1. Regarding skewing of motor bars in a squirrel cage induction motor, (SCIM) which statement is false ?

(a) it prevents cogging

(b) it increases starting torque

(c) it produces more uniform torque

(d) it reduces motor ‘hum’ during its
operation.

2. The principle of operation of a 3-phase.
Induction motor is most similar to that of a

(a) synchronous motor

(b) repulsion-start induction motor

(c) transformer with a shorted secondary

(d) capacitor-start, induction-run motor.

3. The magnetising current drawn by
transformers and induction motors is the cause of their

.........power factor.

(a) zero

(b) unity

(c) lagging

(d) leading.

4. The effect of increasing the length of
air-gap in an induction motor will be to increase the

(a) power factor

(b) speed

(c) magnetising current

(d) air-gap flux.

5. In a 3-phase induction motor, the
relative speed of stator flux with respect to ..........is zero.

(a) stator winding

(b) rotor

(c) rotor flux

(d) space.

6. An eight-pole wound rotor induction
motor operating on 60 Hz supply is driven at 1800 r.p.m. by a prime mover in
the opposite direction of revolving magnetic field. The frequency of rotor current
is

(a) 60 Hz

(b) 120 Hz

(c) 180 Hz

(d) none of the above.

7. A 3-phase, 4-pole, 50-Hz induction motor
runs at a speed of 1440 r.p.m. The rotating field

produced by the rotor rotates at a speed of
....... r.p.m... with respect to the rotor.

(a) 1500

(b) 1440

(c) 60

(d) 0.

8. In a 3-f induction motor, the rotor
field rotates at synchronous speed with respect to

(a) stator

(b) rotor

(c) stator flux

(d) none of the above.

9. Irrespective of the supply frequency, the
torque developed by a SCIM is the same whenever ........ is the same.

(a) supply voltage

(b) external load

(c) rotor resistance

(d) slip speed.

10. In the case of a 3-ö induction motor
having Ns = 1500 rpm and running with s = 0.04

(a) revolving speed of the stator flux is
space is .....rpm

(b) rotor speed is ........rpm

(c) speed of rotor flux relative to the
rotor is .......rpm

(d) speed of the rotor flux with respect to
the stator is ........rpm.

11. The number of stator poles produced in
the rotating magnetic field of a 3-ö induction motor having 3 slots per pole
per phase is

(a) 3

(b) 6

(c) 2

(d) 12

12. The power factor of a squirrel-cage
induction motor is

(a) low at light loads only

(b) low at heavy loads only

(c) low at light and heavy loads both

(d) low at rated load only.

13. Which of the following rotor quantity
in a SCIM does NOT depend on its slip ?

(a) reactance

(b) speed

(c) induced emf

(d) frequency.

14. A 6-pole, 50-Hz, 3-ö induction motor is
running at 950 rpm and has rotor Cu loss of

5 kW. Its rotor input is ......kW.

(a) 100

(b) 10

(c) 95

(d) 5.3.

15. The efficiency of a 3-phase induction
motor is approximately proportional to

(a) (1. s)

(b) s

(c) N

(d) Ns.

16. A 6-pole, 50-Hz, 3-ö induction motor
has a full load speed of 950 rpm. At half-load, its speed would be ......rpm.

(a) 475

(b) 500

(c) 975

(d) 1000

17. If rotor input of a SCIM running with a
slip of 10% is 100 kW, gross power developed by its rotor is ...... kW.

(a) 10

(b) 90

(c) 99

(d) 80

18. Pull-out torque of a SCIM occurs at
that value of slip where rotor power factor equals

(a) unity

(b) 0.707

(c) 0.866

(d) 0.5

19. Fill in the blanks.

When load is placed on a 3-phase induction motor,
its

(i) speed .......

(ii) slip ......

(iii) rotor induced emf ......

(iv) rotor current ......

(v) rotor torque ......

(vi) rotor continues to rotate at that
value of slip at which developed torque equals ...... torque.

20. When applied rated voltage per phase is
reduced by one-half, the starting torque of a SCIM becomes ...... of the
starting torque with full voltage.

(a) 1/2

(b) 1/4

(c) 1/ 2

(d) 3/2

21. If maximum torque of an induction motor
is 200 kg-m at a slip of 12%, the torque at 6% slip would be ...... kg-m.

(a) 100

(b) 160

(c) 50

(d) 40

22. The fractional slip of an induction
motor is the ratio

(a) rotor Cu loss/rotor input

(b) stator Cu loss/stator input

(c) rotor Cu loss/rotor output

(d) rotor Cu loss/stator Cu loss

23. The torque developed by a 3-phase
induction motor depends on the following three factors:

(a) speed, frequency, number of poles

(b) voltage, current and stator impedance

(c) synchronous speed, rotor speed and frequency

(d) rotor emf, rotor current and rotor p.f.

24. If the stator voltage and frequency of
an induction

motor are reduced proportionately, its

(a) locked rotor current is reduced

(b) torque developed is increased

(c) magnetising current is decreased

(d) both (a) and (b)

25. The efficiency and p.f. of a SCIM
increases in proportion to its

(a) speed

(b) mechanical load

(c) voltage

(d) rotor torque

26. A SCIM runs at constant speed only so
long as (a) torque developed by it remains constant

(b) its supply voltage remains constant

(c) its torque exactly equals the
mechanical load

(d) stator flux remains constant

27. The synchronous speed of a linear
induction motor does NOT depend on

(a) width of pole pitch

(b) number of poles

(c) supply frequency

(d) any of the above

28. Thrust developed by a linear induction
motor depends on

(a) synchronous speed

(b) rotor input

(c) number of poles

(d) both (a) and (b)

Answers

1. b 2. c 3. c 4. c 5. c 6. c 7. c 8. a 9. d
10. (i) 1500 (ii) 1440 (iii) 60 (iv) 1500 11. b 12. a 13. b 14. a 15. a 16. c 17.
b 18. b 19. (i) decreases (ii) increases (iii) increases (iv) increases (v) increases
(vi) applied 20. b 21. b 22. a 23. d 24. d 25. b 26. c 27. b 28. d

## No comments:

## Post a Comment