Induction Motor Objective Type Questions

20. In an induction motor, skewing of rotor bars reduces

(a) eddy currents and copper requirement.
(b) noise, vibrations and synchronous cusps.
(c) both (a) and (b).
(d) none of these.

21. Skew of rotor bar eliminates

(a) the effect of space harmonics.
(b) the entire effect of crawling.
(c) magnetic noise.
(d) vibration due to unequal force developed on rotor.

22. Which of the following statements regarding skewing of rotor bars in a squirrel cage induction motor are correct

1. It prevents cogging.
2. It produces more uniform torque.
3. It increases starting torque.
4. It reduces motor ‘hum’ during its operation.

Select the answer using the codes given below:
(a) 2, 3, 4.
(b) 1, 2, 3.
(c) 1, 3, 4.
(d) 1, 2, 4.

23. Jogging of an induction motor is

(a) energizing a motor once or repeatdly to have small movements for mechanisms.
(b) a process of synchronisation.
(c) a method of braking.
(d) none of the above.

24. In 3-phase induction motors, sometimes copper bars are placed deep in the rotor. It is done in order to improve

(a) efficiency.
(b) starting torque.
(c) power factor.
(d) none of these.

25. In a squirrel cage induction motor high starting torque is achieved by using (a) high resistance in series with the rotor circuit.

(b) low resistance across the rotor circuit.
(c) double cage rotor.
(d) none of these.

26. The outer cage of a double squirrel rotor consists of

(a) manganese brass.
(b) red copper.
(c) bronze.
(d) steel.

27. In double cage induction motor the inner cage has

(a) high resistance and high leakage reactance.
(b) high resistance and low leakage reactance.
(c) low resistance and high leakage reactance.
(d) low resistance and low leakage reactance.

28. In the rotor equivalent circuit of a double-cage induction motor, the two rotor cages can be considered

(a) to be in parallel.
(b) to be in se series-parallel.
(c) to be in series.
(d) to be in parallel with stator.

29. The advantage of the double squirrel-cage induction motor over single cage rotor is that its

(a) efficiency is higher.
(b) power factor is higher.
(c) slip is larger.
(d) starting current is lower. [I.E.S. E.E.-11. 2001]

30. In a double squirrel cage induction motor, the resistance of upper cage is ……….. that of inner cage.

(a) equal to
(b) 4 to 5 times of
(c) one-fourth of
(d) none of these.

31. Motor A has-shallow and wider slots. Motor B has deeper and narrow slots. If both are 3-phase 400 V, 50 Hz, 1440 rpm induction motors. It can be concluded that

(a) Motor A has more starting torque as compared to motor B.
(b) Motor B has more starting torque as compare to motor A.
(c) Motor A has more pull-out torque as compared to motor B.
(d) Motor B has more pull-out torque as compared to motor A. [I.E.S. E.E.-II, 1992]

32. The deep-bar rotors or double cage rotors are employed to

(a) increase starting torque.
(b) improve efficiency.
(c) increase pull-out torque.
(d) reduce rotor core loss.

33. During starting of a double cage induction motor, the current induced in the rotor

(a) flows mostly through the lower cage.
(b) flows mostly through the upper cage.
(c) is equally divided between the two windings.
(d) none of the above.

34. Squirrel cage induction motors have the advantages of

(a) cheaper in initial as well as in maintenance costs.
(b) nearly constant speed, high overload capacity, simple starting arrangement and high power factor.
(c) lower copper losses and higher operation efficiency.
(d) all of the above.

35. Squirrel cage induction motors have the disadvantages of

(a) higher starting current and poor starting torque.
(b) high sensitivity to fluctuations in supply voltage and low power factor at light-load.
(c) no possibility of speed regulation.
(d) all of the above.

36. Slip-ring induction motors have the advantages of

(a) high starting torque and high overload capacity.
(b) nearly constant speed.
(c) low starting current in comparison to squirrel cage induction motor.
(d) all of the above.

37. Slip-ring motors have the disadvantages of

(a) low efficiency and low power factor in comparison to squirrel cage induction motors.
(b) low power factor at light load.
(c) sensitivity to fluctuations in supply voltage.
(d) all of the above.

38. For high starting torque, the most suitable 3-phase induction motor is ……… induction motor.

(a) slip-ring
(b) squirrel cage
(c) double cage
(d) deep bar squirrel cage

39. Which of the following ac motor is widely used ?

(a) Squirrel cage induction motor.
(b) Slip-ring induction motor.
(c) Double cage induction motor.
(d) Synchronous motor.