Three Phase Induction Motor MCQs

1. A double squirrel-cage induction motor has

  1. two rotors moving in opposite direction
  2. two parallel windings in the stator
  3. two parallel windings in the rotor
  4. two series windings in the stator

Answer
Answer. c

2. The starting torque of a squirrel-cage induction motor is

  1. low
  2. negligible
  3. same as the full-load torque
  4. slightly more than full-load torque

Answer
Answer. a

3. The number of slip rings on a squirrel cage induction motor is usually

  1. two
  2. three
  3. four
  4. none

Answer
Answer. d

4. The efficiency of a three-phase induction motor can be expected to be nearly

  1. 60 to 90 %
  2. 80 to 90 %
  3. 95 to 98 %
  4. 99 %

Answer
Answer. b

5. If Ns is the synchronous speed and s the slip, then the actual running speed of a three-phase induction motor will be

  1. Ns
  2. sN
  3. (l-s)Ns
  4. (Ns-l)s

Answer
Answer. c

6. A 3-phase 440 V, 50 Hz induction motor has 4 % slip. The frequency of rotor e.m.f. will be

  1. 200 Hz
  2. 50 Hz
  3. 2 Hz
  4. 0.2 Hz

Answer
Answer. c

7. Slip rings are usually made of

  1. copper
  2. carbon
  3. phosphor bronze
  4. aluminium

Answer
Answer. c

8. In case the air gap in a three-phase induction motor is increased

  1. the magnetizing current of the rotor will decrease
  2. the power factor will decrease
  3. the speed of the motor will increase
  4. the windage losses will increase

Answer
Answer. b

9. In squirrel cage induction motors, the rotor slots are usually given slight skew in order to

  1. reduce windage losses
  2. reduce eddy currents
  3. reduce accumulation of dirt and dust
  4. reduce magnetic hum

Answer
Answer. d

10. In medium sized induction motors, the slip is generally around

  1. 0.04 %
  2. 0.4 %
  3. 4 %
  4. 14 %

Answer
Answer. c

11. In a three-phase induction motor, at no-load the slip is generally

  1. less than 1%
  2. 1.5%
  3. 2%
  4. 4%

Answer
Answer. a

12. The shaft of an induction motor is made of

  1. High-speeded steel
  2. stainless steel
  3. carbon steel
  4. cast iron

Answer
Answer. c

13. The shaft of an induction motor is

  1. stiff
  2. flexible
  3. hollow
  4. any of the above

Answer
Answer. a

14. The frame of an induction motor is usually made of

  1. silicon steel
  2. cast iron
  3. aluminium
  4. bronze

Answer
Answer. b

15. Which of the following component is usually fabricated out of silicon steel?

  1. Bearings
  2. Shaft
  3. Stator core
  4. None of the above

Answer
Answer. c

16. A 6 pole, 50 Hz , 3-phase induction motor is running at 950 rpm. It has rotor copper loss of 5 kW. The rotor input will be

  1. 100 kW
  2. 95 kW
  3. 10 kW
  4. 5.3 kW

Answer
Answer. a

17. When N1 is the synchronous speed and N2 is the actual speed of the rotor, the slip is given by

  1. N1−N2
  2. 1−(N1/N2)
  3. 1−(N2/N1)
  4. (N2/N1)−1

Answer
Answer. c

18. In AC rotating machines the generated or speed emf

  1. leads phase by 90°
  2. lags phase by 90°
  3. is in phase with working flux
  4. lags phase by 180°

Answer
Answer. b

19. The crawling in an induction motor is caused by

  1. improper design of the machine
  2. Low voltage supply
  3. High loads
  4. Harmonics developed in the motor

Answer
Answer. d

20. Cogging term is associated with which of the following?

  1. 3-phase transformer
  2. compound motor
  3. dc series motor
  4. induction motor

Answer
Answer. d

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