INDUCTION MOTORS 10

1.Which of the following motors uses a commutator
a) universal motor
b) split phase motor
c) shaded pole motor
d) none of the above
ans. a

2. If a dc series motor is connected to an ac supply
a) the motor will not start
b) the motor will start but then come to stop
c) the motor will start and run but will have poor performance
d) None of these
ans. c

3. If a 3-phase , 4-pole, 50 Hz induction motor runs at a speed of 1440 rpm then the slip is
a) 0.03
b) 0.10
c) 0.04
d) 0.05
ans. c

4. A starting torque of 100 Nm is developed by an auto-transformer starter with a tapping of 40%. If the tapping of auto transformer starter is at 80% , then the starting torque would be
a) 200 Nm
b) 50 Nm
c) 400 Nm
d) 25 Nm
ans. c

5. The complete circle diagram of a 3-phase induction motor can be drawn with the help of
a) running light test alone
b) both running light and blocked rotor tests
c) running light , blocked rotor and stator resistance tests
d) blocked rotor test alone
ans. c

6. The power factor of a delta connected 3-phase , 50 Kw induction motor is 0.4 when delivering 20 % of its rated load. If its stator is reconnected in star , then its pf
a) remains unchanged
b) is improved
c) is worsened
d) none of these
ans. b

7. A 3-phase induction motor runs at 980 rpm at no load. If its squirrel cage rotor is replaced by solid cylinder, then its no load speed would be
a) zero
b) 980 rpm
c) below 980 rpm
d) 490 rpm
ans. c

8. An induction motor in which the stator has the squirrel cage winding is
a) 3-phase squirrel cage motor
b) 1-phase induction motor
c) 1-phase ceiling fan
d) 1-phase table fan
ans. c

9. For satisfactory performance of a 3-phase 420 V , 60 Hz induction motor , the supply voltage at 50 Hz should be equal to
a) 420 V
b) 380 V
c) 350 V
d) 400 V
ans. c

10. A 3-phase wound rotor induction motor with star-connected rotor, runs clockwise. If the phase sequence of the rotor terminals is changed, then the motor would
a) not run
b) run clockwise
c) run anti -clockwise
d) none of these
ans. b

INDUCTION MOTORS 9

1. Rotor slot of squirrel cage induction motor are skewed slightly, so as to
a) increase the mechanical strength of rotor
b) make the rotor construction simplex
c) eliminate locking tendency of the rotor and to reduce the noise
d) save the amount of copper required
ans. c

2. In a 3-phase wound rotor induction motor, 3-phase balanced supply is given to the rotor and stator winding is short circuited. The rotor would
a) not run
b) run in the direction of the field
c) run against the direction of rotating field
d) run at half synchronous speed
ans. c

3. The rotor of an induction motor can't run at synchronous speed, if it did so then
a) rotor emf would be zero
b) rotor current would be zero
c) rotor torque would be zero
d) all of these
ans. d

4. A 3-phase induction motor is running at half full load . If the fuse in one of the phases gets burnt, the motor would
a) come to standstill
b) continue running at the same speed
c) continue running at slightly reduced speed
d) continue running at an increased speed
ans. c

5. A 3-phase , 50 Hz , 4-pole squirrel cage induction motor has its stator rewound for 6 poles without any alternations in the rotor. The motor would now run at a speed
a) less than 1000 rpm
b) less than 1500 rpm
c) less than 3000 rpm
d) zero rpm
ans. a

6. The stator of a 3-phase , 4-pole , slip ring induction motor is fed from 50 Hz source and its rotor from 30 Hz source. The motor will run at
a) 1500 rpm
b) 900 rpm
c) zero rpm
d) 2400 rpm
ans. d

7. The capacitor start motor uses
a) paper capacitor
b) electrolytic capacitor
c) air capacitor
d) none of the above
ans. b

8. The value of capacitor in a capacitor start motor controls the
a) starting value
b) speed of the motor
c) efficiency
d) none of the above
ans. a

9. Under no load conditions, the speed of a universal motor is limited by
a) supply frequency
b) supply voltage
c) windage and friction
d) none of the above
ans. c

10. Which of the following motors does not use a centrifugal switch
a) split phase motor
b) capacitor start capacitor run motor
c) shaded pole motor
d) none of the above
ans. c

