of speed control is most efficient method of speed control in all aspects. We can get constant torque operation and constant power operations as well, with this method.

12. Speed control by Ward Leonard method, can give uniform speed variation _______________

a) In both directions

b) In one direction

c) Below normal speed only

d) Above normal speed only.

Answer: a

Explanation: Speed control by Ward Leonard method, gives uniform speed variation in both the directions and above and below of normal speed as well. Speed reversal is carried out smoothly by this control method.

13. Ward Leonard control is basically a _____________

a) Voltage control method

b) Field diverter method

c) Field control method

d) Armature resistance control method

Answer: a

Explanation: Ward Leonard speed control method is combination of rheostatic series control method and shunted armature control method with field control as well. Thus, it can be called as voltage control method also.

14. In Ward Leonard control of DC motor, the lower limit of speed is imposed by ____________

a) Residual magnetism of the generator

b) Core losses of motor

c) Mechanical losses of motor and generator together

d) Cannot be determined

Answer: a

Explanation: We get the speed below the base value by reducing armature voltage, which is the simple method of reducing back emf which is proportional to the residual magnetism. Thus, lower limit of speed is imposed by residual magnetism of the generator.

15. The disadvantage of the Ward Leonard control method is ___________

a) High initial cost

b) High maintenance cost

c) Low efficiency at high loads

d) High cost, high maintenance and low efficiency

Answer: d

Explanation: Ward Leonard speed control method requires large number of building blocks like generators, motors. Thus, installing cost and maintenance cost of the whole unit is very high. Apart from cost, it gives low efficiency at very high loads.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Braking of DC Motors – 1”.

1. Which of the following is the best braking method?

a) Friction

b) Electromechanical action

c) Eddy-currents

d) Electric braking

Answer: d

Explanation: Braking methods based on friction, electromechanical action, eddy-currents, etc. are independent of the motor but sometimes electric braking is better justified owing to its greater economy and absence of brake wear.

2. DC motor is still widely used in tractions due to its excellent braking properties.

a) True

b) False

Answer: b

Explanation: Dc motor is used in tractions because of its excellent braking characteristics and ability of smooth transition from the motor to the generator mode and vice versa. Also, characteristics suit perfectly for traction application.

3. Which of the following is not the method of electrical braking?

a) Plugging or counter-current

b) Dynamic or rheostatic

c) Regenerative

d) Eddy current

Answer: d

Explanation: Eddy current is the electrical effect or response of the system, which is reflected mechanically at brakes to reduce the speed of the motor. Thus, eddy current is not an electrical brake, it is mechanical one.

4. Which of the following is the plugging method of braking?

a) Reversal of field connections

b) Reversal of armature connections

c) Addition of equal and opposite field

d) Removal of field circuit from current machine circuit

Answer: b

Explanation: Plugging is method where connections are reversed at a given instant. Because of the problem of interrupting highly inductive field current and the time needed for the field current to build up in opposite direction, it is a common practice to reverse armature connections.

5. Which of the following is correct formula for braking torque in plugging?

a) n (ka 2 /R b )

b) n 2 (ka 2 /Rb)

c) n -1 (ka 2 /Rb)

d) (ka 2 /Rb)

Answer: a

Explanation: Braking torque is equal to braking power divided by speed of the motor.

T= [ 2 /Rb]/n. As, Braking power is equal to Ea 2 /Rb/n. By solving for the braking torque from the above equation, we get n (ka 2 /Rb).

6. Electrical braking of any variety becomes less effective as ________________

a) Speed increases

b) Speed decreases

c) Independent of speed

d) Depends on supply voltage

Answer: b

Explanation: Braking torque of the DC machine is given by n . Here, braking torque is directly proportional to the speed of the motor, so as the speed decreases the efficiency of electrical brakes which is dependent on braking torque decreases.

7. Plugging is applied in a motor, if we don’t make the switch OFF what will happen?

a) Motor will come to rest as a result of plugging

b) Motor will come to rest and will start rotating in another direction

c) Motor will burn

d) Nothing will happen

Answer: b

Explanation: If the switch is kept ON near to zero speed, motor will have braking torque acting in opposite direction greater than the electromechanical torque. Thus, motor will come to rest and for the next instant motor will start rotating in opposite direction.

8. Plugging is used in ____________

a) Small motors only

b) Small and medium powered

c) Only in large heavy machines

d) Everywhere

Answer: a

Explanation: Plugging is used in small scale applications only. The large initial current and high mechanical stress restrict the application of plugging in large machines. So, in order to balance stress this method is used in small machines only.

