Induction Motor MCQ PDF

1. The rotor of an induction motor never runs at synchronous speed, because then the relative speed between the rotating flux and rotor will be

(a) maximum and hence, torque will he maximum.
(b) maximum and hence, torque will be zero.
(c) zero and hence, torque will be maximum.
(d) zero and hence, torque will be zero. [A.M.I.E. Sec. B. Elec. Machines Summer 2004]

2. The voltage actually used for setting up of the useful flux in the air gap of 3-phase induction motor is

(a) = applied voltage.
(b) > applied voltage.
(c) < applied voltage.
(d) = rotor induced emf. [I.E.S. E.E.-II, 2006]

3. If N_s is the synchronous speed, N is the rotor speed and s is the slip then relation is

(a) N_s = (1 — s) N
(b) N = sN_s
(c) N = (s — 1) N_s
(d) None of these.

4. The maximum possible speed of a 3-phase squirrel cage induction motor running at a slip of 4% is

(a) 2,880 rpm.
(b) 3,000 rpm.
(c) 1,440 rpm.
(d) 960 rpm.

5. A three-phase 6 pole, 50 Hz, induction motor is running at 5% slip. What is the speed of the motor?

(a) 850 rpm.
(b) 900 rpm.
(c) 950 rpm.
(d) 1000 rpm. [I.E.S. E.E.-II, 2009]

6. At the instant of starting, the per unit slip of the 3-phase induction motor is

(a) 0.05
(b) 0.1
(c) 1.0
(d) 0.5

7. A balanced 3-phase induction motor runs at slip s. If ω_s is its synchronous speed, what is the relative speed between the stator mmf and rotor mmf ?

(a) sω_s
(b) (1 — s)ω_s
(c) ω_s
(d) zero [I.E.S. E.E.-II, 2008]

8. A three-phase 440 V, 6 pole, 50 Hz, squirrel cage induction motor is running at a slip of 5%. The speed of stator magnetic field with respect to rotor magnetic field and speed of rotor with respect to stator magnetic field are

(a) zero, —50 rpm
(b) zero, 950 rpm
(c) 1,000 rpm, —50 rpm
(d) 1,000 rpm, 950 rpm [GATE E.E., 2011]

9. The rotor of a three phase, 5 kW, 400 V, 50 Hz, slip ring induction motor is wound for 6 poles while its stator is wound for 4 poles. The approximate average no load steady state speed when this motor is connected to 400 V, 50 Hz supply is

(a) 1,500 rpm.
(b) 500 rpm.
(c) 0 rpm.
(d) 1,000 rpm. [GATE E.E. 2002]

10. If the full-load speed of a 6-phase, 50 Hz induction motor is 950 rpm, what is its half-load speed nearly equal to ?

(a) 1,000 rpm.
(b) 450 rpm.
(c) 1,900 rpm.
(d) 975 rpm.

11. The direction of rotation of a 3-phase induction motor is clockwise when it is supplied with 3-phase sinusoidal voltage having phase sequence A-B-C. For counterclockwise rotation of the motor, the phase sequence of the power supply should be

(a) B-C-A
(b) C-A-B
(c) A-C-B
(d) B-C-A and C-A-B [GATE E.E. 2004]

12. The mmf produced by the current of a 3-phase induction motor

(a) rotates at the speed of rotor in the air gap.
(b) is standstill with respect to stator mmf.
(c) rotates at slip speed with respect to stator mmf.
(d) rotates at synchronous speed with respect to rotor. [I.E.S. E.E.-11, 2000]

13. The mmf produced by the rotor currents of a 3-phase induction motor

(a) rotates at the speed motor in the air gap.
(b) is at standstill with respect to stator mmf.
(c) rotates at slip speed with respect to stator mmf.
(d) rotates at synchronous speed with respect to rotor. [I.E.S. E.E.-II, 2010]

14. A 6-pole, 50 Hz wound rotor induction motor when supplied at the rated voltage and frequency with slip-rings open circuited, developed a voltage of 100 V between any two slip rings. If the rotor is driven by an external means at 1,000 rpm opposite to the direction of stator field, the frequency of voltage across slip rings will be

(a) zero.
(b) 50 Hz
(c) 100 Hz
(d) 200 Hz [I.E.S. E.E.-II, 2000]

15. In an induction motor the slip will be negative when

(a) stator magnetic field and rotor rotate in opposite directions.
(b) rotor rotates at a speed less than synchronous speed and in the direction of rotation of stator field.
(c) rotor rotates at a speed more than synchronous speed and in the direction of rotation of stator field.
(d) none of the above.