INDUCTION MOTORS 8

1. In an induction motor, the value of average flux density in air gap should be small

a) to get good pf

b) to get poor pf

c) to achieve good efficiency

d) for minimum cost

ans. a

2. An increase in the value of air gap flux density in an induction motor

a) increase iron-loss

b) increase efficiency

c) decrease efficiency

d) both a and c are correct

ans. d

3. The pulsation losses and noise can be reduced in an induction motor by using 

a) a large number of narrow slots

b) less number of narrow slots

c) large number of semi slots

d) none of these 

ans. a

4. With increase in length of the air gap in an induction motor, the pulsation losses becomes

a) more 

b) less

c) uncharged

d) none of these 

ans. b

5.  The large number of slots in an induction motor has

a) high over load capacity

b) low over load capacity

c) no effect no overload capacity

d) none of these 

ans. a

6.A short air gap in an induction motor is necessary to achieve

a) to increased overload capacity

b) to reduce pulsation loss

c) to reduce the noise

d) a good pf

ans. d

7. The large value of air gap length in an induction motor results in 

a) to increased overload capacity

b) to reduce pulsation loss

c) providing better cooling

d) reducing the noise

ans. a

8. In an induction motor , the motor fails to start when number of rotor slots is equal to number of stator slots this is known as

a) crawling

b) cogging

c) synchronous cusps

d) none of these

ans. b

9. The simplest way to eliminate the harmonic induction torques is

a) integral slot winding

b) skewing 

c) chording

d) none of these

ans. c

10. The motor noise, vibrations, cogging and synchronous cusps can be reduced by

a) chording

b) integral slot winding

c) skewing

d) increasing the air gap length

ans. c

INDUCTION MOTORS 7

1. Imbalance in shaft of the induction motor occurs due to
a) air gap is not uniform
b) overheating of the winding
c) rugged construction
d) slip rings.
ans. a

2. The noise and tooth pulsation losses can be minimized by using
a) large number of open slots in stator
b) small number of open slots in stator
c) large number of narrow slots in stator
d) small number of narrow slots in stator
ans. c

3. Dispersion coefficient σ is the ratio of
a) magnetizing current to supply voltage
b) magnetizing current to ideal short circuit current
c) open circuit voltage to short circuit for the same excitation
d) none of these
ans. b

4. If two phases open accidentally of 3-phase supply of a running 3-phase induction motor, the motor will
a) stop
b) run at high speed
c) continue to run depending on load
d) none of these
ans. c

5. If 3-phase induction motor is switched on with one phase disconnected, it
a) is likely to burn out
b) will run at very low speed
c) will not run at all
d) none of these
ans. a

6. In squirrel-cage induction motor, if the rotor is open, the rotor
a) will not run
b) will make noise
c) will run at very slow speed
d) will run at very high speed
ans. a

7. If the input voltage and the frequency of supply is halved, to the 3-phase induction motor
a) the maximum torque is halved
b) the maximum torque remains the same
c) the air-gap flux is halved
d) none of these
ans. a

8. An induction motor crawls at lower speed. This is due to
a) skewing
b) harmonic induction torques
c) high secondary reactance
d) low secondary reactance
ans. b

9. Semi-closed stator slot and not open slot is used in induction motor
a) to increase the pull out torque
b) to decrease the slot reactance
c) to decrease magnetizing current
d) both a and b are correct
ans. c

10. In an induction motor, skew of the rotor bar reduces
a) noise
b) vibration
c) synchronous cusps
d) all of the these
ans. d

INDUCTION MOTORS 6

1. In an induction motor, the ratio of torque exerted at 4% slip to maximum torque, when slip at maximum torque is 12% will be
a) 3/5
b) 5/3
c) 1/3
d) none of these
ans. a

2. In an induction motor the rotor resistance is equal to stand-still reactance then the maximum torque is
a) less than starting torque
b) equal to starting torque
c) more than starting torque
d) none of these
ans. b

3. The complete circle diagram of an induction motor can be drawn with the help of data found from
a) no-load test
b) blocked rotor test
c) a and b
d) a, b and stator resistance test
ans. d

4. The power scale of circle diagram of an induction motor can be found from
a) no-load test only
b) short-circuit test only
c) stator resistance test
d) none of these
ans. b

5. Which of the following is not determined by circle diagram?
a) output
b) efficiency
c) power factor
d) frequency
ans. d

6. The torque of an induction motor in synchronous watt is equal to
a) rotor output
b) stator output
c) stator input
d) watts at rotor speeds
ans. b

7. The shape of torque/slip curve of an induction motor is
a) hyperbola
b) parabola
c) straight line
d) rectangular parabola
ans. d

8. When the frequency of the rotor of an induction motor is small it can be measured by
a) galvanometer
b) dc moving coil millivoltmeter
c) dc moving coil ammeter
d) ac voltmeter
ans. b

9. The stator frame in an induction motor is used
a) as a return path for the flux
b) to hold the armature stamping/stator
c) to protect the whole machine
d) to ventilate the armature
ans. a

10. The speed of a 3-phase cage rotor induction motor depends on
a) frequency of the supply only
b) number of poles only
c) number of poles and frequency of the supply
d) input voltage
ans. c 

INDUCTION MOTORS 5

1. Which of the following motors is used most frequently ?
a) DC shunt motor
b) AC induction motor
c) 3-phase commutator motor
d) 3-phase induction motor
ans. d

2. What is the advantage of the slip ring induction motor over the squirrel cage induction motor?
a) It is suitable for higher speeds
b) Its efficiency is higher
c) Its power factor is higher
d) It can be started with the help of rotor resistances
ans. d

3. The starting torque of the slip ring induction motor can be increased by
a) adding resistance to the rotor
b) adding resistance to the stator
c) adding resistance to both
d) none of these
ans.