9. Which of the following is dynamic braking?

a) Reversal of field connections

b) Reversal of armature connections

c) Addition of equal and opposite field

d) Removal of armature circuit from current machine circuit

Answer: d

Explanation: Reversal of the connections of armature is the method called plugging. In dynamic braking we remove the armature circuit and connect it to different resistor, with field circuit still connected to the external supply.

10. Braking time in the dynamic braking is the function of _____________

a) System inertia

b) Load torque

c) Motor rating

d) All- system inertia, load torque and motor rating

Answer: d

Explanation: In dynamic braking, when brakes are applied the armature is disconnected from machine circuit and connected to the braking resistor. Now, at this point motor is driven by kinetic energy gained earlier, dissipating power in braking resistor.

11. In dynamic braking, when braking is applied system acts as ___________

a) Freely running machine

b) Motor with slow speed

c) Generator

d) Motor with same speed in opposite direction

Answer: c

Explanation: The armature is disconnected from the supply and then a braking resistor Rb is immediately connected across it. The motor acts as a generator, driven by the inertia and stored kinetic energy dissipating power in Rb. This is a simple method of bringing a motor nearly to a standstill.

12. In which of the following electrical braking method, energy is supplied back to the supply?

a) Plugging

b) Dynamic braking

c) Regenerative braking

d) In all electrical braking

Answer: c

Explanation: In plugging energy is wasted in braking resistance which is equal to starting resistance while running as a motor. In dynamic braking energy is generated but it is not fed back to supply. In regenerative method energy is sent back for reuse.

This set of DC Machines Question Bank focuses on “Braking of DC Motors – 2”.

1. Regenerative braking is used when duty cycle ____________

a) Requires braking of machine

b) Requires accelerating of machine

c) Requires constancy of machine

d) Cannot comment on duty cycle

Answer: a

Explanation: Regenerative braking is used specially where the duty cycle requires the braking or slowing of the machine more frequently and is most useful in holding a descending load of high potential energy at a constant speed.

2. Regeneration is not easily possible for ____________

a) DC shunt motor

b) Separately excited motor

c) Compounding motor with weak series compounding

d) DC series motor

Answer: d

Explanation: Regeneration is possible with a shunt and separately excited motors and with compound motors with weak series compounding. Series motors need a reversal of either the field or the armature connections.

3. Which of the following method is not used for regeneration?

a) Increasing field current

b) Increasing armature speed

c) Increasing supply voltage

d) Reducing supply voltage

Answer: c

Explanation: Regeneration is achieved by either increasing field current, increasing armature speed, or by reducing supply voltage. Increasing supply voltage is not the method which is employed in regeneration process.

4. If the terminals of armature of DC motor are interchanged, this action will offer following kind of electrical braking ______________

a) Regenerative

b) Plugging

c) Dynamic braking

d) Depends on other parameters

Answer: b

Explanation: Plugging is electrical braking method, where field or armature connections are reversed technically. But field reversal is not employed as results obtained from field reversal are not good compare to armature reversal.

5. The plugging braking gives the _____________

a) Zero torque braking

b) Smallest torque braking

c) Highest torque braking

d) Variable torque braking

Answer: c

Explanation: In electrical braking called plugging direct reversal of connections is done, which causes maximum torque to act on shaft but in opposite direction. As the speed decreases this torque also starts decreasing.

6. Regenerative method of braking is based on ___________

a) Back emf is less than the applied voltage

b) Back emf is equal to the applied voltage

c) Back emf of rotor is more than the applied voltage

d) Cannot be determined

Answer: c

Explanation: The condition for regeneration is that the rotational emf is more than the applied voltage so that the current is reversed and the mode of operation changes from motoring to generating.

7. During regenerative braking of DC motors ____________

a) Motor will run as a generator

b) Motor will reverse in direction

c) Motor will run at reduced speed

d) Motor will run as free rotating shaft

Answer: a

Explanation: In regenerative method of electrical braking, motor is suddenly forced to act as a generator, all the energy then obtained is pushed back into the supply unlike in dynamic braking this energy is wasted.

8. Where dynamic braking is used?

a) Shunt motors

b) Series motors

c) Compound motors

d) All DC motors

Answer: d

Explanation: Dynamic braking is used in all DC motors though its implantation in series DC motor requires one more additional step of reversal of connections. Only care taken is, addition of braking resistance, armature resistance and series field resistance is lower than the critical resistance at that speed.

