Three phase induction motor part -1

1. Which of the following component is usually fabricated out of silicon steel ?
(a) Bearings
(b) Shaft
(c) Statorcore
(d) None of the above

Ans: c

2. The frame of an induction motor is usually made of
(a) silicon steel
(b) cast iron
(c) aluminium
(d) bronze

Ans: b

3. The shaft of an induction motor is made of
(a) stiff
(b) flexible
(c) hollow
(d) any of the above

Ans: a

4. The shaft of an induction motor is made of
(a) high speed steel
(b) stainless steel
(c) carbon steel
(d) cast iron

Ans: c

5. In an induction motor, no-load the slip is generally
(a) less than 1% (b) 1.5%
(c) 2%
(d) 4%

Ans: a
 

6. In medium sized induction motors, the slip is generally around (a) 0.04%
(b) 0.4%
(c) 4%
(d) 14%

Ans: c

7. In squirrel cage induction motors, the rotor slots are usually given slight skew in order to
(a) reduce windage losses
(b) reduce eddy currents
(c) reduce accumulation of dirt and dust
(d) reduce magnetic hum

Ans: d

8. In case the air gap in an induction motor is increased
(a) the magnetising current of the rotor will decrease
(b) the power factor will decrease
(c) speed of motor will increase
(d) the windage losses will increase

Ans: b

9. Slip rings are usually made of
(a) copper
(b) carbon
(c) phospor bronze
(d) aluminium

Ans: c

10. A 3-phase 440 V, 50 Hz induction motor has 4% slip. The frequency of rotor e.m.f. will be
(a) 200 Hz
(b) 50 Hz
(c) 2 Hz
(d) 0.2 Hz

Ans: c

11. In Ns is the synchronous speed and s the slip, then actual running speed of an induction motor will be
(a) Ns
(b) s.N,
(c) (l-s)Ns
(d) (Ns-l)s

Ans: c

12.The efficiency of an induction motor can be expected to be nearly
(a) 60 to 90%
(b) 80 to 90%
(c) 95 to 98%
(d) 99%

Ans: b

13. The number of slip rings on a squirrel cage induction motor is usually
(a) two
(b) three
(c) four
(d) none

Ans: d

14. The starting torque of a squirrel-cage induction motor is
(a) low
(b) negligible
(c) same as full-load torque
(d) slightly more than full-load torque

Ans: a

15. A double squirrel-cage induction motor has
(a) two rotors moving in oppsite direction
(b) two parallel windings in stator
(c) two parallel windings in rotor
(d) two series windings in stator

Ans: c

16. Star-delta starting of motors is not possible in case of
(a) single phase motors
(b) variable speed motors
(c) low horse power motors
(d) high speed motors

Ans: a

17. The term 'cogging' is associated with
(a) three phase transformers
(b) compound generators
(c) D.C. series motors
(d) induction motors

Ans: d

18. In case of the induction motors the torque is
(a) inversely proportional to (Vslip)
(b) directly proportional to (slip)2
(c) inversely proportional to slip
(d) directly proportional to slip

Ans: d

19. An induction motor with 1000 r.p.m. speed will have
(a) 8 poles
(b) 6 poles
(c) 4 poles
(d) 2 poles

Ans: b

20. The good power factor of an induction motor can be achieved if the average flux density in the air gap is
(a) absent
(b) small
(c) large
(d) infinity

Ans: b