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Q Bank & Answer Electrical Engineer - INDUCTION MOTOR

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

a. Bearings 

b. Shaft 

c. Stator core 

d. None of the above

Answer - c. Stator core 


2. The frame of an induction motor is usually made of [ b ]

a. Silicon steel 

b. cast iron 

c. aluminium 

d. bronze

Answer -b. cast iron 


3. The shaft of an induction motor is made of [ c ]

a. high speed steel 

b. stainless steel

 c. carbon steel 

d. cast iron

Answer  -  c. carbon steel 


4. In squirrel cage induction motors, the rotor slots are usually given slight skew in order to [ d ]

a. reduce windage losses 

b. reduce eddy currents

c. reduce accumulation of dirt and dust 

d. reduce magnetic hum

Answer  -  d. reduce magnetic hum


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

a. the magnetizing current of the 

b. the power factor will decrease rotor will decrease

c. speed of motor will increase 

d. the windage losses will increase

Answer  - b. the power factor will decrease rotor will decrease


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

a. Ns 

b. s.Ns 

c. (1-s)Ns 

d. (Ns-1)s

Answer  - c. (1-s)Ns 


7. Slip rings are usually made of [ c ]

a. copper 

b. Carbon 

c. phosphor bronze 

d. aluminium

Answer  - c. phosphor bronze 


8. The efficiency of an induction motor can be expected to be nearly [ b ]

a. 60 to 90% 

b. 80 to 90%

c. 95 to 98% 

d. 99%

Answer  - b. 80 to 90%


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

a) 3 

b) 1 

c) 6 

d) 0

Answer  - d) 0


10. Running torque of the squirrel-cage induction motor on full load is [ a ]

a. low 

b. negligible 

c. same as full-load torque 

d. slightly more than full-load torque

Answer  -  a. low 


11. Star-delta starting of motors is not possible in case of [ a ]

a. single phase motors 

b. variable speed motors

c. low horse power motors 

d. high speed motors 

Answer  - a. single phase motors 


12. An induction motor with 1000 r.p.m. speed will have [ b ]

a. 8 poles 

b. 6 poles 

c. 4 poles 

d. 2 poles

Answer  - b. 6 poles 


13. The crawling in the induction motor is caused by [ c ]

a. low voltage supply 

b. high loads

c. harmonics developed in the motor 

d. improper design of machine

Answer  - c. harmonics developed in the motor 


14. It is advisable to avoid line starting of induction motor and use starter because [ a ]

a. motor take five to seven time its 

b. it will pick up very high speed and full load current may go out of step

c. it will run in reverse direction 

d. starting torque is very high

Answer  - a. motor take five to seven time its 


15. Rotor rheostat control method of speed control is used for [ b ]

a. squirrel-cage induction motors 

b. slip ring induction motors only only

c. both (a) and (b) 

d. none of the above

Answer  - b. slip ring induction motors only only


16. If any two phases for an induction motor are interchanged [ a ]

a. the motor will run in reverse direction 

b. the motor will run at reduced speed direction

c. the motor will not run 

d. the motor will burn

Answer  -  a. the motor will run in reverse direction 


17. An induction motor is [ c ]

a. self-starting with zero torque 

b. self starting with high torque

c. self starting with low torque 

d. non self starting

Answer  -  c. self starting with low torque 


18. In three-phase squirrel-cage induction motors [ b ]

a. rotor conductor ends are short- 

b. rotor conductors are short-circuited circuited through slip rings through end rings

c. rotor conductors are kept open 

d.rotor conductors are connected to insulation

Answer  - b. rotor conductors are short-circuited circuited through slip rings through end rings