9. Which method of braking is generally used in elevators?

a) Plugging

b) Regenerative braking

c) Rheostatic braking

d) Mechanical braking

Answer: a

Explanation: Plugging braking provides maximum torque in opposite direction at the instant of braking, this characteristic of braking suits perfectly with the application that is in elevators. If switch is kept ON, we get reverse rotation also.

10. For which of the following motor dynamic braking is very effective?

a) Shunt motors

b) Separately excited motors

c) Series motors

d) Differential compound motors

Answer: b

Explanation: Dynamic braking is very effective for separately excited DC motors. As in separately excited motors the direction of field can be very easily altered by altering the terminals of the field, which is the condition in dynamic braking.

11. When is the dynamic braking is employed?

a) Non-reversing drive

b) Reversing drive

c) Both Reversing and Non-reversing

d) Cannot tell

Answer: c

Explanation: Dynamic braking is employed to brake both reversing drives and non-reversing drives.

In dynamic braking, the electrical energy generated during stopping action is released as heat through a voltage regulated transistor and resistor.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Efficiency of DC Machine- 1”.

1. The efficiency of the DC motor at maximum power will be ___________________

a) 100%

b) Around 90%

c) Anywhere between 75% and 90%

d) Less than 50%

Answer: d

Explanation: For getting maximum power, derivative of power with respect to current is equal to 0. This is practically impossible to achieve as, current required is much more than its normal rated value. Large heat will be produced in a machine and efficiency of motor will be less than 50 %.

2. The hysteresis loss in a DC machine least depends on _____________

a) Frequency of magnetic reversals

b) Maximum value of flux density

c) Volume and grade of iron

d) Rate of flow of ventilating air

Answer: d

Explanation: As iron core of the armature is rotating in magnetic field, some losses occurs in the core which is called core losses. These losses are categorized as Hysteresis loss and Eddy current loss. They depend on all quantities listed above.

3. Which of the following is not the effect of iron loss?

a) Loss of efficiency

b) Excessive heating of core

c) Increase in terminal voltage

d) Rise in temperature of ventilating air

Answer: c

Explanation: Iron loss causes excessive heat production in the core of a machine, which will rise the temperature of ventilating air, as it acts as heat exchanger. Thus, terminal voltage rise is not an effect of any loss.

4. Which of the following loss is likely to have highest proportion at rated load of the DC generator?

a) Hysteresis loss

b) Field copper loss

c) Armature copper loss

d) Eddy current loss

Answer: c

Explanation: Armature copper loss is directly proportional to the square of armature current multiplied by the armature resistance and also the series field resistance if present any. As, at loaded condition armature current is very high.

5. Which of the following loss in a DC generator varies significantly with the load current?

a) Field copper loss

b) Windage loss

c) Armature copper loss

d) Cannot be determined

Answer: c

Explanation: Armature copper loss is directly proportional to the square of armature current, as load current varies armature current varies, which is reflected significantly in loss as a square of it. Thus, armature copper loss can be detected.

6. Which of the following methods is likely to result in reduction of hysteresis loss in a DC generator?

a) Providing laminations in armature core

b) Providing laminations in stator

c) Using non-magnetic material for frame

d) Using material of low hysteresis co-efficient for armature core material

Answer: d

Explanation: Providing laminations will work for reducing eddy current losses, but hysteresis loss is dependent on the material chosen. Thus, using different material for core of armature will definitely work.

7. Which of the following loss/losses in a DC generator is dissipated in the form of heat?

a) Mechanical loss

b) Core loss

c) Copper loss

d) Mechanical, Copper and Core

Answer: d

Explanation: All the losses listed above dissipate the heat. This, dissipated heat due to various losses results in increasing the temperature of ventilating air. These losses are dangerous in long running of a machine, can reduce efficiency also.

8. Which of the following losses are significantly reduced by laminating the core of a DC generator?

a) Hysteresis losses

b) Eddy current losses

c) Copper losses

d) Windage losses

Answer: b

Explanation: Hysteresis losses can be minimized by using material with lower hysteresis coefficient. Eddy current losses can be minimized by using laminated sheets of core structed together. To reduce other losses, current should be minimized, can’t be reduced as it also has lower limit.

9. The total losses in a well-designed DC generator of 10 kW will be nearly equal to ________

a) 100 W

b) 500 W

c) 1000 W

d) 1500 W

Answer: b

Explanation: Total losses in a DC machine can be approximated to 4-5% of its rating from the experimental observations. Thus, 5% of 10 kW is equal to 500 W. It’s an approximation formed on various observations.

