Induction Motor

 Induction Motor

1. Which of the following component is usually fabricated out of silicon steel ?

1. Bearings
2. Shaft
3. Stator core
4. None of the above

c. Stator core

2. The frame of an induction motor is usually made of

1. Silicon steel
2. cast iron
3. aluminium
4. bronze

b. cast iron

3. The shaft of an induction motor is made of

1. high speed steel
2. stainless steel
3. carbon steel
4. cast iron

c. carbon steel

4. 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

d. reduce magnetic hum

5. In case the air gap in an induction motor is increased

1. the magnetizing current of the rotor will decrease
2. the power factor will decrease
3. speed of motor will increase
4. the windage losses will increase

b. the power factor will decrease

6. In Ns is the synchronous speed and s the slip, then actual running speed of an induction motor will be

1. Ns
2. s.Ns
3. (1-s)Ns
4. (Ns-1)s

c. (1-s)Ns

7. Slip rings are usually made of

1. copper
2. carbon
3. phosphor bronze
4. aluminium

c. phosphor bronze

8. 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. 0.99

b. 80 to 90%

9. The number of slip rings on a squirrel-cage induction motor is usually

1. low
2. negligible
3. same as full-load torque
4. slightly more than full-load torque

a. low

10. 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

a. single phase motors

11. An induction motor with 1000 r.p.m. speed will have

1. 8 poles
2. 6 poles
3. 4 poles
4. 2 poles

b. 6 poles

12. The crawling in the induction motor is caused by

1. low voltage supply
2. high loads
3. harmonics developed in the motor
4. improper design of machine

c. harmonics developed in the motor

13. It is advisable to avoid line starting of induction motor and use starter because

1. motor take five to seven time 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

a. motor take five to seven time its full load current

14. Rotor rheostat control method of speed control is used for

1. squirrel-cage induction motors only
2. slip ring induction motors only
3. both (a) and (b)
4. none of the above

b. slip ring induction motors only

15. If any two phases for an induction motor are interchanged

1. the motor will run in reverse direction
2. the motor will run at reduced speed
3. the motor will not run
4. the motor will burn

a. the motor will run in reverse direction

16. An induction motor is

1. self-starting with zero torque
2. self starting with high torque
3. self starting with low torque
4. non self starting

c. self starting with low torque

17. In three-phase squirrel-cage induction motors

1. rotor conductor ends are shortcircuited through slip rings
2. rotor conductors are short-circuited through end rings
3. rotor conductors are kept open
4. rotor conductors are connected to insulation

b. rotor conductors are short-circuited through end rings

18. In a three-phase induction motor, the number of poles in the rotor winding is always

1. zero
2. more than the number of poles in stator
3. less than number of poles in stator
4. equal to number of poles in stator

d. equal to number of poles in stator

19. DOL starting of induction motors is usually restricted to

1. low horsepower motors
2. variable speed motors
3. high horsepower motors
4. high speed motors

a. low horsepower motors

20. The power factor of an induction motor under no-load conditions will be closer to

1. 0.2 lagging
2. 0.2 leading
3. 0.5 leading
4. unity

a. 0.2 lagging

21. Size of a high speed motor as compared to low speed motor for the same H.P. will be

1. bigger
2. smaller
3. same
4. any of the above

b. smaller

22. 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

d. all above features are required

23. Which type of bearing is provided in small induction motors to support the rotor shaft

1. ball bearings
2. cast iron bearings
3. bush bearings
4. non of the above

a. ball bearings

24. Lubricant used for ball bearing is usually

1. graphite
2. grease
3. mineral oil
4. molasses

b. grease

25. If the rotor circuit of a squirrel cage induction motor is open, the rotor will

1. run at very high speed
2. run at very low speed
3. make noise
4. not run

d. not run

26. The advantage of a slip-ring induction motor over a squirrel cage induction motor is that

1. it has higher efficiency
2. it has higher power factor
3. it can be started with help of rotor resistance stater
4. non of the above

c. it can be started with help of rotor resistance stater

27. A 3-phase slip-ring induction motor is always started with

1. a starting winding
2. squirrel cage winding
3. no external resistance in rotor circuit
4. full external resistance in rotor circuit

d. full external resistance in rotor circuit

28. The synchronous speed of a 3- phase induction motor is given by the formula

1. Ns =120f/P
2. Ns =120P/f
3. Ns =120 fP
4. Ns = f P/120

a. Ns =120f/P

29. If single-phasing occurs on the running position in an induction motor, the motor will

1. fail to carry load
2. produce peculiar noise
3. draw unbalanced and excessive currents
4. not start

