Model Quiz Polyphase Induction Motors Questions and Answers test2

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1. As load on an induction motor goes on increasing _____.
its power factor goes on decreasing	its power factor remains constant
its power factor goes on increasing even after full load	its power factor goes on increasing upto full load and then it falls again

2. If a 3-phase supply is given to the stator and rotor is short circuited rotor will move ______.
in the opposite direction as the direction of the rotating field	in the same direction as the direction of the field
in any direction depending upon phase squence of supply	None of the Above

3. It is advisable to avoid line starting of induction motor and use starter because ______.
it will run in reverse direction	it will pick up very high speed and may go out of step
motor takes five to seven times its fullload current	starting torque is very high

4. The speed characteristics of an induction motor closely resemble the speedload characteristics of which of the following machines
D.C. series motor	D.C. shunt motor
universal motor	none of the above

5. Which type of bearing is provided in small induction motors to support the rotor shaft ?
Ball bearings	Cast iron bearings
Bush bearings	None of the above

6. A pump induction motor is switched on to a supply 30% lower than its rated voltage. The pump runs. What will eventually happen ? It will _____.
stall after sometime	stall immediately
continue to run at lower speed without damage	get heated and subsequently get damaged

7. 5 H.P., 50-Hz, 3-phase, 440 V, induction motors are available for the following r.p.m. Which motor will be the costliest ?
730 r.p.m.	960 r.p.m.
1440 r.p.m.	2880 r.p.m.

8. A 3-phase slip ring motor has _____.
double cage rotor	wound rotor
short-circuited rotor	any of the above

9. The starting torque of a 3-phase squirrel cage induction motor is ______.
twice the full load torque	1.5 times the full load torque
equal to full load torque	None of the above

10. Short-circuit test on an induction motor cannot be used to determine
windage losses	copper losses
transformation ratio	power scale of circle diagram

11. In a three-phase induction motor _____.
iron losses in stator will be negligible as compared to that in rotor	iron losses in motor will be negligible as compared to that in rotor
iron losses in stator will be less than that in rotor	iron losses in stator will be more than that in rotor

12. In case of 3-phase induction motors, plugging means _____.
pulling the motor directly on line without a starter	locking of rotor due to harmonics
starting the motor on load which is more than the rated load	interchanging two supply phases for quick stopping

13. Which is of the following data is required to draw the circle diagram for an induction motor ?
Block rotor test only	No load test only
Block rotor test and no-load test	Block rotor test, no-load test and stator resistance test

14. In three-phase induction motors sometimes copper bars are placed deep in the rotor to _____.
improve starting torque	reduce copper losses
improve efficiency	improve power factor

15. In a three-phase induction motor
power factor at starting is high as compared to that while running	power factor at starting is low as compared to that while running
power factor at starting in the same as that while running	None of the Above

16. The voice of transformation ratio of an induction motor can be found by _____.
open-circuit test only	short-circuit test only
stator resistance test	none of the above

17. The power scale of circle diagram of an induction motor can be found from _____.
stator resistance test	no-load test only
short-circuit test only	noue of the above

18. The shape of the torque/slip curve of induction motor is _____.
parabola	hyperbola
rectangular parabola	straigth line

19. A change of 4% of supply voltage to an induction motor will produce a change of appromimately _____.
4% in the rotor torque	8% in the rotor torque
12% in the rotor torque	16% in the rotor torque

20. The stating torque of the slip ring induction motor can be increased by adding _____.
external inductance to the rotor	external resistance to the rotor
external capacitance to the rotor	both resistance and inductance to rotor