This set of DC Machines Questions and Answers for Entrance exams focuses on “Efficiency of DC Machine – 2”.

1. The condition for maximum efficiency for a DC generator is __________

a) Eddy current losses = stray losses

b) Hysteresis losses = eddy current losses

c) Copper losses = 0

d) Variable losses = constant losses

Answer: d

Explanation: All losses in a given DC machine can be categorized into variable losses and constant losses. Variable losses are proportional to the square of armature current while constant losses are almost constant for a given DC machine throughout its application.

2. DC generators are normally designed for maximum efficiency at or near ____________

a) Full-load

b) Minimum load

c) Rated voltage

d) At all loads

Answer: c

Explanation: The efficiency of a machine is different at different values of power output. All electrical machines are generally designed to give maximum efficiency at or near the rated output of the machine. Thus, maximum efficiency occurs at rated voltage.

3. In a DC generator, the iron losses mainly take place in ____________

a) Yoke

b) Commutator

c) Armature conductors

d) Armature rotor

Answer: d

Explanation: Iron losses take place in the form of hysteresis loss and eddy current loss. These losses are maximum, where field is maximum. Thus, when armature is rotated in presence of flux we get maximum iron loss.

4. If DC generators are located near load centres, which losses can be minimised?

a) Iron losses

b) Eddy current losses

c) Line losses

d) Corona losses

Answer: c

Explanation: Line losses occur in long transmission lines while sending output power to the loading stations. Thus, by locating the generators near loading stations losses occurring due transmission line can be eliminated.

5. Nature of efficiency curve of DC machine is _________________

a) First decreases then increases

b) First constant then decreases

c) First constant then increases

d) First increases then decreases

Answer: d

Explanation: The efficiency of a DC machine is different at different values of power output. As the output increases, the efficiency increases till it reaches to a maximum value. As the output is further increased, the efficiency starts decreasing slowly.

6. Why retardation test is carried on DC machine?

a) To find stray losses

b) To find eddy current losses

c) To find field copper losses

d) To find windage losses

Answer: a

Explanation: This test is generally employed to shunt generators and shunt motors. From this method we can get stray losses of a machine. Thus, if armature and shunt copper losses at any given load current are known then efficiency of a DC machine can be easily estimated.

7. In the DC motor the iron losses occur in __________

a) Field

b) Rotor

c) Brushes

d) Commutator

Answer: b

Explanation: Armature winding in a DC machine is located on rotor. Iron losses occur in an armature, hence in rotor. Iron losses are hysteresis loss and eddy current loss, which are seen prominently in armature rotor.

8. Which of the following losses is not under constant losses?

a) Friction and windage losses

b) No load core losses

c) Shunt field losses

d) Hysteresis losses

Answer: d

Explanation: All the losses that is friction and windage losses, no load core losses, shunt field core losses in shunt field and compound motors come under the category of constant losses, while iron losses come under category of variable losses.

9. Variable losses are proportional to ________

a) Armature current

b) Square of armature current

c) Inverse of armature current

d) Inverse of square of armature current

Answer: b

Explanation: Variable losses include losses in armature resistance and losses in series resistance, which are directly proportional to the square of armature current. It also includes stray load loss  which is proportional to square of armature current.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Swinburne’s Test”.

1.Swinburne’s test can be carried out on all DC motors.

a) True

b) False

Answer: b

Explanation: Swinburne’s test is a no-load test; thus, it is performed only on shunt and compound motors. DC series motor is not advisable to start at no-load so, this test can’t be performed on series motor.

2. Which of the following test will be suitable for testing two similar DC series motors of large capacity?

a) Swinburne’s test

b) Hopkinson’s test

c) Field test

d) Brake test

Answer: c

Explanation: As Swinburne’s test and Hopkinson’s test are no-load test, one can’t perform these tests on DC series motor. For DC series motor with large capacities, we conduct field test to find various losses occurring in a machine.

3. Which losses can be identified from Swinburne’s test?

a) No-load core loss

b) Windage and friction loss

c) No-load and windage and friction loss

d) Stray load loss

Answer: c

Explanation: We get total rotational losses occurring in a machine, which are equal to no-load core losses  and windage and friction losses. We also get shunt field losses and variable loss occurring in armature resistance.

4. While carrying out Swinburne’s test at rated armature voltage motor will run at ____________

a) Speed equal to rated speed

b) Speed greater than rated speed

c) Speed less than rated speed

d) Can run anywhere

Answer: b

Explanation: The motor is set to run on a rated speed while conducting Swinburne’s test by adjusting field current to rated value. When rated armature voltage is applied, motor runs with slightly greater speed than the rated one as some o the losses are not taken into account.

