Quiz  /  Polyphase Induction Motors Electrical Engineering MCQ Questions and Answers PDF

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

  • Bearings
  • Shaft
  • Statorcore
  • None of the above

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

  • silicon steel
  • cast iron
  • aluminium
  • bronze

  • 3. 
    The shaft of an induction motor is made of

  • stiff
  • flexible
  • hollow
  • any of the above

  • 4. 
    The shaft of an induction motor is made of

  • high speed steel
  • stainless steel
  • carbon steel
  • cast iron

  • 5. 
    In an induction motor, no-load the slip is generally

  • less than 1%
  • 1.5%
  • 2%
  • 4%

  • 6. 
    In medium sized induction motors, the slip is generally around

  • 0.04%
  • 0.4%
  • 4%
  • 14%

  • 7. 
    In squirrel cage induction motors, the rotor slots are usually given slight skew in order to

  • reduce windage losses
  • reduce eddy currents
  • reduce accumulation of dirt and dust
  • reduce magnetic hum

  • 8. 
    In case the air gap in an induction motor is increased

  • the magnetising current of the rotor will decrease
  • the power factor will decrease
  • speed of motor will increase
  • the windage losses will increase

  • 9. 
    Slip rings are usually made of

  • copper
  • carbon
  • phospor bronze
  • aluminium

  • 10. 
    A 3-phase 440 V, 50 Hz induction motor has 4% slip. The frequency of rotor e.m.f. will be

  • 200 Hz
  • 50 Hz
  • 2 Hz
  • 0.2 Hz

  • 11. 
    In Ns is the synchronous speed and s the slip, then actual running speed of an induction motor will be

  • Ns
  • s.N,
  • (l-s)Ns
  • (Ns-l)s

  • 12. 
    The efficiency of an induction motor can be expected to be nearly

  • 60 to 90%
  • 80 to 90%
  • 95 to 98%
  • 99%

  • 13. 
    The number of slip rings on a squirrel cage induction motor is usually

  • two
  • three
  • four
  • none

  • 14. 
    The starting torque of a squirrel-cage induction motor is

  • low
  • negligible
  • same as full-load torque
  • slightly more than full-load torque

  • 15. 
    A double squirrel-cage induction motor has

  • two rotors moving in oppsite direction
  • two parallel windings in stator
  • two parallel windings in rotor
  • two series windings in stator

  • 16. 
    Star-delta starting of motors is not possible in case of

  • single phase motors
  • variable speed motors
  • low horse power motors
  • high speed motors

  • 17. 
    The term ‘cogging’ is associated with

  • three phase transformers
  • compound generators
  • D.C. series motors
  • induction motors

  • 18. 
    In case of the induction motors the torque is

  • inversely proportional to (Vslip)
  • directly proportional to (slip)2
  • inversely proportional to slip
  • directly proportional to slip

  • 19. 
    An induction motor with 1000 r.p.m. speed will have

  • 8 poles
  • 6 poles
  • 4 poles
  • 2 poles

  • 20. 
    The good power factor of an induction motor can be achieved if the average flux density in the air gap is

  • absent
  • small
  • large
  • infinity

  • 21. 
    An induction motor is identical to

  • D.C. compound motor
  • D.C. series motor
  • synchronous motor
  • asynchronous motor

  • 22. 
    The injected e.m.f. in the rotor of induction motor must have

  • zero frequency
  • the same frequency as the slip frequency
  • the same phase as the rotor e.m.f.
  • high value for the satisfactory speed control

  • 23. 
    Which of the following methods is easily applicable to control the speed of the squirrel-cage induction motor ?

  • By changing the number of stater poles
  • Rotor rheostat control
  • By operating two motors in cascade
  • By injecting e.m.f. in the rotor circuit

  • 24. 
    The crawling in the induction motor is caused by

  • low voltage supply
  • high loads
  • harmonics developed in the motor
  • improper design of the machine
  • none of the above

  • 25. 
    The auto-starters (using three auto transformers) can be used to start cage induction motor of the following type

  • star connected only
  • delta connected only
  • (a) and (b) both
  • none of the above

  • 26. 
    When the equivalent circuit diagram of double squirrel-cage induction motor is constructed the two cages can be considered

  • in series
  • in parallel
  • in series-parallel
  • in parallel with stater

  • 27. 
    It is advisable to avoid line-starting of induction motor and use starter because

  • motor takes five to seven times its full load current
  • it will pick-up very high speed and may go out of step
  • it will run in reverse direction
  • starting torque is very high

  • 28. 
    Stepless speed control of induction motor is possible by which of the following methods ?

