Three Phase Induction Motors MCQs with Explanatory Answers

 3-Phase Induction Motors – Objective Multiple Choice Questions with Explanatory Answers

1. The induction motor is also known as ________.

1. Asynchronous motor
2. Synchronous motor
3. DC motor
4. Servo motor

2. If the frequency of 3-phase supply to the stator of 3-phase induction motor is increased, then synchronous speed is ________?

1. Increased
2. Decreased
3. Remain unchanged
4. None of the above

3. The rotating magnetic field in a three-phase induction motor is produced by ________.

1. Rotor current
2. Stator current
3. Exciter
4. Permanent magnets

4. The speed of a three-phase induction motor depends on ________.

1. Rotor resistance
2. Stator voltage
3. Supply frequency and number of poles
4. Rotor reactance

5. Slip in an induction motor is defined as ________.

1. Difference between rotor and stator current
2. Ratio of rotor speed to synchronous speed
3. Difference between synchronous and rotor speed divided by synchronous speed
4. Ratio of stator to rotor voltage

6. At synchronous speed, the slip of an induction motor is ________.

1. 0
2. 1
3. Between 0 and 1
4. Infinity

7. At no-load, the slip of an induction motor is typically ________.

1. 0.1 to 0.5
2. 0.001 to 0.03
3. 0.2 to 0.4
4. 1

8. The rotor frequency in an induction motor is given by ________.

1. Supply frequency
2. Slip × Supply frequency
3. Rotor speed / Supply frequency
4. Constant always

9. If slip is zero, the rotor induced EMF is ________.

1. Maximum
2. Zero
3. Equal to supply voltage
4. Infinite

10. The torque produced in an induction motor is proportional to ________.

1. Slip
2. Square of slip
3. Frequency
4. Synchronous speed

11. Maximum torque of an induction motor occurs when ________.

1. Slip = 0
2. Rotor resistance = Rotor reactance
3. Rotor resistance = Stator resistance
4. Slip = 1

12. Which part of a squirrel cage induction motor resembles a “rotating transformer”?

1. Stator
2. Rotor
3. Shaft
4. Bearings

13. The starting current of a squirrel cage induction motor is ________.

1. 1 to 2 times full-load current
2. 5 to 7 times full-load current
3. Equal to full-load current
4. Zero

14. In slip ring induction motors, external resistance is added to ________.

1. Stator winding
2. Rotor winding
3. Shaft
4. Air gap

15. The power transferred across the air gap is called ________.

1. Input power
2. Rotor copper loss
3. Air-gap power
4. Mechanical power

16. Mechanical power developed in rotor = ________.

1. Air-gap power – Rotor copper loss
2. Air-gap power + Stator copper loss
3. Rotor copper loss only
4. None

17. If supply frequency increases, synchronous speed ________.

1. Increases
2. Decreases
3. Remains constant
4. Becomes zero

18. Which of the following is NOT a starting method for squirrel cage induction motors?

1. Direct-on-line (DOL)
2. Star-delta starter
3. Rotor resistance starter
4. Autotransformer starter

19. The efficiency of a three-phase induction motor is generally ________.

1. 40–50%
2. 60–70%
3. 85–95%
4. 99%

20. The relation between rotor copper loss and slip is ________.

1. Rotor copper loss = s × Rotor input
2. Rotor copper loss = (1 – s) × Rotor input
3. Rotor copper loss = s × Mechanical power
4. None

21. At full load, slip is usually ________.

1. 0–1%
2. 2–6%
3. 10–15%
4. 50%

22. The torque-speed characteristic of an induction motor is ________.

1. Linear
2. Parabolic
3. Non-linear
4. Constant

23. If rotor resistance is increased, the maximum torque ________.

1. Increases
2. Decreases
3. Remains same
4. Becomes infinite

24.The no-load current of an induction motor is about ________.

