Induction Motor Objective Type Questions

1. Presence of 5th harmonics in induction motor causes

(a) cogging.
(b) crawling.
(c) small reverse braking torque.
(d) hunting.

2. The cogging occurs in induction motors due to

(a) harmonic induction torques.
(b) harmonic synchronous torques.
(c) vibration torques.
(d) both (a) and (b).

3. Cogging of induction motors occurs at

(a) high voltage and when the number of stator teeth and rotor teeth are equal.
(b) high voltage and when the number of stator teeth and rotor teeth are not equal.
(c) low voltage and when the number of stator teeth and rotor teeth are equal.
(d) low voltage and when the number of stator teeth and rotor teeth are not equal.

4. Cogging and crawling are phenomenon associated with

(a) cage induction machines and they are essentially the same.
(b) squirrel cage induction machines, the former during starting and the latter at a fraction of its rated speed.
(c) squirrel cage induction machines the former at a fraction of its rated speed and the latter during starting.
(d) wound rotor induction machines and they are reduced by skewing, chording and distribution of windings. [U.P.S.C. I.E.S. E.E.-11, 1995]

5. In an induction motor, if the air gap is increased

(a) speed will reduce.
(b) efficiency will improve.
(c) power factor will be lowered.
(d) breakdown torque will reduce. [GATE E.E. 1996]

6. If two induction motors A and B are identical except that the air-gap of motor ‘A’ is 50% greater than that of motor ‘B’ then

(a) the no-load power factor of A will be better than that of B.
(b) the no-load power factor of A will be poorer than that of B.
(c) the core losses of A will be more than those of B.
(d) the operating flux of A will be smaller than that of B. [U.P.S.C. I.E.S. E.E.-II, 1997]

7. An increase in number of poles of an induction motor results in

(a) decrease in maximum pf.
(b) increase in maximum pf.
(c) no change in maximum pf.
(d) cannot be predicted. [A.M.I.E. Sec B. Principles of Elec. Design Winter 1993]

8. Increase in length of air gap in an induction motor causes

(a) decrease in power factor.
(b) decrease in pulsation losses.
(c) increase in ampere-turns required to produce same flux density in the air gap.
(d) all of the above.

9. An induction motor with larger number of slots has ………… overload capacity.

(a) low
(b) large
(c) no effect on
(d) none of these.

10. The 3-phase induction motor provided with open slots has

(a) reduced leakage reactance.
(b) increased starting current, starting torque and breakdown torque.
(c) better pf.
(d) improved efficiency.
(e) both (a) and (b).

11. Semi-closed or totally closed slots are used in induction motors essentially to

(a) improve pull-out torque.
(b) increase pull-out torque.
(c) increase efficiency.
(d) reduce magnetizing current and improve power factor. [U.P.S.C. I.E.S. E.E.-II, 1994]

12. The stator of a small (up to 5 hp) induction motor is provided with

(a) open slots with parallel teeth.
(b) open slots with tapered teeth.
(c) semi-closed slots with parallel teeth.
(d) totally closed slots with parallel teeth.

13. “Cogging” in induction motor occurs when

(a) number of stator teeth — number of rotor teeth = odd number.
(b) number of stator teeth — number of rotor teeth = even number.
(c) number of stator teeth — number of rotor teeth = zero.
(d) number of stator teeth — number of rotor teeth = negative number.

14. To avoid cogging in a squirrel cage induction motor the following stator slot (Z₁) and rotor slot (Z₂) combination must be avoided

(a) Z₁ = Z₂
(b) Z₁ – Z₂ = 2P + 1
(c) Z₁ – Z₂ = 3P + 1
(d) Z₁ – Z₂ = 3P – 1 [A.M.LE. Sec B. Elec. Machine Design Summer 1997]

15. In a 3-phase induction motor, the stator slots are

(a) equal to rotor slots.
(b) exact multiple of rotor slots.
(c) not exact multiple of rotor slots.
(d) none of the above.

16. In a 3-phase induction motor, the number of slots on stator is not kept an exact multiple of the number of rotor slots because it

(a) facilitates cooling.
(b) avoids magnetic locking between stator field and rotor.
(c) improves efficiency.
(d) improves pf.

17. The difference between the number of stator slots and that of rotor slots in an induction motor should not be equal to P, 2P or 5P. It is essential in order to avoid

(a) synchronous cusps.
(b) crawling.
(c) magnetic locking.
(d) noise and vibrations.

18. In induction machines, it is usually a standard practice to employ

(a) integral-slot winding with full pitch coils.
(b) integral slots winding with chorded coils.
(c) fractional slot winding with fractional pitch coils.
(d) fractional slot winding with full pitch coils.

19. The rotor slots in a 3-phase induction motor are kept inclined. This phenomenon is known as

(a) skewing.
(b) crawling.
(c) cogging.
(d) none of these.