  • e.m.f. injection in rotor eueuit
  • Changing the number of poles
  • Cascade operation
  • None of the above

  • 29. 
    Rotor rheostat control method of speed control is used for

  • squirrel-cage induction motors only
  • slip ring induction motors only
  • both (a) and (b)
  • none of the above

  • 30. 
    In the circle diagram for induction motor, the diameter of the circle represents

  • slip
  • rotor current
  • running torque
  • line voltage

  • 31. 
    For which motor the speed can be controlled from rotor side ?

  • Squirrel-cage induction motor
  • Slip-ring induction motor
  • Both (a) and (b)
  • None of the above

  • 32. 
    If any two phases for an induction motor are interchanged

  • the motor will run in reverse direction
  • the motor will run at reduced speed
  • the motor will not run
  • the motor will burn

  • 33. 
    An induction motor is

  • self-starting with zero torque
  • self-starting with high torque
  • self-starting with low torque
  • non-self starting

  • 34. 
    The maximum torque in an induction motor depends on

  • frequency
  • rotor inductive reactance
  • square of supply voltage
  • all of the above

  • 35. 
    In three-phase squirrel-cage induction motors

  • rotor conductor ends are short-circuited through slip rings
  • rotor conductors are short-circuited through end rings
  • rotor conductors are kept open
  • rotor conductors are connected to insulation

  • 36. 
    In a three-phase induction motor, the number of poles in the rotor winding is always

  • zero
  • more than the number of poles in stator
  • less than number of poles in stator
  • equal to number of poles in stator

  • 37. 
    DOL starting of induction motors is usually restricted to

  • low horsepower motors
  • variable speed motors
  • high horsepower motors
  • high speed motors

  • 38. 
    The speed of a squirrel-cage induction motor can be controlled by all of the following except

  • changing supply frequency
  • changing number of poles
  • changing winding resistance
  • reducing supply voltage

  • 39. 
    The power factor of an induction motor under no-load conditions will be closer to

  • 0.2 lagging
  • 0.2 leading
  • 0.5 leading
  • unity

  • 40. 
    The ‘cogging’ of an induction motor can be avoided by

  • proper ventilation
  • using DOL starter
  • auto-transformer starter
  • having number of rotor slots more or less than the number of stator slots (not equal)

  • 41. 
    If an induction motor with certain ratio of rotor to stator slots, runs at 1/7 of the normal speed, the phenomenon will be termed as

  • humming
  • hunting
  • crawling
  • cogging

  • 42. 
    Slip of an induction motor is negative when

  • magnetic field and rotor rotate in opposite direction
  • rotor speed is less than the synchronous speed of the field and are in the same direction
  • rotor speed is more than the synchronous speed of the field and are in the same direction
  • none of the above

  • 43. 
    Size of a high speed motor as compared to low speed motor for the same H.P. will be

  • bigger
  • smaller
  • same
  • any of the above

  • 44. 
    A 3-phase induction motor stator delta connected, is carrying full load and one of its fuses blows out. Then the motor

  • will continue running burning its one phase
  • will continue running burning its two phases
  • will stop and carry heavy current causing permanent damage to its winding
  • will continue running without any harm to the winding

  • 45. 
    A 3-phase induction motor delta connected is carrying too heavy load and one of its fuses blows out. Then the motor

  • will continue running burning its one phase
  • will continue running burning its two phase
  • will stop and carry heavy current causing permanent damage to its winding
  • will continue running without any harm to the winding

  • 46. 
    Low voltage at motor terminals is due to

  • inadequate motor wiring
  • poorely regulated power supply
  • any one of the above
  • none of the above

  • 47. 
    In an induction motor the relationship between stator slots and rotor slots is that

  • stator slots are equal to rotor slots
  • stator slots are exact multiple of rotor slots
  • stator slots are not exact multiple of rotor slots
  • none of the above