19. In a three-phase induction motor, the number of poles in the rotor winding is always [ d ]

a. zero stator

b. more than the number of poles in stator

c. less than number of poles in stator

d. equal to number of poles in stator

Answer  - d. equal to number of poles in stator


20. DOL starting of induction motors is usually restricted to [ a ]

a. low horsepower motors 

b. variable speed motors

c. high horsepower motors 

d. high speed motors 

Answer  - a. low horsepower motors 


21. The power factor of an induction motor under no-load conditions will be closer to [ a ]

a. 0.2 lagging 

b. 0.2 leading

c. 0.5 leading 

d. unity

Answer  -  a. 0.2 lagging 


22. Size of a high speed motor as compared to low speed motor for the same H.P. will be [ b ]

a. bigger 

b. smaller 

c. same 

d. any of the above

Answer  - b. smaller 


23. Slip ring motor is recommended where [ d ]

a. speed control is required

frequenting,showstopping required

c. high starting torque is needed 

d. all above features are required

Answer  - d. all above features are required


24. Which type of bearing is provided in small induction motors to support the rotor shaft [ a ]

a. ball bearings 

b. cast iron bearings

c. bush bearings 

d. non of the above

Answer  - a. ball bearings 


25. Lubricant used for ball bearing is usually [ b ]

a. graphite 

b. grease

c. mineral oil 

d. molasses

Answer  - b. grease


26. If the rotor circuit of a squirrel cage induction motor is open, the rotor will [ d ]

 a. run at very high speed 

b. run at very low speed

c. make noise 

d. not run

Answer  - d. not run


27. The advantage of a slip-ring induction motor over a squirrel cage induction motor is that [ c ]

a. it has higher efficiency 

b. it has higher power factor

c. it can be started with help of rotor 

d. non of the above  resistance starter

Answer  - c. it can be started with help of rotor 


28. A 3-phase slip-ring induction motor is always started with [ d ]

a. a starting winding

b.squirrel cage winding

c. no external resistance in rotor circuit

d. full external resistance in rotor circuit

Answer  -  d. full external resistance in rotor circuit


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

a. Ns =120f/P 

b. Ns =120P/f

c. Ns =120 fP 

d. Ns = f P/120

Answer  -   a. Ns =120f/P 


30. If single-phasing occurs on the running position in an induction motor, the motor will [ a ]

a. fail to carry load 

b. produce peculiar noise

c. draw unbalanced and excessive currents 

d. not start 

Answer  - a. fail to carry load 


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

a. the pump will get heated and consequentlyget damaged

b. the pup will stall after sometimes

c. the pump will continue to run at lower speed without damage

 d. None

Answer  -  a. the pump will get heated and consequentlyget damaged


32. If there is an open circuit in the rotor of a squirrel cage induction motor [ d ]

a. rotor will overheat 

b. line fuses will blow

c. motor will be noisy 

d. motor will not start

Answer  -  d. motor will not start


33. The principle of operation of a 3phase induction motor is most similar to that of [ a ]

a. transformer with a shorted secondary 

b. synchronous motor

c. capacitor start induction run motor 

d. repulsion start motor induction motor

Answer  -  a. transformer with a shorted secondary 


34. 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 [ d ]

a. repulsion motor 

b. synchronous motor

c. squirrel cage 

d. slip ring induction motor

Answer  -  d. slip ring induction motor


35. Two of the power supply terminals to a 3-phase induction motor get inter-changed during re connection after maintenance of the motor. When put back into service, the motor will [ d ]

a. get heated up and damaged

b. rotate in the same direction as it was prior to maintenance

c. fail to rotate

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

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


36. The frame of an induction motor is made of [ b ]

a. carbon 

b. closed grained cast iron

c. aluminium 

d. stainless steel

Answer  -  b. closed grained cast iron


37. Slip rings for induction motors are made of [ a ]

a. phosphor bronze 

b. aluminium

c. carbon 

d. cobalt steel

Answer  - a. phosphor bronze 


38. The shaft, on which the rotor of an induction motor is mounted is made of [ d ]

a. high speed steel 

b. chrome vanadium steel

c. cast-iron 

d. carbon steel

e. aluminium 

Answer  - e. aluminium 


39. Which of the following type of bearing is generally used to support the rotor of an induction motor [ a ]

a. Ball bearing 

b. Needle bearing

c. Plummer block 

d. Bush bearing

Answer  - a. Ball bearing 


40. Under which method of starting an induction motor is expected to take largest starting current? [ c ]

a. star-delta starting 

b. auto-transformer starting

c. direct on line starting

 d. stator rotor starting

Answer  -  c. direct on line starting


41. The direction of rotation of a 3-phase induction motor can be reversed by [ a ]

a. interchanging any two phases 

b. supplying low voltage

c. reducing load 

d. reducing frequency

Answer  -  a. interchanging any two phases 


42. The number of slip rings on a squirrel cage induction motor is [ d ]

a. four 

b. three 

c. two 

d. none

Answer  -  d. none


43. The starting torque of the slip ring induction motor can be increased by [ b ]

a. adding resistance to the stator 

b. adding resistance to the rotor

c. adding resistance to stator as well as the rotor 

d. none of the above

Answer  -  b. adding resistance to the rotor


44. If the rotor is open in a squirrel cage motor, it [ c ]

a. will run at very high speed 

b. will run at very slow speed

c. will not run 

d. will make noise

Answer  -  c. will not run 


45. The value of average flux density in air gap in an induction motor, should be small [ c ]

a. to achieve good efficiency 

b. to get poor power factor

c. to get good power factor

 d. for minimum cost

Answer  -  c. to get good power factor


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

a. 8 poles 

b. 6 poles 

c . 4 poles 

d.2 poles

Answer  -  a. 8 poles 


47. During starting if an induction motor hums, the probable cause could be [ d ]

a. open circuit 

b. unequal phase resistance

c. inter-turn short circuit on rotor 

d. any of the above

Answer  - d. any of the above


48. The probable reason for an induction motor running too hot could be [ d ]

a. low voltage 

b. uneven air gap

c. clogged ventilating ducts 

d. any of the above

Answer  -  d. any of the above


49. In case single phasing occurs in delta connected motor [ a ]

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

b. two phases will be seriously over-loaded and there will be no current in the third phase

c. there will be no current in one phases

d. there will be no current in two phases

Answer  -  a. one phase will be seriously over-loaded and two others will be slightly overloaded


50. Synchronous speed is defined as [ b ]

a. the speed of a synchronous motor

b. the actual speed at which a magnetic field rotates

c. the speed of the rotor of an induction

d. the speed of an induction motor at no motor load

Answer  -  b. the actual speed at which a magnetic field rotates


51. The speed of three phase cage-rotor induction motor depends on [ d ]

a. number of poles only 

b. input voltage

c. frequency of supply only 

d. number of poles and frequency of supply

e. none of the above

Answer  - d. number of poles and frequency of supply


52. The two important parts of a 3-phase induction motor are [ b ]

a. rotor and armature 

b. rotor and stator

c. slip ring and brushes 

d. stator and field

Answer  - b. rotor and stator


53. Phase advancers are used with induction motors to [ d ]

a. reduce noise 

b. reduce vibrations

c. reduce copper losses

 d. improve power factor 

Answer  -  d. improve power factor 

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