16. A 3-phase induction motor is operating at slip s. If its two supply leads are interchanged, then its slip at that instant will be

(a) 2 — s
(b) 2 + s
(c) 1 + s
(d) 1 — s

17. The speed of rotating field due to rotor currents relative to rotor surface in an induction motor is

(a) N_s
(b) s N_s
(c) N
(d) s N

18. The relationship between rotor frequency f₂, slip s and the stator supply frequency f₁ is given by

(a) f₁ = sf₂
(b) f₂ = sf₁
(c) f₂ = f₁(1—s)
(d) f₂ = √(sf) [A.M.I.E. Sec B. Elec. Machines Summer 1995]

19. What is the frequency of rotor current of a 50 Hz induction motor operating at 2% slip ?

(a) 1 Hz
(b) 100 Hz
(c) 2 Hz
(d) 50 Hz [I.E.S. E.E.-II, 2007]

20. An induction motor having 8 poles runs at 727.5 rpm. If the supply frequency is 50 Hz, the emf in the rotor will have a frequency of

(a) 1.5 Hz
(b) 48.5 Hz
(c) 5.15 Hz
(d) 75 Hz [I.E.S. E.E.-II, 2003]

21. A 6-pole, 3-phase alternator running at 1,000 rpm supplies to an 8-pole, 3-phase induction motor which has a rotor current of frequency 2 Hz. The speed at which the motor operates is

(a) 1,000 rpm.
(b) 960 rpm.
(c) 750 rpm.
(d) 720 rpm. [I.E.S. E.E.-II, 2002]

22. A voltmeter gives 120 oscillations per minute when connected to the rotor of an induction motor. The frequency is 50 Hz. The slip of the motor is

(a) 2%
(b) 4%
(c) 5%
(d) 25% [I.E.S. E.E.-1-1, 1995, 2000]

23. The frequency of rotor currents at standstill is equal to

(a) zero
(b) 2f
(c) f
(d) sf

24. The rotor of a 4-pole, 50 Hz, 3-phase slip-ring induction motor runs in clockwise direction when its stator terminals 1,2,3 are connected to supply terminals A, B, C respectively. If 1,2,3, are connected to A, C, B, respectively of supply terminals and rotor runs in a clockwise direction at synchronous speed, then the frequency of induced emf across the open-circuited rotor terminals is

(a) 50 Hz
(b) zero
(c) 25 Hz
(d) 100 Hz [A.M.I.E. Elec. Science Winter 1993]

25. In a slip-ring induction motor, the frequency of rotor currents can be measured with a

(a) galvanometer.
(b) wattmeter
(c) dc moving coil milli-voltmeter.
(d) none of the above.

26. A centre zero ammeter connected in the rotor circuit of a 6-pole, 50 Hz induction motor makes 30 oscillations in one minute. The rotor speed is

(a) 970 rpm.
(b) 990 rpm.
(c) 1010 rpm.
(d) 1030 rpm. [U.P.S.C. 1995]

27. For an induction motor under operating condition, the emf induced/phase in rotor circuit is its standstill rotor induced emf per phase.

(a) equal to
(b) s times
(c) (1—s) times
(d) 1/s times.

28. The direction of rotor current produced in an induction motor can be determined by

(a) Lenz’s law.
(b) Induction law.
(c) Fleming’s right hand rule.
(d) Fleming’s left hand rule.

29. In case of an induction motor the leakage flux is more in comparison to that of a transformer. This is due to

(a) revolving rotor.
(b) air gap between rotor and stator.
(c) higher flux densities in induction motor.
(d) none of the above.

30. In an induction motor the phase reactance is ………… in comparison to phase resistance.

(a) quite high
(b) very small
(c) slightly high
(d) almost same

31. In a 3-phase induction motor reactance under running condition is less than its standstill value. This is due to reduction in

(a) rotor inductance.
(b) stator magnetic flux.
(c) frequency of rotor emf.
(d) mutual flux linking the stator and rotor.

32. In an induction motor under running condition, the rotor reactance per phase is ……….. its standstill phase reactance.

(a) s times
(b) equal to
(c) 1/s times
(d) (1 – s) times

33. If E₂ is the standstill rotor phase emf, I₂ is the standstill rotor phase current and cosφ₂ is the rotor power factor then torque developed by a 3-phase induction motor varies as

(a) E₂I₂
(b) E₂I₂ cosφ₂
(c) E₂I₂ ÷ cosφ₂
(d) E₂ ÷ I₂cosφ₂

34. The torque developed by a 3-phase induction motor is approximately proportional to

(a) √s
(b) s²
(c) s
(d) 1/s

35. Insertion of resistance in the rotor circuit of an induction motor to develop a given torque causes ………… in rotor current.

(a) almost no change
(b) increase
(c) decrease
(d) none of these

36. In an induction motor if the flux density is reduced to one-half of its normal value then the torque will

(a) reduce to one half.
(b) reduce to one-fourth.
(c) remain unchanged.
(d) increase four times.

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