d. not start

30. A pump induction motor is switched on to a supply 25 percent lower than its rated voltage. The pump runs; eventually

1. the pump will get heated and consequently get damaged
2. the pump will stall after sometimes
3. the pump will continue to run at lower speed without damage
4. None

a. the pump will get heated and consequently get damaged

31. If there is an open circuit in the rotor of a squirrel cage induction motor

1. rotor will overheat
2. line fuses will blow
3. motor will be noisy
4. motor will not start

d. motor will not start

32. The principle of operation of a 30phase induction motor is most similar to that of

1. transformer with a shorted secondary
2. synchronous motor
3. capacitor start induction run motor
4. repulsion start motor induction motor

a. transformer with a shorted secondary

33. The A.C. motor which would be best suited to drive a centrifugal pump for discharging a variable quantity of water against a fixed head is the

1. repulsion motor
2. synchronous motor
3. squirrel cage
4. slip ring induction motor

d. slip ring induction motor

34. Two of the power supply terminals to a 3-phase induction motor get inter-changed during reconnection after maintenance of the motor. When put back into service, the motor will

1. get heated up and damaged
2. rotate in the same direction as it was prior to maintenance
3. fail to rotate
4. rotate in the reverse direction to that prior to maintenance

d. rotate in the reverse direction to that prior to maintenance

35. The frame of an induction motor is made of

1. carbon
2. closed grained cast iron
3. aluminium
4. stainless steel

b. closed grained cast iron

36. Slip rings for induction motors are made of

1. phosphor bronze
2. aluminium
3. carbon
4. cobalt steel

a. phosphor bronze

37. The shaft, on which the rotor of an induction motor is mounted is made of

1. high speed steel
2. chrome vanadium steel
3. cast-iron
4. mild steel

d. mild steel

38. Which of the following type of bearing is generally used to support the rotor of an induction motor

1. Ball bearing
2. Needle bearing
3. Plummer block
4. Bush bearing

a. Ball bearing

39. Under which method of starting an induction motor is expected to take largest starting current?

1. star-delta starting
2. auto-transformer starting
3. direct on line starting
4. stator rotor starting

c. direct on line starting

40. The direction of rotation of a 3-phase induction motor can be reversed by

1. interchanging any two phases
2. supplying low voltage
3. reducing load
4. reducing frequency

a. interchanging any two phases

41. The number of slip rings on a squirrel cage induction motor is

1. four
2. three
3. two
4. none

d. none

42. The starting torque of the slip ring induction motor can be increased by

1. adding resistance to the stator
2. adding resistance to the rotor
3. adding resistance to stator as well as the rotor
4. none of the above

b. adding resistance to the rotor

43. If the rotor is open in a squirrel cage motor, it

1. will run at very high speed
2. will run at very slow speed
3. will not run
4. will make noise

c. will not run

44. The value of average flux density in air gap in an induction motor, should be small

1. to achieve good efficiency
2. to get poor power factor
3. to get good power factor
4. for minimum cost

c. to get good power factor

45. An induction motor has a rated speed of 720 r.p.m. How many poles has its rotating magnetic field?

1. 8 poles
2. 6 poles
3. 4 poles
4. 2 poles

a. 8 poles

46. During starting if an induction motor hums, the probable cause could be

1. open circuit
2. unequal phase resistance
3. inter-turn short circuit on rotor
4. any of the above

d. any of the above

47. The probable reason for an induction motor running too hot could be

1. low voltage
2. uneven air gap
3. clogged ventilating ducts
4. any of the above

d. any of the above

48. In case single phasing occurs in delta connected motor

1. one phase will be seriously overloaded and two others will be slightly overloaded
2. two phases will be seriously over-loaded and there will be no current in the third phase
3. there will be no current in one phases
4. there will be no current in two phases

a. one phase will be seriously overloaded and two others will be slightly overloaded

49. Synchronous speed is defined as

1. the speed of a synchronous motor
2. the natural speed at which a magnetic field rotates
3. the speed of the rotor of an induction motor
4. the speed of an induction motor at no load

b. the natural speed at which a magnetic field rotates

50. The speed of three phase cage-rotor induction motor depends on

1. number of poles only
2. input voltage
3. frequency of supply only
4. number of poles and frequency of supply

d. number of poles and frequency of supply

51. The two important parts of a 3-phase induction motor are

1. rotor and armature
2. rotor and stator
3. slip ring and brushes
4. stator and field

b. rotor and stator

52. Phase advancers are used with induction motors to

1. reduce noise
2. reduce vibrations
3. reduce copper losses
4. improve power factor

d. improve power factor