3 Phase Induction Motor Multiple Choice Questions And Answers In English
1. In an induction motor, no-load the slip is generally
(1) less than 1%
(2) 1.5%
(3) 2%
(4) 4%
Ans: (1)
2. Slip ring motor is recommended where
(1) speed control is required
(2) frequent starting, stopping and reversing is required
(3) high starting torque is needed
(4) all above features are required
3. If a 3-phase supply is given to the stator and rotor is short
circuited rotor will move
(1) in the opposite direction as the direction of the rotating
field
(2) in the same direction as the direction of the field
(3) in any direction depending upon phase squence of supply
(4) None of the above
Ans: (2)
(1) its power factor goes on decreasing
(2) its power factor remains constant
(3) its power factor goes on increasing even after full load
(4) its power factor goes on increasing up to full load and then
it falls again
Ans: (4)
5. In squirrel cage induction motors, the rotor slots are usually
given slight skew in order to
(1) reduce windage losses
(2) reduce eddy currents
(3) reduce accumulation of dirt and dust
(4) reduce magnetic hum
Ans: (4)
6. Slip rings are usually made of
(1) copper
(2) carbon
(3) phosphor bronze
(4) aluminium
Ans: (3)
7. A 3-phase 440 V, 50 Hz induction motor has 4% slip. The
frequency of rotor e.m.f. will be
(1) 200 Hz
(2) 50 Hz
(3) 2 Hz
(4) 0.2 Hz
Ans: (3)
8. In Ns is the synchronous speed and s the slip, then actual
running speed of an induction motor will be
(1) Ns
(2) s.N,
(3) (l-s)Ns
(4) (Ns-l)s
Ans: (3)
9. In case the air gap in an induction motor is increased
(1) the magnetising current of the rotor will decrease
(2) the power factor will decrease
(3) speed of motor will increase
(4) the windage losses will increase
Ans: (2)
10. The efficiency of an induction motor can be expected to be
nearly
(1) 60 to 90%
(2) 80 to 90%
(3) 95 to 98%
(4) 99%
Ans: (2)
11. The number of slip rings on a squirrel cage induction motor is
usually
(1) two
(2) three
(3) four
(4) none
Ans: (4)
12. Star-delta starting of motors is not possible in case of
(1) single phase motors
(2) variable speed motors
(3) low horse power motors
(4) high speed motors
Ans: (1)
13. The term ‘cogging’ is associated with
(1) three phase transformers
(2) compound generators
(3) D.C. series motors
(4) induction motors
Ans: (4)
(1) low
(2) negligible
(3) same as full-load torque
(4) slightly more than full-load torque
Ans: (1)
15. An induction motor with 1000 r.p.m. speed will have
(1) 8 poles
(2) 6 poles
(3) 4 poles
(4) 2 poles
Ans: (2)
16. An induction motor is identical to
(1) D.C. compound motor
(2) D.C. series motor
(3) synchronous motor
(4) asynchronous motor
Ans: (4)
17. The torque developed in the cage induction motor with auto
start is
(1) k/torque with direct switching
(2) K x torque with direct switching
(3) K2 x torque with direct switching
(4) k2/torque with direct switching
Ans: (3)
18. Rotor rheostat control method of speed control is used for
(1) squirrel-cage induction motors only
(2) slip ring induction motors only
(3) both (1) and (2)
(4) none of the above
Ans: (2)
19. It is advisable to avoid line-starting of induction motor and
use starter because
(1) motor takes five to seven times its full load current
(2) it will pick-up very high speed and may go out of step
(3) it will run in reverse direction
(4) starting torque is very high
Ans: (1)
(1) high loads
(2) low voltage supply
(3) improper design of machine
(4) harmonics developed in the motor
Ans: (4)