5. In order to run motor on rated speed while carrying out Swinburne’s test we add ______________

a) Resistance in parallel with armature

b) Resistance in series with armature

c) Inductor in series with armature

d) Capacitor in parallel with armature

Answer: b

Explanation: The machine would run at higher than rated speed with a rated armature voltage. Therefore, a series in the armature circuit is employed to reduce voltage applied to the motor armature such that it runs at rated speed.

6. What is the purpose of performing retardation test after Swinburne’s test?

a) To find stray load loss

b) To find variable losses

c) To separate out windage and friction losses

d) To find shunt field losses

Answer: c

Explanation: While performing Swinburne’s test we get addition of all losses. So, if we want to treat each loss individually, one needs to separate them. Retardation test is thus used by disconnecting both armature and field of the running motor.

7. Efficiency calculated by Swinburne’s test is _____________

a) Exactly equal

b) Over-estimated

c) Under-estimated

d) Depends on the manual errors

Answer: b

Explanation: The stray-load loss cannot be determined by this test and hence efficiency is over-estimated. Correction can be applied after assuming the stray-load loss to be half the no-load loss, which is done generally. Also, temperature may effect on resistance value which is not considered in this test.

8. Which of the following is not a disadvantage of a Swinburne’s test?

a) The stray-load losses can’t be determined by this test

b) Steady temperature rise can’t be determined

c) Does not give results about satisfactory commutation

d) Machine gets damaged

Answer: d

Explanation: For increasing life and durability of a machine we carry out such tests. But stray load losses, steady temperature rise is not determined by this method. Also, results about satisfactory commutation are not given by this method.

9. While carrying out retardation test, if t is equal to time constant then _____________

a) Speed increases to 36.8% of its initial value

b) Speed reduces to 36.8% of its initial value

c) Speed reduces to 26.8% of its initial value

d) Speed reduces to 46.8% of its initial value

Answer: b

Explanation: While carrying out retardation test, speed of the motor decreases. At time t equal to time constant then speed reduced is given by 36.8% of its initial value. Retardation test result is used to determine initial slope of ω.

10. In retardation test _____________

a) Motor switch is made ON and various speed readings are taken

b) At rated speed various speed readings are taken out at different times

c) Motor switch is made OFF at rated speed and various speed readings are taken

d) Some readings are taken while speed is building up and some readings while speed is lowering down

Answer: c

Explanation: The motor is run to rated speed  and the supply is switched-off. As the motor decelerates , several speed-time readings are taken, by a speedometer and watch with seconds hand. Initial readings are taken at small time intervals and the time interval is increased as the motor slows down.

11. Retardation curve is ______________

a) Starting from origin

b) Starts from some positive value and increasing

c) Starts from some positive curve and stays constant

d) Starts from some positive value and decreases

Answer: d

Explanation: Retardation curve is a plot of speed vs. time at various retarding speeds. Thus, curve starts at some positive value and shows exponentially decaying nature with time. From various values of speed we calculate windage and friction loss at each point.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “Hopkinson’s Test”.

1. Hopkinson’s test of D.C. machines is conducted at _______________

a) No-load

b) Part load

c) Full-load

d) Overload

Answer: c

Explanation: Unlike Swinburne’s test Hopkinson’s test is carried out at loaded condition. Thus, we get stray load loss also, while finding out the efficiency. Hence, efficiency is not over-estimated like Swinburne’s test.

2. Hopkinson’s test requires ____________

a) One DC machine on which test is carried out

b) Two different DC machines

c) Two identical DC machines

d) Can be worked with one or two machines

Answer: c

Explanation: This is a regenerative test in which two identical dc shunt machines are coupled mechanically and tested simultaneously. One of the machines is used as a motor driving while the other one acts as a generator which supplies electric power to motor.

3. In Hopkinson’s test, two machines are connected in ______________

a) Series

b) Parallel

c) Can be connected in parallel or series

d) Two machines are not required

Answer: b

Explanation: The two machines are made parallel by means of switch S after checking that similar polarities of the machine are connected across the switch. Here, one machine is driving another machine.

4. What will happen if field current of generator in Hopkinson’s test is increased?

a) Current through motor armature will increase

b) Current through motor armature will decrease

c) Current through motor armature will remain constant

d) Motor armature current cannot be determined

Answer: a

Explanation: If field current through generator is increased, back emf of generator will increase thus it’ll become greater than back emf of a motor, so to compensate this effect armature current in generator will increase thus, motor armature current will also increase.

