301 MCQ Questions on Induction Motors

1. The normal 3-phase slip-ring induction motor, the developed starting torque will

(a) Increase with the increase in the value of the rotor resistance.
(b) Decrease with the increase in the value of rotor resistance.
(c) Not depend upon the value of the rotor resistance.
(d) Increase with increase in the value of the rotor resistance upto a certain value and then decrease with further increase in rotor resistance.

2. A 3-phase induction machine operates on 3-phase fixed frequency ac mains at a per unit slip of -1.5 p.u.

Consider the following statements regarding the operating condition of the machine.

1. It draws electric power from the mains.
2. It draws mechanical power through the shaft.
3. It delivers electrical power to the mains.
4. It delivers mechanical power through the shaft.

Of these statements:
(a) 1 and 2 are correct
(b) 1 and 4 are correct
(c) 2 and 3 are correct
(d) 3 and 4 correct

3. The magnetising current component of the no load current of an induction motor is much larger than a corresponding transformer because of

(a) Additional friction and windage loss in motor.
(b) Different winding configurations on stator and rotor.
(c) Increased flux requirement.
(d) An air gap in the magnetic circuit.

4. If the load on the squirrel cage induction motor, operating on a constant voltage, constant frequency ac supply is increased

(a) Power factor angle will increase and slip will decrease.
(b) Power factor angle will decrease and slip will increase.
(c) Both power factor angle and slip will decrease.
(d) Both power factor angle and slip will increase.

5. A 3-phase, 4-pole, slip-ring induction motor is supplied with appropriate voltage at 50Hz to its stator and voltage at 10Hz to its rotor through slip-rings. The phase sequence of both the voltage is same. The motor will produce steady torque at a speed of

(a) 1500 rpm
(b) 1200 rpm
(c) 300 rpm
(d) zero

6. Consider the following statements about the operation of a three-phase induction motor running at full load:

If one of the supply line fuse blows off, the motor will

1. Stall.
2. Continue to run with increased slip.
3. Continue to run with normal supply current.
4. Continue to run with excessive supply current.

Of these statements
(a) 1 alone is correct
(b) 3 alone is correct
(c) 2 and 3 are correct
(d) 2 and 4 are correct

7. The phenomenon of crawling is usually not found in slip-ring because these motors have

(a) No space harmonics.
(b) High resistance rotor windings.
(c) No zig-zag leakage.
(d) Comparatively large air-gap.

8. Consider the following statements:

As a 3-phase induction motor is loaded from no-load to rated load,

1. There is an improvement in the power factor.
2. The torque increases almost in proportion to slip.
3. The air-gap flux falls sharply.

Of these statements:
(a) 1, 2 and 3 are correct
(b) 1 and 2 are correct
(c) 2 and 3 are correct
(d) 1 and 3 are correct

9. A 3-phase induction motor requires a starter

(a) Because there is no back emf induced in the stator winding during starting.
(b) Because the motor does not possess an inherent starting torque.
(c) Because the induced secondary voltage and hence the currents in the windings are abnormally high during starting.
(d) To increase the starting torque.

10. For a rotor-fed 3-phase induction motor, the absolute speed of the rotating magnetic field in space is

(a) The synchronous speed.
(b) The difference between the synchronous speed and the rotor speed.
(c) The rotor speed.
(d) Zero

11. Consider the following types of drive

1. Slip energy recovery schemes with converter cascade in the rotor circuit.
2. Stator voltage control using ac voltage controller.
3. Constant V/f control (V = applied voltage, f=frequency). Variable voltage and variable frequency schemes with air-gap flux constant.

Those suitable for blowers and centrifugal pumps (T α N²), include
(a) 1, 3
(b) 1, 2
(c) 1
(d) 2, 3

12. A 3-phase induction motor has a full-load slip of 3 percent at normal voltage. Which one of the following will be the value of the slip of the motor if it develops the same torque theoretically while operating at 110% of its normal voltage?

(a) 2.5%
(b) 0.25%
(c) 0.5%
(d) 5.0%

13. A 6-pole, 3-phase, 60 Hz induction motor runs at 1000 rpm developing maximum torque. Rotor resistance per phase is 1.2 ohms. Neglecting stator impedance, then for developing maximum starting torque, the external resistance to be connected in series with each rotor phase will be

(a) 7.2 ohms
(b) 6 ohms
(c) 1.44 ohms
(d) 1.2 ohms

14. If a 3-phase induction motor and a 1-phase induction motor of the same kW rating and same winding voltage are compared, then

(a) Both will be of same size and efficiency.
(b) The 3-phase induction motor will be bigger in size and less efficient.
(c) The 3-phase induction motor will be smaller in size and more efficient.
(d) The 3-phase induction motor will be bigger in size and more efficient.

15. In the slip power recovery method of speed control of a 3-phase wound rotor induction motor, to operate it in the sub synchronous region of speed, a voltage has to be injected across the rotor terminals

(a) In phase with the rotor induced emf.
(b) In phase opposition to the rotor induced emf.
(c) Either (a) and (b)
(d) Lagging the rotor induced emf by 90°

16. A 3-phase induction motor develops a torque “T” when driven from a balanced 3-phase supply. The terminal voltage and frequency are halved so that the air-gap flux remains the same while the load torque is kept constant. Then the slip speed

(a) Is reduced to half.
(b) Remains same as the previous value.
(c) Speed is reduced to half.
(d) Speed remains the same.