  • 48. 
    As load on an induction motor goes on increasing

  • its power factor goes on decreasing
  • its power factor remains constant
  • its power factor goes on increasing even after full load
  • its power factor goes on increasing up to full load and then it falls again

  • 49. 
    It is advisable to avoid line starting of induction motor and use starter because

  • it will run in reverse direction
  • it will pick up very high speed and may go out of step
  • motor takes five to seven times its full load current
  • starting torque is very high

  • 50. 
    The speed characteristics of an induction motor closely resemble the speedload characteristics of which of the following machines

  • D.C. series motor
  • D.C. shunt motor
  • universal motor
  • none of the above

  • 51. 
    Which type of bearing is provided in small induction motors to support the rotor shaft ?

  • Ball bearings
  • Cast iron bearings
  • Bush bearings
  • None of the above

  • 52. 
    A pump induction motor is switched on to a supply 30% lower than its rated voltage. The pump runs. What will eventually happen ? It will

  • stall after sometime
  • stall immediately
  • continue to run at lower speed without damage
  • get heated and subsequently get damaged

  • 53. 
    5 H.P., 50-Hz, 3-phase, 440 V, induction motors are available for the following r.p.m. Which motor will be the costliest ?

  • 730 r.p.m.
  • 960 r.p.m.
  • 1440 r.p.m.
  • 2880 r.p.m.

  • 54. 
    Short-circuit test on an induction motor cannot be used to determine

  • windage losses
  • copper losses
  • transformation ratio
  • power scale of circle diagram

  • 55. 
    In case of 3-phase induction motors, plugging means

  • pulling the motor directly on line without a starter
  • locking of rotor due to harmonics
  • starting the motor on load which is more than the rated load
  • interchanging two supply phases for quick stopping

  • 56. 
    Which is of the following data is required to draw the circle diagram for an induction motor ?

  • Block rotor test only
  • No load test only
  • Block rotor test and no-load test
  • Block rotor test, no-load test and stator resistance test

  • 57. 
    In three-phase induction motors sometimes copper bars are placed deep in the rotor to

  • improve starting torque
  • reduce copper losses
  • improve efficiency
  • improve power factor

  • 58. 
    The vafcie of transformation ratio of an induction motor can be found by

  • open-circuit test only
  • short-circuit test only
  • stator resistance test
  • none of the above

  • 59. 
    The shape of the torque/slip curve of induction motor is

  • parabola
  • hyperbola
  • rectangular parabola
  • straigth line

  • 60. 
    A change of 4% of supply voltage to an induction motor will produce a change of appromimately

  • 4% in the rotor torque
  • 8% in the rotor torque
  • 12% in the rotor torque
  • 16% in the rotor torque

  • 61. 
    The stating torque of the slip ring induction motor can be increased by adding

  • external inductance to the rotor
  • external resistance to the rotor
  • external capacitance to the rotor
  • both resistance and inductance to rotor

  • 62. 
    A 500 kW, 3-phase, 440 volts, 50 Hz, A.C. induction motor has a speed of 960 r.p.m. on full load. The machine has 6 poles. The slip of the machine will be

  • 0.01
  • 0.02
  • 0.03
  • 0.04

  • 63. 
    In the squirrel-cage induction motor the rotor slots are usually given slight skew

  • to reduce the magnetic hum and locking tendency of the rotor
  • to increase the tensile strength of the rotor bars
  • to ensure easy fabrication
  • none of the above

  • 64. 
    The torque of a rotor in an induction motor under running condition is maximum

  • at the unit value of slip
  • at the zero value of slip
  • at the value of the slip which makes rotor reactance per phase equal to the resistance per phase
  • at the value of the slip which makes the rotor reactance half of the rotor

  • 65. 
    What will happen if the relative speed between the rotating flux of stator and rotor of the induction motor is zero ?

  • The slip of the motor will be 5%
  • The rotor will not run
  • The rotor will run at very high speed
  • The torque produced will be very large

  • 66. 
    The circle diagram for an induction motor cannot be used to determine

  • efficiency
  • power factor
  • frequency
  • output

  • 67. 
    Blocked rotor test on induction motors is used to find out

  • leakage reactance
  • power factor on short circuit
  • short-circuit current under rated voltage
  • all of the above

  • 68. 
    Lubricant used for ball bearing is usually

  • graphite
  • grease
  • mineral oil
  • molasses

  • 69. 
    An induction motor can run at synchronous speed when

  • it is run on load
  • it is run in reverse direction
  • it is run on voltage higher than the rated voltage
  • e.m.f. is injected in the rotor circuit

  • 70. 
    Which motor is preferred for use in mines where explosive gases exist ?