4. What will happen to rotor copper losses of the frequency of the input to an induction motor increases?
a) Increases
b) Decreases
c) Remains same
d) None of these
ans. a

5. In an induction motor, if the phase sequence of applied voltage is changed
a) no back emf will be developed in rotor
b) no torque will be developed by rotor
c) speed will drop to zero
d) direction of rotation of motor will change
ans. d

6. If a rotor of induction motor could be theoretically rotated at synchronous speed, the voltage that would be generated in the rotor is
a) zero
b) equal to applied voltage
c) more than applied voltage
d) none of these
ans. a

7. In an induction motor, if the rotor is locked, then the rotor frequency of the induction motor will be
a) more than the supply frequency
b) zero
c) equal to the supply frequency
d) less than the supply frequency
ans. c

8. When measuring the no load input to an induction motor, using two wattmeter method, it is necessary to subtract wattmeter readings , because the  no load power factor is
a) lagging
b) less than 0.5
c) unity
d) zero
ans. b

9. The purpose of skewing of rotor slots in induction motor is
a) to reduce the magnetic hum of motor
b) to increase the distribution factor
c) to reduce the locking tendency of rotor
d) to increase the breadth factor
ans. c

10. If the speed is 460 rpm then the number of poles of a 50 Hz induction motor are
a) 4
b) 8
c) 12
d) 18
ans. c

DC MACHINES 4

31. The power stated on the name plate of any motor is always
a) the output power at the shaft.
b) the power drawn in kVA.
c) the power drawn in kW.
d) the gross power.
ans. a

32. The exciting coils of any d.c. machine are wound
a) in the machine
b) in the armature slots
c) around the poles
d) separately
ans. c

33. A starter is necessary to start a d.c. motor because
a) it starts the motor.
b) it limits the speed of the motor.
c) it limits the back emf to a safe value.
d) it limits the starting current to a safe value.
ans. d

34. The efficiency of any electrical machine will be high , if
a) its losses are minimum.
b) its losses are more.
c) voltage applied is constant.
d) the current drawn is minimum.
ans. a

35. The direction of rotation of a d.c. shunt motor is reversed by
a) reversing armature connections.
b) interchanging the armature and field connection.
c) adding resistance to the field circuit.
d) reversing supply connections.
ans. a

36. The armature of d.c.motors is laminated
a) to reduce the hysteresis losses.
b) to reduce the eddy current losses.
c) to reduce the inductivity of armature.
d) to reduce the mass of the armature.
ans. b

37. Which of the following is a correct statement about a series motor?
a) Its field winding consists of thicker wire and less turns.
b) It can run easily without load.
c) It has almost constant speed.
d) It has poor torque.
ans. a

38. Which of the following d.c. motors has the least drop in the speed between no load to nominal load?
a) Shunt motor with commutating poles.
b) Series motor without commutating poles.
c) Compound motor without commutating poles.
d) Series motor with commutating poles.
ans. a

39. What happens if the field winding of a running shunt motor suddenly breaks open?
a) Its speed slows down
b) Its speed becomes dangerously high.
c) It gives out sparks.
d) It stops at once.
ans. b

40. The speed of a series motor at no load is
a) zero
b) 3000 rpm.
c) 3600 rpm.
d) infinity.
ans. d

DC MACHINES 3

21. The function of a commutator in a d.c. generator is
a) to collect current from conductors.
b) to change d.c. to a.c.
c) to conduct the current to the brushes.
d) to change a.c. to d.c.
ans. d

22. The current flowing in the conductors of a d.c. motor is
a) a.c.
b) d.c.
c) a.c as well as d.c.
d) transients.
ans. a

23. As the load is increased the speed of a d.c. shunt motor will
a) remain constant.
b) increase proportionately.
c) increase slightly.
d) reduce slightly.
ans. d

24. If the current in the armature of d.c. series motor is reduced to 5% , the torque of the motor will become
a) 50% of the previous value.
b) 25% of the previous value.
c) 150% of the previous value.
d) 125% of the previous value.
ans. b

25. In d.c. shunt motor, if the terminal voltage is reduced to half and torque remains the same. then,
a) speed will be half and armature current also will be half.
b) speed will be half but armature current remains the same.
c) speed will be half and armature current becomes double.
d) speed and armature current will remain the same.
ans. c

