Induction Motor Objective Questions

1. In a 3-phase induction motor, the variable mechanical load is represented electrically by a variable ………… only.

(a) inductance
(b) capacitance
(c) resistance
(d) any of these

2. The stator referred resistance in the equivalent circuit of an induction motor, representing mechanical output is

(a) r₂/s
(b) r’₂(1/s – 1)
(c) r₂/s
(d) r₂²(1/s – 1) [A.M.LE. Sec B. Electrical. Machines 1994]

3. If the load on an induction motor is increased from no load to full load, its slip and the power factor will, respectively:

(a) decrease, decrease.
(b) decrease, increase.
(c) increase, decrease.
(d) increase, increase. [U.P.S.C. I.E.S. E.E.-II, 2006]

4. The output power of a 3-phase induction motor will be maximum when the equivalent load resistance is equal to the standstill leakage ………. of the motor.

(a) impedance, Z₁’
(b) reactance, X₁’
(c) resistance
(d) capacitance

5. The power factor of an induction motor operating at no load will have a value around

(a) 0.9 lag.
(b) 0.2 lead.
(c) 0.2 lag.
(d) 0.9 lead. [I.E.S. E.E.-II, 2010]

6. The power factor of an induction motor at full load is likely to be

(a) unity.
(b) 0.85 lead.
(c) 0.85 lag.
(d) 0.5 lag.

7. The power factor of a 3-phase induction motor is likely to be maximum when it will operate at

(a) full load.
(b) no-load.
(c) maximum slip.
(d) maximum torque.

8. The power factor of a lightly loaded induction motor is quite low because

(a) the current drawn is largely a magnetizing current due to air gap.
(b) of the current due to air gap.
(c) the current drawn is largely a magnetizing component due to laminated core.
(d) the current drawn is largely an energy component due to laminated core.

9. In a 3-phase induction motor when the load is increased from light load

(a) rotor pf increases but stator pf decreases.
(b) stator pf increases but the rotor pf decreases.
(c) both stator and rotor pf increase.
(d) both stator and rotor pf decrease.

10. An induction motor always operates on lagging power factor. This is due to

(a) stator reactance.
(b) rotor leakage reactance.
(c) the large reactive lagging magnetizing current essential to produce the magnetic flux.
(d) all of the above.

11. In which of the following tests to be conducted on induction motors, reduced voltage is applied.

(a) Blocked rotor test.
(b) No-load test.
(c) Both (a) and (b).
(d) None of these.

12. Blocked rotor test of an induction motor corresponds, in case of a transformer, to

(a) full load.
(b) half-full load.
(c) no load.
(d) short-circuit operation. [A.M.I.E. Elec. Science Summer 1994]

13. No-load test on a 3-phase induction motor was conducted at different supply voltages and a plot of input power versus voltage was drawn. This curve was extrapolated to intersect the y-axis. This intersection point yields

(a) core loss.
(c) stray load loss.
(b) stator copper loss.
(d) friction and windage loss. [GATE E.E. 2003]

14. What is the shunt resistance component in equivalent circuit obtained by no-load test of an induction motor representative of?

(a) Windage and frictional losses only.
(b) Core losses only.
(c) Core, windage and frictional losses.
(d) Copper losses. [I.E.S. E.E.-11, 2005; GATE E.E. 2003]

15. Under no-load condition, if the applied voltage to an induction motor is reduced from the rated voltage to half the rated value.

(a) the speed decreases and the stator current increases.
(b) both the speed and the stator current decrease.
(c) the speed and the stator current remain practically constant.
(d) there is negligible change in the speed but the stator current decreases. [GATE E.E. 2005]

16. The core losses, and friction and windage losses in case of an induction motor are determined from the ………. test.

(a) no-load
(b) blocked rotor
(c) load
(d) stator resistance

17. Short-circuit test is performed on an induction motor to determine

(a) short-circuit current under rated voltage.
(b) equivalent resistance and reactance.
(c) transformation ratio.
(d) power factor on short-circuit.
(e) all of the above.

18. Circle diagram is employed to determine the performance of a/an

(a) synchronous motor.
(b) induction motor.
(c) dc motor.
(d) transformer.

19. The power scale of circle diagram of an induction motor is determined from ……. test data only.

(a) open circuit
(b) stator resistance
(c) short circuit
(d) slip

20. In the circle diagram of an induction motor the diameter of the circle represents the

(a) rotor current.
(b) line voltage.
(c) operating torque.
(d) maximum torque.