Squirrel Cage Induction Motor MCQ

1. The torque developed in an induction motor is nearly proportional to

(a) 1/V
(b) V
(c) V²
(d) none of these.

2. If a 400 V, 50 Hz star-connected, 3-phase squirrel cage induction motor is operated from a 400 V, 75 Hz supply, the torque that the motor can now provide while drawing rated current from the supply

(a) reduces.
(b) increases.
(c) remains the same.
(d) increases or reduces depending upon the rotor resistance. [GATE E.E. 2002]

3. For a constant torque load, the supply voltage of a squirrel cage induction motor is reduced by a factor of 1/√2, its rotor current is modified by a factor

(a) 2
(b) 1/√2
(c) √2
(d) none. [A.M.I.E. Sec. B. Elec. Machines Summer 1999]

4. The starting torque of a 3-phase induction motor varies as

(a) V²
(b) V
(c) √V
(d) 1/V

5. The starting torque of a squirrel cage induction motor is

(a) very large.
(b) very low.
(c) slightly more than full-load torque.
(d) zero.

6. For achieving high starting torque and high operation efficiency an induction motor should have ……….. rotor circuit resistance at starting and ……… circuit resistance under operating condition.

(a) high, low
(b) low, high
(c) high, high
(d) low, low

7. When the applied voltage per phase is reduced to one-half, the starting torque of a three-phase squirrel cage induction motor becomes

(a) 1/2 of the initial value.
(b) 1/4 of the initial value.
(c) twice of the initial value.
(d) 4 times of the initial value. [I.E.S. E.E.11, 2006]

8. In a 3-phase induction motor, the starting torque will be maximum when

(a) R₂ = 1/X₂
(b) R₂ = X₂
(c) R₂ = X₂²
(d) R₂ = √X₂

9. in a 3-phase induction motor, the maximum, torque

(a) is independent of rotor circuit resistance.
(b) varies as rotor resistance.
(c) varies as the square of rotor resistance.
(d) varies inversely as rotor circuit resistance.

10. For a slip-ring induction motor, if the rotor resistance is increased, then

(a) starting torque and efficiency increase.
(b) starting torque decreases but efficiency increases.
(c) starting torque increases but efficiency decreases.
(d) starting torque and efficiency decrease. [I.E.S. E.E.-II, 2005]

11. An induction motor has a rotor resistance of 0.002 ohm/phase. If the resistance is increased to 0.004 ohm/phase then the maximum torque will

(a) reduce to half.
(b) increase by 100%.
(c) increase by 200%.
(d) remain unaltered. [U.P.S.C. I.E.S. 1994]

12. In an induction motor, maximum torque varies as

(a) V/X₂
(b) V²/X₂
(c) V/R₂
(d) V/X₂²

13. Breakdown torque of a 3-phase induction motor of negligible stator impedance is

(a) directly proportional to the rotor resistance.
(b) inversely proportional to the rotor resistance.
(c) directly proportional to the reactance.
(d) inversely proportional to the rotor leakage reactance. [I.E.S. E.E.-II, 1993, 2008]

14. Which one of the following statements is correct in respect of an induction motor?

(a) The maximum torque will depend on rotor resistance.
(b) Although the maximum torque does not depend on rotor resistance, yet the speed at which maximum torque is produced depends on rotor resistance.
(c) The maximum torque will not depend on standstill rotor reactance.
(d) The slip of induction motor decreases as torque increases. [I.E.S. 2003]

15. In a 3-phase induction motor, the torque developed is maximum when the rotor circuit resistance per phase is equal to

(a) rotor leakage reactance per phase at standstill.
(b) slip times the rotor leakage reactance per phase at standstill.
(c) stator resistance per phase.
(d) stator leakage reactance per phase. [ I.E.S. E.E.-II 2004]

16. A 4-pole, 50 Hz, 3-phase induction motor has blocked rotor reactance per phase which is four times the rotor resistance per phase. The speed at which maximum torque develops is

(a) 1,125 rpm.
(b) 1,500 rpm.
(c) 1,050 rpm.
(d) 1,210 rpm. [U.P.S.C. I.E.S. E.E.-II, 2006]

17. The supply voltage to an induction motor is reduced by 10%. By what percentage, approximately, will the maximum torque decrease?

