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

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

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

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

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 %

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

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

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

7. Slip rings are usually made of

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

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

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

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

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

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%

12. The shaft of an induction motor is made of

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

13. The shaft of an induction motor is

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

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

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

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

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

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

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°

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

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

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