17. Consider the following statements:

A ceiling fan fails to start because

1. Main winding is open-circuited.
2. Auxiliary winding is open-circuited.
3. Capacitor is short-circuited.
4. Supply terminals are reversed.
5. Main and auxiliary winding connections are interchanged.

Which of these statements are correct?
(a) 1, 2 and 5
(b) 2, 3, 4 and 5
(c) 1, 2 and 3
(d) 1, 3 and 4

18. In a 3-phase induction motor, if ‘P_g’ is the power transferred across the air gap and ‘s’ is the slip, then ratio of rotor copper loss to the mechanical power developed is given by

(a) s/(1-s)
(c) s
(b) 1/s
(d) (1-s)/s

19. The following performance characteristics are attributed to a 3-phase cage induction motor without skew:

1. Lower magnetizing current compared to a slip ring motor.
2. Low efficiency.
3. High p.f.
4. Possible cogging and crawling.

Which of these statements are correct?
(a) 1, 2 and 3
(b) 2, 3 and 4
(c) 1, 3 and 4
(d) 1, 2 and 4

20. The power input to an induction motor is 50 kW when it is running at 4% slip. If the stator resistance and core losses are neglected, the rotor copper loss is equal to

(a) 38 kW
(b) 20 kW
(c) 4 kW
(d) 2 kW

21. In order to control the speed of a slip ring 3-phase induction motor through injected voltage in its rotor circuit, this voltage and the rotor voltage should essentially be

(a) In quadrature.
(b) In phase opposition.
(c) of same frequency.
(d) In same phase.

22. Stator winding of a slip-ring induction motor is fed by 3-phase ac supply at a frequency f. If the rotor speed is N_r and synchronous speed is N_s, then the absolute speed of resultant flux in space is

(a) N_s, irrespective of rotor speed N_r.
(b) (N_s+N_r) if the flux rotates in the same direction as the rotor.
(c) (N_s – N_r) if the flux rotates in the same direction as the rotor.
(d) N_r irrespective of N_s.

23. A 3-phase induction motor draws 1000 kVA at a pf of 0.8 lag. A synchronous condenser is connected in parallel to draw an additional 750 KVA to 0.6 pf lead. The pf of the total load supplied by the mains is

(a) unity
(b) 0.707 lag
(c) 0.707 lead
 (d) zero

24. An induction generator

(a) Takes reactive power from the mains and supplies active power to the mains.
(b) Takes active power from the mains and gives reactive power to the mains.
(c) Supplies both active and reactive powers to the mains.
(d) Consumes both active and reactive powers from the mains.

25. The approximate value of efficiency of a three phase induction motor running at a slip ‘s’ is given by

(a) 1/(1+s)
(b) s/(1+s)
(c) (1-s)/(1+s)
(d) 1-s

26. Consider the following statements:

The emf induced in the rotor of a 3-phase induction motor is proportional to the

1. Relative velocity between stator field and rotor conductors.
2. Voltage applied to the stator.
3. Slip.

Of these statements are correct:
(a) 1, 2 and 3
(b) 1 and 2
(d) 1 and 3
(c) 2 and 3

27. A single-phase induction motor with only the main winding excited would produce zero torque at

(a) Synchronous speed only.
(b) Starting and synchronous speed.
(c) Starting only.
(d) Starting and a speed slightly below synchronous speed.

28. A polyphase slip-ring induction machine has the stator and rotor windings wound for

(a) The same number of phase but different number of pairs of Poles.
(b) The same number of pairs of poles but different number of Phases.
(c) The same number of phases with number of pairs of poles either equal or different.
(d) The same number of pairs of poles with number of phases either equal or different.

29. The single-phase induction motor can not be started with

(a) Capacitor connection.
(b) Inductor connection.
(c) Resistor connection.
(d) Shaded pole arrangement.

30. Low voltage starting of induction motors is prescribed because

(a) High starting current at full voltage would damage the windings.
(b) It facilitates smooth acceleration.
(c) It increases the life of the motor.
(d) It is mandated by electric supply authorities to prevent voltage dips at other consumer premises.

31. Crawling of an induction motor is due to

(a) Space harmonics in the stator flux.
(b) The number of teeth on stator being equal to the number of teeth on the rotor.
(c) The fluctuation in load torque.
(d) Unequal distribution of winding.

32. Consider the following statements regarding the equivalent circuit parameters of an induction motor:

1. Leakage reactance is dependent on supply current
2. Magnetising reactance is dependent on the air-gap flux.
3. Core loss is dependent on the input voltage and frequency.
4. Rotor resistance is dependent on the speed.

