Induction Motor Question Bank

1. The circle diagram for an induction motor cannot be used to determine

(a) efficiency
(b) power factor
(c) frequency
(d) output

2. Blocked rotor test on induction motor is used to find out

(a) leakage reactance
(b) power factor on short circuit
(c) short-circuit current under rated voltage
(d) all of the above.

3. We can run an Induction motor at synchronous speed by

(a) reverseing the direction
(b) applying a higher voltage than the rated voltage
(c) injecting an emf in the rotor circuit.

4. The torque developed by a 3-phase induction motor least depends on

(a) rotor current.
(b) rotor power factor
(c) rotor E.M.F.
(d) shaft diameter

5. In an induction motor if air-gap is increased

(a) the power factor will be low
(b) windage losses will be more
(c) bearing friction will reduce
(d) copper loss will reduce

6. When R₂ is the rotor resistance. X₂, the rotor reactance at supply frequency and s the slip, then the condition for maximum torque under running conditions will be

(a) sR₂X₂ = 1
(b) sR₂ = X₂
(c) R₂ = sX₂

7. In case of a double cage
induction motor, the outer cage has

(a) high inductance and low resistance
(b) low inductance and high resistance
(c) low inductance and low resistance
(d) high resistance and low inductance

8. The low power factor of induction motor is due to

(a) rotor leakage reactance
(b) stator reactance
(c) the reactive lagging magnetizing current necessary to generate the magnetic flux
(d) all of the above

9. Insertion of reactance in the rotor circuit

(a) reduces starting torque as well as maximum torque
(b) increases starting torque as well as maximum torque
(c) increases starting torque but maximum torque remains unchanged
(d) increases starting torque but maximum torque decreases

10. Insertion of resistance in the rotor of an induction motor to develop a given torque

(a) decreases the rotor current
(b) increases the rotor current
(c) rotor current becomes zero
(d) rotor current remains same

12. To driving high inertia loads, best type of induction motor is

(a) slip-ring type
(b) squirrel cage type
(c) any of the above
(d) none of the above

13. Temperature of the stator winding of a three-phase induction motor is obtained by

(a) resistance rise method
(b) thermometer method
(c) embedded temperature method
(d) all above methods

14. In a squirrel cage motor the induced E.M.F., is

(a) dependent on the shaft loading
(b) dependent on the number of slots
(c) slip times the stand still E.M.F. induced in the rotor
(d) none of the above.

15. Less maintenance troubles are experienced in case of

(a) slip ring induction motor
(b) squirrel cage induction motor
(c) both
(d) none of the above

16. A squirrel cage induction motor is not selected when

(a) initial cost is the main consideration
(b) maintenance cost is to be kept low
(c) higher starting torque is the main consideration
(d) all above considerations are involved

17. Reduced voltage starter can be used with

(a) slip-ring motor only but not with squirrel cage induction motor
(b) squirrel cage induction motor only but not with slip-ring motor
(c) squirrel cage as well as slip-ring induction motor
(d) none of the above

18. A star-delta starter applies ……..to the induction motor.

(a) reduced voltage
(b) 57.7% of the line voltage
(c) 37.7% of the line voltage

19. The torque of an induction motor is

(a) directly proportional to slip
(b) inversely proportional to slip
(c) proportional to the square of the slip
(d) none of the above

20. Insertion of resistance in the stator of an induction motor

(a) increases the load torque
(b) decreases the starting torque
(c) increases the starting torque
(d) none of the above

21. The starting torque of a 3-phase squirrel cage induction motor is

(a) twice the full load torque
(b) 1.5 times the full load torque
(c) equal to full load torque

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