  • Air motor
  • Induction motor
  • D.C. shunt motor
  • Synchronous motor

  • 71. 
    The torque developed by a 3-phase induction motor least depends on

  • rotor current
  • rotor power factor
  • rotor e.m.f.
  • shaft diameter

  • 72. 
    In an induction motor if air-gap is increased

  • the power factor will be low
  • windage losses will be more
  • bearing friction will reduce
  • copper loss will reduce In an induction motor

  • 73. 
    In induction motor, percentage slip depends on

  • supply frequency
  • supply voltage
  • copper losses in motor
  • none of the above

  • 74. 
    When /?2 is the rotor resistance, .X2 the rotor reactance at supply frequency and s the slip, then the condition for maximum torque under running conditions will be

  • sR2X2 = 1
  • sR2 = X2
  • R2 = sX2
  • R2 = s2X2

  • 75. 
    In case of a double cage induction motor, the inner cage has

  • high inductance arid low resistance
  • low inductance and high resistance
  • low inductance and low resistance
  • high inductance and high resistance

  • 76. 
    The low power factor of induction motor is due to

  • rotor leakage reactance
  • stator reactance
  • the reactive lagging magnetizing current necessary to generate the magnetic flux
  • all of the above

  • 77. 
    Insertion of reactance in the rotor circuit

  • reduces starting torque as well as maximum torque
  • increases starting torque as well as maximum torque
  • increases starting torque but maxi-mum torque remains unchanged
  • increases starting torque but maxi-mum torque decreases

  • 78. 
    Insertion of resistance in the rotcir of an induction motor to develop a given torque

  • decreases the rotor current
  • increases the rotor current
  • rotor current becomes zero
  • rotor current rernains same

  • 79. 
    For driving high inertia loods best type of induction motor suggested is

  • slip ring type
  • squirrel cage type
  • any of the above
  • none of the above

  • 80. 
    Temperature of the stator winding of a three phase induction motor is obtained by

  • resistance rise method
  • thermometer method
  • embedded temperature method
  • all above methods

  • 81. 
    The purpose of using short-circuit gear is

  • to short circuit the rotor at slip rings
  • to short circuit the starting resistances in the starter
  • to short circuit the stator phase of motor to form star
  • none of the above

  • 82. 
    In a squirrel cage motor the induced e.m.f. is

  • dependent on the shaft loading
  • dependent on the number of slots
  • slip times the stand still e.m.f. induced in the rotor
  • none of the above

  • 83. 
    A squirrel cage induction motor is not selected when

  • initial cost is the main consideration
  • maintenance cost is to be kept low
  • higher starting torque is the main consideration
  • all above considerations are involved

  • 84. 
    Reduced voltage starter can be used with

  • slip ring motor only but not with squirrel cage induction motor
  • squirrel cage induction motor only but not with slip ring motor
  • squirrel cage as well as slip ring induction motor
  • none of the above

  • 85. 
    Slip ring motor is preferred over squirrel cage induction motor where

  • high starting torque is required
  • load torque is heavy
  • heavy pull out torque is required
  • all of the above

  • 86. 
    In a star-delta starter of an induction motor

  • resistance is inserted in the stator
  • reduced voltage is applied to the stator
  • resistance is inserted in the rotor
  • applied voltage perl stator phase is 57.7% of the line voltage

  • 87. 
    The torque of an induction motor is

  • directly proportional to slip
  • inversely proportional to slip
  • proportional to the square of the slip
  • none of the above

  • 88. 
    The rotor of an induction motor runs at

  • synchronous speed
  • below synchronous speed
  • above synchronous speed
  • any of the above

  • 89. 
    . The starting torque of a three phase induction motor can be increased by

  • increasing slip
  • increasing current
  • both (a) and (b)
  • none of the above

  • 90. 
    . Insertion of resistance in the stater of an induction motor

  • increases the load torque
  • decreases the starting torque
  • increases the starting torque
  • none of the above
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