Of these statements
(a) 2 and 3 are correct
(b) 1, 3 and 4 are correct
(c) 1, 2, 3 and 4 are correct
(d) 1, 2 and 4 are correct

33. In the equivalent circuit of a 3-phase induction motor, the mechanical shaft load is represented by the resistance

(a) R2
(b) (R’₂/s) + R’₂
(c) R₂/s
(d) (R’₂/s) – R’₂

34. A reduction of the voltage applied to a 3-phase induction motor

(a) Increases the breakdown torque as well as the starting torque.
(b) Increases the breakdown torque but decreases the starting torque.
(c) Decreases the breakdown torque but the starting torque remains unchanged.
(d) Decreases both breakdown and starting torques.

35. A star/delta starter of a 3-phase induction motor is equivalent to an auto transformer starter having a tapping of

(a) √3/2
(b) 1/2
(c) 1/√3
(d) √3

36. A 3-phase slip ring induction motor with negligible stator impedance operates on a balanced 3-phase ac supply, Consider the following statements in this regard:

The maximum torque developed by the motor

1. Is independent of the value of the rotor circuit resistance.
2. Is directly proportional to the square of the supply voltage.
3. Occurs at a slip whose value is independent of the value of the rotor resistance.

Of these statements are correct
(a) 1, 2 and 3
(b) 1 and 2
(c) 2 and 3
(d) 1 and 3

37. A 4-pole, 50 Hz induction motor operates at 5% slip. The frequency of e.m.f. induced in the rotor will be………….

(a) 25 Hz
(b) 50 Hz
(c) 2.5 Hz
(d) none of these

38. In FHP induction motors, splitting the phase is done using:

(a) Only capacitor
(b) Only resistance
(c) Resistance/capacitor
(d) Resistance/inductance [DMRC JE – 2017]

39. A 3-phase induction motor operates on a variable frequency, variable voltage supply such that V/f is constant. The breakdown torque of the drive will

(a) Decrease with decrease in frequency.
(b) Remain constant irrespective of the frequency.
(c) Be slightly lower at very low frequencies.
(d) Increase with decrease in frequency.

40. Consider the following statements:

The steady-state speed of a 3-phase cage induction motor delivering a load torque depends upon

1. Supply voltage
2. Supply frequency
3. Load torque speed characteristic

Of these statements are correct:
(a) 1, 2 and 3
(b) 1 and 2
(c) 2 and 3
(d) 1 and 3

41. Which one of the following 3-phase induction motor starting methods will give the highest starting torque ?

(a) Auto-transformer starter
(b) Direct on-line starting
(c) Reactor starter
(d) Star delta starter

42. In an induction motor, what will happen if air- gap is increased?

(a) Speed will reduce
(b) Efficiency will improve
(c) p.f. will be lowered
(d) Break-down torque will increase

43. A 440V, 6-Pole, 50Hz, three-phase induction motor is operating from a 440V supply and driving a grinding mill, supplying rated HP. The rpm of the motor is approximately equal to

(a) 960
(b) 1000
(c) 1440
(d) 1500

44. The stator of a 415V, 50 Hz, 6-pole, 970 rpm, 3- phase slip ring induction motor is fed with normal voltage. The rotor with slip rings open-circuited is rotated at 500 rpm along the normal direction of its rotation. The frequency of the voltage across slip rings will be

(a) 1.5 Hz
(b) 25 Hz
(c) 26.5 Hz
(d) 75 Hz

45. Vacuum cleaners use

(a) Permanent magnet motors
(b) Single-phase capacitor motor with direct-on-line start
(c) Reluctance motor
(d) Universal motor

46. Consider the following types of machines:

1. 2-phase A.C. servomotor
2. Conventional 2-phase induction motor
3. Stepper motor
4. Tachogenerator

The rotors are generally of drag cup construction in the case of
(a) 1, 2 and 4
(b) 1 and 3
(c) 3 and 4
(d) 1 and 4

47. Consider the following statements about single- phase reluctance motor:

1. The starting torque is a function of rotor position.
2. The torque developed at sub synchronous speeds varies sinusoidally.
3. Starting is asynchronous but running is synchronous.
4. It pulls synchronism on switching on de excitation.

Of these statements:
(a) 1, 2 and 3 are correct
(b) 1, 3 and 4 are correct
(c) 2, 3 and 4 are correct
(d) 1 and 4 are correct

48. Match List I with List II and select the correct answer using the codes given below the lists:

List I

1. Rolling mill drive
2. Power drill drive
3. Hoist drive

List II

1. Adjustable speed drive
2. Variable speed drive
3. Constant speed drive

(a) A-1 B-2 C-3
(c) A-3 B-1 C-2
 (b) A-2 B-3 C-1
(d) A-3 B-2 C-3

49. Consider the following statements:

Addition of resistance to the rotor of a two-phase ac servomotor will

1. Increase the overall torque.
2. Increase the stability of operation.
3. Linearized its torque-speed characteristics.
4. Lead to attainment of maximum stalling torque.

Of these statements:
(a) 1, 2 and 4 are correct
(b) 2, 3 and 4 are correct
(c) 2 and 3 are correct
(d) 1, 3 and 4 are correct

50. Which one of the following motors running under the specified conditions will have a leading power factor?

(a) Slip ring induction motor at no-load.
(b) Under excited synchronous motor.
(c) Over excited synchronous motor.
(d) Double-cage induction motor at load.