5. What will happen if field current of motor in Hopkinson’s test is decreased?

a) Current through motor armature will increase

b) Current through motor armature will decrease

c) Current through motor armature will remain constant

d) Motor armature current can’t be determined

Answer: a

Explanation: If field current through motor is decreased, speed of the motor will increase due to inverse proportionality, back emf of generator will increase thus it’ll become greater than back emf of a motor, so to compensate this effect armature current in generator will increase thus, motor armature current will also increase.

6. Hopkinson’s test is a regenerative test.

a) True

b) False

Answer: a

Explanation: Hopkinson’s test is a regenerative test, because the power drawn from the mains is only that needed to supply losses. The test is, therefore, economical for long duration test like a “heat run”.

7. For carrying out load test on Hopkinson’s test setup _________________

a) Actual load is needed

b) By changing field currents in two machines load can be changed

c) Can’t carry out

d) By changing the armature current test is carried out

Answer: b

Explanation: There is no need to arrange for actual load  which apart from the cost of energy consumed, would be prohibitive in size for large-size machines. By merely adjusting the field currents of the two machines, the load can be easily changed and a load test conducted over the complete load range in a short time.

8. Hopkinson’s test gives _______________

a) Combined iron losses of two machines which can be separated

b) Combined iron losses of two machines which can’t be separated

c) Doesn’t include iron losses

d) Depends on actual setup

Answer: b

Explanation: From Hopkinson’s test both machines are not loaded equally and this crucial in small machines. Thus, it is important to know the separate iron losses for given machines. But, test gives combined losses which are different for different machines as excitation differs.

9. Hopkinson’s test is suitable for ______________

a) Small machines only

b) Small and medium machines

c) All machines

d) Only large machines

Answer: d

Explanation: A large variation of field currents is required for small machines, the full-load set speed is usually higher than the rated speed and the speed varies with load. The full load in small machines cannot obtained by cutting out all the external resistances present in the generator field. Sufficient reduction in the motor field current is necessary to achieve full-load conditions resulting in speeds greater than the rated value.

10. Why field test is conducted even if Hopkinson’s test is present?

a) Instability of an operation

b) Possibility of run-away speed

c) Both instability and possibility of run-away speed

d) Field test is not conducted

Answer: c

Explanation: Regenerative test on two identical series motors is not feasible because of instability of such an operation and the possibility of run-away speed. Therefore, we have toconduct a loading test.

11. In field’s test generator field and motor field are connected in ______________

a) Series

b) Parallel

c) Alternatively, series and parallel

d) Not connected

Answer: a

Explanation: The generator field is connected in series with motor field circuit. The generator is thus separately excited and its excitation is identical to that of motor at all loads. This ensures that the iron-loss of both the machines are always equal.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “BLDC Motors”.

1. In BLDC motor field winding is kept on _______________

a) Stator

b) Rotor

c) Can be placed anywhere

d) Absent

Answer: b

Explanation: Unlike conventional DC motor, BLDC motor has its field winding rotating. Unlike PMDC motor field winding is present. Thus, it is placed on rotor. In BLDC motor we have fixed commutators, with brush gear rotating with speed of rotor field.

2. By electronic circuitry BLDC can be controlled for both constant and variable torque operation.

a) True

b) False

Answer: a

Explanation: In BLDC because of rotor position feedback triggering of thyristors/transistors, the stator and rotor field always remain in synchronism as the frequency of triggering automatically adjusts to motor speed. The length of on-time of the transistors determines motor torque magnitude.

3. Which of the following is not an advantage of BLDC motor over conventional DC motor.

a) Less maintenance

b) Long life

c) No risk of explosion or possibility of RF radiation

d) Low cost

Answer: d

Explanation: The brushless DC motor being more expensive compare to DC motor of same kW ratings, serves many other advantages like long life, less maintenance, rapid response, linear characteristic, no sparking and many more.

4. In BLDC motor driver module, we do not require ______________

a) SCRs

b) Power transistors

c) FETs

d) Transistors

Answer: d

Explanation: The stator is connected to a variable voltage current source through an inductor and an inverter comprising six SCRs . In place of SCRs, power transistor or FETs could be used according to power rating of the motor.

5. In medical fields which DC motor is widely used?

a) PMDC

b) BLDC

c) Brushed DC motor

d) Cannot be determined

Answer: b

Explanation: BLDC motors are widely used in various applications of medical industry. Sensor less BLDC motor and with sensor BLDC motors are used because of easy operation and high reliability compare to conventional motors.