26. One d.c.motor drives another d.c.motor . The second d.c. motor when excited and driven,it
a) runs
b) does not run as generator.
c) runs as a generator
d) also runs as a motor.
ans. c

27. A 4-pole lap-wound armature has 480 conductors and a flux per pole of 25 m Wb. The emf generated, when running at 600 rpm, will be
a) 240 V
b) 120 V
c) 60 V
d) 30 V
ans. b

28. The torque of a motor is
a) force is N-m acting on the rotor.
b) the product of tangential force on the motor and its radius.
c) the electrical power in kW.
d) the power given to load being driven by the motor
ans. b

29. The armature current drawn by any d.c. motor is proportional to
a) the voltage applied to the motor.
b) the torque required .
c) the flux required in the motor.
d) the speed of the motor.
ans.

30. The output power of any electrical motor is taken from
a) the armature
b) the coupling mounted on the shaft
c) the conductors.
d) the poles.
ans. b

DC MACHINES 2

11.  In a d.c. motor the iron losses occur in
a) the yoke
b) the armature
c) the field.
d) None of these.
ans. a

12. The dummy coil in d.c. machines used to
a) eliminate reactance voltage.
b) eliminate armature reaction.
c) bring about mechanical balance of armature.
d) eliminate harmonics developed in the machine.
ans. c

13. A simple method of increasing the voltage of an d.c. generator is
a) to decrease the air gap flux density
b) to increase the speed of rotation.
c) to decrease the speed of rotation.
d) to increase the length of the armature.
ans. b

14. The function of starter in a d.c. machine is
a) to avoid the excessive current at starting
b) to control the speed.
c) to avoid armature reaction.
d) to increase the length of the armature.
ans. b

15. An external resistance is added in the series with the field of a d.c. shunt motor. When the motor runs, the effect of resistance is
a) to reduce the speed of the motor
b) to increase the speed of the motor.
c) to reduce the armature current drawn by the motor
d) to reduce the losses.
ans. b

16. An external resistance is added in the field of a d.c. shunt generator will
a) increase the speed of the generator.
b) increase the voltage of the generator.
c) decrease the voltage of the generator.
d) increase the power delivered.
ans. c

17. The speed of a d.c. shunt motor is required to be more than F.L. speed. This is possible by
a) increasing the armature current.
b) decreasing the armature current.
c) increasing the excitation current.
d) reducing the field current.
ans. d

18. An emf is induced in the windings of an armature of a d.c. generator when the armature rotates in
a) alternating magnetic flux.
b) magnetic field.
c) electrostatic field.
d) electromagnetic flux.
ans. b

19. If the speed of  d.c. generator is increased, the generated emf
a) increases
b) decreases
c) remains constant
d) decreases and then increases.
ans. a

20. If the speed of a d.c. shunt motor increases, the back emf
a) increases
b) decreases
c) remains constant
d) first decreases and then increases.
ans. a

DC MACHINES

1. What is the effect produced by the electric current in an electric motor?
a) magnetic effect only.
b) magnetic as well as heating effect.
c) heating effect only.
d) heating as well as chemical effect.
ans. b

2. The rotating part of a d.c. motors is known as
a) pole
b) stator
c) armature
d) carbon brush.
ans. c

3. Why is the airgap between the yoke and armature of an electric motor kept smaller?
a) To achieve a stronger magnetic field.
b) To avoid overheating of the machine.
c) To make station easier.
d) None of these.
ans. a

4. The function of the commutator in a d.c. machine is
a) to change alternating current to direct current.
b) to improve commutation.
c) for easy speed control.
d) to change alternating voltage to direct voltage.
ans. d

5. Carbon brushes are used in electric motors to
a) brush off carbon deposits on the commutator.
b) provide a path for flow of current.
c) prevent overheating of armature windings.
d) prevent sparking during commutation.
ans. b

6. D.C. shunt generator has
a) slightly drooping characteristics.
b) appreciably rising characteristics.
c) constant voltage characteristics.
d) appreciably falling characteristics.
ans. a

7. which one of the following is used for charging batteries?
a) compound generator.
b) shunt generator.
c) series generator.
d) tacho generator.
ans. b

8. The speed of a d.c. motor is
a) directly proportional to back emf and inversely proportional to flux.
b) inversely proportional to back emf and directly proportional to flux.
c) directly proportional to emf as well as flux.
d) inversely proportional to emf as well as flux.
ans. a

9. In any d.c. generator the emf generated in the armature is maximum when
a) rate of change of flux linked is minimum.
b) rate of change of flux linked is maximum.
c) flux linked with conductor is maximum.
d) flux linked with conductor is minimum.
ans. b

10. The highest speed attained by a d.c. shunt motor is
a) equal to infinity at rated flux.
b) higher than no load speed at rated flux.
c) equal to no load speed at rated flux.
d) lower than no load speed at rated flux.
ans. c