1. 1–2% of full-load current
2. 5–10% of full-load current
3. 25–40% of full-load current
4. 50–60% of full-load current

25. Which losses are present in the rotor at standstill?

1. Only copper losses
2. Only core losses
3. Copper + core losses
4. Mechanical losses

26. The starting torque of a squirrel cage induction motor is usually ________.

1. High
2. Low
3. Zero
4. Infinite

27. The crawling phenomenon in induction motor occurs due to ________.

1. Rotor resistance
2. Harmonics in supply
3. Improper rotor design
4. Supply voltage unbalance

28. Cogging in induction motor is ________.

1. Running at high speed
2. Running at synchronous speed
3. Failure to start due to magnetic locking
4. Sudden stalling

29. Which rotor has better starting torque?

1. Squirrel cage
2. Slip ring
3. Permanent magnet rotor
4. Salient pole rotor

30. The torque under blocked rotor test is proportional to ________.

1. Supply voltage
2. Square of supply voltage
3. Slip
4. Resistance

31. Rotor copper loss is maximum when slip is ________.

1. 0
2. 0.5
3. 1
4. Between 0 – 1

32. Which of the following is not a part of induction motor losses?

1. Stator copper loss
2. Rotor copper loss
3. Stray losses
4. Commutation losses

33. The frequency of rotor current at standstill is ________.

1. Zero
2. Equal to supply frequency
3. Half of supply frequency
4. Twice supply frequency

34. When the motor is running close to synchronous speed, rotor frequency is: ________.

1. Zero
2. High
3. Very low
4. Equal to stator frequency

35. Double cage induction motor is designed to improve ________.

1. Speed regulation
2. Efficiency
3. Starting torque
4. Power factor

36. Which starting method gives the lowest starting current for squirrel cage motors?

1. DOL starter
2. Star-delta starter
3. Autotransformer starter
4. None

37. Slip at maximum torque is directly proportional to ________.

1. Stator resistance
2. Rotor resistance
3. Synchronous speed
4. Stator leakage reactance

38. For constant torque load, the motor draws ________.

1. Same current at all speeds
2. More current at lower speeds
3. More current at higher speeds
4. Zero current

39. The induction motor operates in braking region when ________.

1. Slip is positive and < 1
2. Slip > 1
3. Slip is negative
4. Slip = 0

40. The rotor bars of a squirrel cage motor are usually made of ________.

1. Copper or aluminum
2. Brass
3. Steel
4. Silver

41. The stator winding of a 3-phase induction motor is similar to ________.

1. Armature of DC machine
2. Primary of transformer
3. Rotor winding
4. Exciter

42. Which factor mainly decides synchronous speed?

1. Supply voltage
2. Frequency and poles
3. Stator current
4. Rotor resistance

43. If supply frequency is halved, the synchronous speed will ________.

1. Remain constant
2. Be halved
3. Double
4. Zero

44. The main reason for using laminated cores in stator is ________.

1. Reduce copper losses
2. Reduce eddy current losses
3. Reduce hysteresis losses
4. Increase efficiency

45. The type of bearing mostly used in induction motors is ________.

1. Thrust bearing
2. Ball/roller bearing
3. Magnetic bearing
4. Hydrostatic bearing

46. Induction motor torque decreases if ________.

1. Supply voltage decreases
2. Supply voltage increases
3. Frequency decreases
4. Poles increase

47. The induced emf in the rotor is due to ________.

1. Direct supply
2. Relative speed between rotor and RMF
3. External excitation
4. Residual magnetism

48. If number of poles increases, synchronous speed ________.

1. Increases
2. Decreases
3. Remains constant
4. Becomes zero

49. The main advantage of squirrel cage motor is ________.

1. High starting torque
2. Simple and rugged construction
3. Variable speed operation
4. Easy to add external resistance

50. The ratio of starting torque to full load torque is highest in ________.

1. Squirrel cage motor
2. Slip ring motor
3. Double cage motor
4. Star-delta starting

51. Which of the following applications use slip-ring induction motors?

1. Ceiling fans
2. Pumps
3. Cranes and hoists
4. Mixers