6. Construction of BLDC is exactly similar to the ______________

a) Conventional DC motor

b) Induction motor

c) Permanent magnet synchronous motor

d) Totally different construction

Answer: c

Explanation: Construction of BLDC motor is exactly similar to the PMSM i.e. permanent magnet synchronous motor. BLDC is considered as machine working like DC with construction similar to the AC synchronous machine.

7. Typical brushless motor doesn’t have ______________

a) Commutator

b) Permanent magnet

c) Electronic controller

d) Fixed armature

Answer: a

Explanation: A typical brushless motor has permanent magnets which rotate around a fixed armature. An electronic controller replaces the brush-commutator assembly of the brushed conventional DC motor, which continually switches the phase to the windings to keep the motor turning.

8. BLDC can be used instead ____________

a) Synchronous motor

b) Normal brushed DC motor

c) Induction motor

d) Air motor

Answer: b

Explanation: Brushless motors fulfil almost all functions originally performed by brushed DC motors, but cost and control complexity prevent brushless motors from replacing brushed motors completely in the lowest-cost areas. Brushless motors are now dominating many applications, particularly devices such as computer hard drives and CD/DVD players.

This set of DC Machines Multiple Choice Questions & Answers  focuses on “DC Motor Control Through Converters”.

1. Half wave converters are used for controlling DC motor of ________________

a) Below 400 W

b) 400 W – 4000W

c) More than 4000W

d) Anywhere

Answer: a

Explanation: Half wave converter requires a single thyristor and a free-wheeling diode. In half wave converter circuit the motor current will always be discontinuous. Ultimately it results in poor motor performance. This type of converter is employed only for motors below 400W.

2. How many thyristors does we need in half wave converter?

a) 1

b) 2

c) 3

d) Many

Answer: a

Explanation: Only one thyristor is sufficient for the half wave converter purpose, this is employed for small motors only. This thyristor is connected to the single-phase AC supply and with parallel combination of free-wheeling diode with motor armature.

3.Semi-converters can’t be used when regeneration is required.

a) True

b) False

Answer: a

Explanation: According to the one-quadrant converter, it gives voltage and current of one polarity at DC terminals. Therefore, it does not provide support for regenerative braking, i.e. power flow from DC motor to the AC supply.

4. How many thyristors does we need in semi-converter?

a) 1

b) 2

c) 3

d) Many

Answer: b

Explanation: Semi-converter requires two thyristors. Thyristor Th1 is fired at angle of α and Th2 at angle π+α with respect to the supply voltage v and the process is repeated continuously. Circuit also requires two diodes and one free-wheeling diode.

5. When armature current becomes discontinuous?

a) Small firing angles

b) Large firing angles

c) Infinite firing angle

d) Does not depend on firing angle

Answer: b

Explanation: There are various reasons when armature current becomes discontinuous armature current, prominent reasons are large values of firing angle, high speed and sometimes low torque can also affect.

6. Which of the following is the effect of non-uniform armature current?

a) Ratio of peak to average and rms to average armature current decreases

b) Ratio of peak to average and rms to average armature current increases

c) Ratio of peak to average increases and rms to average armature current decreases

d) Ratio of peak to average decreases and rms to average armature current increases

Answer: b

Explanation: The armature current sometimes due to some reasons may become non-uniform. In this conditions motor performance deteriorates with discontinuous armature current. So, the ratio of peak current to average current increases. Also, rms to average armature current increases.

7. How many quadrants does full-converter work?

a) One

b) Half

c) Two

d) Can be any

Answer: c

Explanation: A full-converter is a two-quadrant converter in which the voltage polarity of the output can reverse, but the current remains unidirectional because of the unidirectional thyristors. It requires no free-wheeling diode.

8. How many thyristors are required for full converter?

a) 1

b) 2

c) 3

d) 4

Answer: d

Explanation: Full converter requires 4 thyristors. The only different thing about the full converter is, it does not require free-wheeling diode. A full converter can be used to feed a separately excited DC motor. Each thyristor is fired accordingly.

9. Full-converter can be used in DC motor for regenerative braking in ______________

a) Constant operation

b) Variable operation

c) Inversion operation

d) Opposite operation

Answer: c

Explanation: When α is greater than 900, the average motor voltage becomes negative. If the motor terminals are reversed, then the circuit will act as a generator in order to feed power back to the AC supply. This is the inversion operation of the converter and used in regenerative braking.