(a) 50%
(b) 10%
(c) 20%
(d) 40% [I.E.S. E.E.-II, 2004]

18. If the rotor circuit resistance is increased in an induction motor, the maximum torque will occur at

(a) lower speed.
(b) high speed.
(c) the same speed.
(d) none of these.

19. Beyond the point of maximum torque any further increase in load will cause

(a) increase in torque developed and so the motor will run at a high speed.
(b) no change in the torque developed and so the motor will run at the same speed as before.
(c) decrease in torque developed and the motor will slow down.
(d) increase in current drawn from the supply mains resulting in damage to it.

20. The torque-slip characteristic of a poly-phase induction motor becomes almost linear at small values of slip, because in this range of slips

(a) the effective rotor circuit resistance is very large compared to the rotor reactance.
(b) the rotor resistance is equal to the stator resistance.
(c) the rotor resistance is equal to the rotor reactance.
(d) the rotor resistance is equal to the stator reactance. [I.E.S. E.E.-II, 1993]

21. In a 3-phase induction motor if the leakage reactance is reduced by using open slots

(a) starting torque and starting current will decrease but power factor will increase.
(b) starting torque and starting current both will increase but power factor will decrease.
(c) pull-out torque will decrease.
(d) starting current will increase but starting torque will decrease. [AMIE. Sec B. Elec. Machine Design 1996]

22. An increase in rotor circuit reactance of a 3-phase induction motor will

(a) reduce starting torque as well as maximum torque.
(b) increase starting torque as well as maximum torque.
(c) increase starting torque and reduce maximum torque.
(d) increase maximum torque and reduce starting torque.

23. The torque-slip characteristic of an induction motor is approximately a/an

(a) straight line.
(b) rectangular parabola.
(c) parabola.
(d) hyperbola.

24. Stable operation of an induction motor is

(a) between zero slip and unity/slip.
(b) between 0.5 slip and 0.95 slip.
(c) between zero slip and slip corresponding to maximum torque.
(d) between slip corresponding to maximum torque and unity slip.

25. With the increase in load on a squirrel cage induction motor

(a) stator current increases.
(b) power factor improves.
(c) slip increases.
(d) the torque developed by the motor increases till it becomes equal to the load torque.
(e) all of the above.

26. Whenever any poly-phase induction motor is loaded

(a) motor speed decreases.
(b) stator flux cuts the rotor bars more rapidly.
(c) emf induced in the rotor as well as its frequency increases.
(d) current in rotor bars increases.
(e) all of the above.

27. The losses that occur in an induction motor are

(a) stator copper loss.
(b) stator iron loss.
(c) rotor copper loss.
(d) windage and friction losses.
(e) all of the above.

28. In a 3-phase induction motor iron loss mainly occurs in

(a) stator and rotor.
(b) rotor core and rotor teeth.
(c) stator core and stator teeth.
(d) stator winding.

29. Iron loss in the rotor of a 3-phase induction motor is negligible. This is due to

(a) very low frequency of emf induced in the rotor.
(b) very low emf induced in the rotor.
(c) very low flux density in rotor parts.
(d) the fact that the rotor core is laminated.
(e) constant magnitude of flux linking the rotor core.

30. Rotor input of a three phase induction motor is equal to

(a) input to motor — stator copper and iron losses.
(b) mechanical power developed — windage and friction losses.
(c) input to motor — windage and friction losses.
(d) input to motor — stator iron losses — windage and friction losses.

31. In an induction motor, what is the ratio of rotor copper loss and rotor input ?

(a) 1/s
(b) s
(c) (1 — s)
(d) s/(1 — s) [U.P.S.C. I.E.S. 2007]

32. For an induction motor, operating at slip s, the ratio of gross power output to air gap power is equal to

(a) (1 — s)²
(b) (1 — s)
(c) √(1 — s)
(d) (1 — √s) [GATE E.E. 2005]

33. The synchronous watt is

(a) the torque which under synchronous speed would develop a power of 1 watt or power input to rotor in watts.
(b) shaft output in watts.
(c) input power to stator.
(d) the unit of ratings of synchronous machines.

34. The no-load current in an induction motor is ……….. in comparison to that of a power transformer.

(a) much lower
(b) much higher
(c) slightly less
(d) slightly more

35. The equivalent circuit per phase of a three phase transformer is similar to that of a three phase induction motor but the transformer does not develop any torque. This is due to

(a) insufficient voltage.
(b) low supply frequency.
(c) non-fulfillment of condition of space condition of winding.
(d) none of the above.

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