10. Dual converter operates in _________________

a) 1

b) 2

c) 3

d) 4

Answer: d

Explanation: The dual-converter is the modified one and can operate in all the four quadrants. Its circuit contains two full-converters converting to the DC in either direction. The dual converter provides virtually instantaneous reversal of voltage at DC terminals.

11. Which converter/s can be used for DC series motor control?

a) Semi-converters

b) Half-wave converter

c) Full-converter

d) Semi converters and full converter

Answer: d

Explanation: Semi-converters and full-converters are also employed for control of a DC series motor. The total circuit inductance is high because of the series field, as a consequence of which the armature current is mostly continuous.

12. Three phase converters are employed for _______________

a) Large kW motors

b) Small kW motors

c) In all motors

d) Never used

Answer: a

Explanation: Three phase converters are employed when DC motor of very high ratings is used. In three phase converters, the advantage is the ripple frequency of the motor terminal voltage is generally higher than that in single-phase converters.

This set of DC Machines Questions and Answers for Campus interviews focuses on “DC Motor Control Through Choppers”.

1. Which of the following method is adopted for controlling a DC motor?

a) Resistance control

b) Motor-generator set

c) Inverter Rectifier

d) DC chopper

Answer: d

Explanation: All methods listed above are the methods which can be used to control DC motor, but resistive method makes lots of wastage of energy, while motor-generator set and inverter rectifier set are bulky and costly.

2. Which of the following method is employed when regenerative braking is necessary?

a) DC chopper

b) Variable resistor

c) Inverter rectifier

d) Motor-generator

Answer: a

Explanation: A chopper is essentially an electronic switch that turns on the fixed-voltage DC source for short time intervals and applies source potential to motor terminals in series of pulses. This process controls the average voltage applied to the motor.

3. Which of the following statement is not true regarding to DC chopper?

a) Cheap

b) Fast response

c) Regeneration

d) AC to DC control

Answer: d

Explanation: The chopper provides stepless and efficient control of DC voltage and it is less expensive. It is having another advantage of faster response and regeneration. The chopper is relatively new technology and equivalent to DC-DC transformer.

4. Which of the following correctly suits with chopper?

a) Thyristor in series with load

b) Thyristor in parallel with load

c) Thyristor switch in series with load

d) Thyristor switch in parallel with load

Answer: c

Explanation: A chopper is essentially a thyristor switch in series with the load. A shunting diode is provided across the load because free-wheeling the load current when thyristor is off. The thyristor can be shown enclosed by a dotted square can be turned-off over a time period and the cycle is repeated.

5. The average value of chopper output waveform is given by _____________

a) αV

b) V/α

c) α2V

d) V/α2

Answer: a

Explanation: Here the average value of the chopper output voltage waveform is given by Va=V tON/ = V  =αV. So, by varying duty cycle α one can vary the average voltage at output.

6. How output voltage of chopper can be varied?

a) By using constant frequency system

b) By using variable frequency system

c) By using constant and variable frequency systems

d) By using constant or variable frequency systems

Answer: c

Explanation: The average voltage of the chopper output depends on α, thus, α can be varied by either changing the complete time period or by changing an on time or off time. These two methods are named as constant and variable frequency system.

7. What is the formula for step up chopper?

a) V

b) V/

c) αV

d) v/α

Answer: b

Explanation: With the help of conventional chopper one can reduce the average voltage, so by doing slight modification in the circuit one can use chopper method to increase the average voltage applied, by the formula V/.

8. Which of the following is not the method of auxiliary commutation?

a) Voltage commutation

b) Resistance commutation

c) Current commutation

d) Load commutation

Answer: b

Explanation: Thyristor in a chopper has to be turned off by auxiliary commutation. This can be done by two methods-Forced commutation and Load commutation. Forced commutation can also be done by two ways, current and voltage.

9. In voltage commutation, in chopper circuit we use _____________

a) 2 auxiliary thyristors

b) 2 diodes

c) 1 auxiliary thyristor

d) Many diodes

Answer: a

Explanation: Two auxiliary thyristors are connected in the chopper circuit along with one diode, one inductor, one capacitor in series with the voltage supply which is then connected in parallel with diode and DC motor armature.

10. In current commutation, in chopper circuit we use _____________

a) 2 auxiliary thyristors

b) 2 diodes

c) 1 auxiliary thyristor

d) 2 auxiliary thyristors and 2 diodes

Answer: d

Explanation: Two auxiliary thyristors are connected in the chopper circuit along with two diodes, one inductor, one capacitor in series with the voltage supply which is then connected in parallel with diode and DC motor armature.