101 TOP Polyphase Induction Motors Multiple choice Questions and Answers pdf free download

Real Time Polyphase Induction Motors Online Quiz Questions and Answers

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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

21. An induction motor is identical to
(a) D.C. compound motor
(b) D.C. series motor
(c) synchronous motor
(d) asynchronous motor
Ans: d


22. The injected e.m.f. in the rotor of induction motor must have
(a) zero frequency
(b) the same frequency as the slip frequency
(c) the same phase as the rotor e.m.f.
(d) high value for the satisfactory speed control
Ans: b

23. Which of the following methods is easily applicable to control the speed of the squirrel-cage induction motor ?
(a)     By changing the number of stater poles
(b)     Rotor rheostat control
(c) By operating two motors in cascade
(d) By injecting e.m.f. in the rotor circuit
Ans: a

24. The crawling in the induction motor is caused by
(a)     low voltage supply
(b)     high loads
(c) harmonics developed in the motor
(d) improper design of the machine
(e) none of the above
Ans: c

25. The auto-starters (using three auto transformers) can be used to start cage induction motor of the following type
(a) star connected only
(b) delta connected only
(c) (a) and (b) both
(d) none of the above
Ans: c

26. The  torque  developed  in  the  cage induction motor with auto starter is
(a)   K/torque with direct switching
(6)   K x torque with direct switching
(c) K2 x torque with direct switching
(d) k2/torque with direct switching
Ans: c

27. When the equivalent circuit diagram of double squirrel-cage induction motor is constructed the two cages can be considered
(a)     in series
(b)     in parallel
(c) in series-parallel
(d) in parallel with stater
Ans: b

28. It is advisable to avoid line-starting of induction motor and use starter because
(a) motor takes five to seven times its full load current
(b) it will pick-up very high speed and may go out of step
(c) it will run in reverse direction
(d) starting torque is very high
Ans: a

29. Stepless speed control of induction motor is possible by which of the following methods ?
(a)     e.m.f. injection in rotor eueuit
(b)     Changing the number of poles
(c) Cascade operation
(d) None of the above
Ans: b

30. Rotor rheostat control method of speed control is used for
(a)     squirrel-cage induction motors only
(b)     slip ring induction motors only
(c) both (a) and (b)
(d) none of the above
Ans: b

31. In the circle diagram for induction motor, the diameter of the circle represents
(a) slip
(b) rotor current
(c) running torque
(d) line voltage
Ans: b

32. For which motor the speed can be controlled from rotor side ?
(a) Squirrel-cage induction motor
(b) Slip-ring induction motor
(c) Both (a) and (b)
(d) None of the above
Ans: b

33. If any two phases for an induction motor are interchanged
(a)     the motor will run in reverse direction
(b)     the motor will run at reduced speed
(c) the motor will not run
(d) the motor will burn
Ans: a

34. An induction motor is
(a) self-starting with zero torque
(b) self-starting with high torque
(c) self-starting with low torque
(d) non-self starting
Ans: c

35. The maximum torque in an induction motor depends on
(a)     frequency
(b)     rotor inductive reactance
(c) square of supply voltage
(d) all of the above
Ans: d

36. In three-phase squirrel-cage induction motors
(a) rotor conductor ends are short-circuited through slip rings
(b) rotor conductors are short-circuited through end rings
(c) rotor conductors are kept open
(d) rotor conductors are connected to insulation
Ans: b

37. In a three-phase induction motor, the number of poles in the rotor winding is always
(a) zero
(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
Ans: d

38. DOL starting of induction motors is usually restricted to
(a)     low horsepower motors
(b)     variable speed motors
(c) high horsepower motors
(d) high speed motors
Ans: a

39. The speed of a squirrel-cage induction motor can be controlled by all of the following except
(a) changing supply frequency
(b) changing number of poles
(c) changing winding resistance
(d) reducing supply voltage
Ans: c

40. The 'crawling" in an induction motor is caused by
(a)     high loads
(6)     low voltage supply
(c) improper design of machine
(d) harmonics developed in the motor
Ans: d

41. The power factor of an induction motor under no-load conditions will be closer to
(a) 0.2 lagging      
(b) 0.2 leading
(c) 0.5 leading      
(d) unity
Ans: a

42. The 'cogging' of an induction motor can be avoided by
(a)     proper ventilation
(b)     using DOL starter
(c) auto-transformer starter
(d) having number of rotor slots more or less than the number of stator slots (not equal)
Ans: d

43. 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
(a) humming
(b) hunting
(c) crawling
(d) cogging
Ans: c

44. Slip of an induction motor is negative when
(a)     magnetic field and rotor rotate in opposite direction
(b)     rotor speed is less than the syn-chronous speed of the field and are in the same direction
(c) rotor speed is more than the syn-chronous speed of the field and are in the same direction
(d) none of the above
Ans: c

45. Size of a high speed motor as compared to low speed motorfor the same H.P. will be
(a) bigger
(b) smaller
(c) same
(d) any of the above
Ans: b

46. A 3-phase induction motor stator delta connected, is carrying full load and one of its fuses blows out. Then the motor
(a) will continue running burning its one phase
(b) will continue running burning its two phases
(c) will stop and carry heavy current causing permanent damage to its winding
(d) will continue running without any harm to the winding
Ans: a

47. A 3-phase induction motor delta connected is carrying too heavy load and one of its fuses blows out. Then the
motor
(a) will continue running burning its one phase
(b) will continue running burning its two phase
(c) will stop and carry heavy current causing permanent damage to its winding
(d) will continue running without any harm to the winding
Ans: c

48. Low voltage at motor terminals is due to
(a)     inadequate motor wiring
(b)     poorely regulated power supply
(c) any one of the above
(d) none of the above
Ans: c

49. In an induction motor the relationship between stator slots and rotor slots is that
(a)     stator slots are equal to rotor slots
(b)     stator slots are exact multiple of rotor slots
(c) stator slots are not exact multiple of rotor slots
(d) none of the above
Ans: c

50. Slip ring motor is recommended where
(a)     speed control is required
(6)     frequent starting, stopping and reversing is required
(c) high starting torque is needed
(d) all above features are required
Ans: d

51. As load on an induction motor goes on increasing
(a) its power factor goes on decreasing
(b) its power factor remains constant
(c) its power factor goes on increasing even after full load
(d) its power factor goes on increasing upto full load and then it falls again
Ans: d

52. If a 3-phase supply is given to the stator and rotor is short circuited rotor will move
(a) in the opposite direction as the direction of the rotating field
(b) in the same direction as the direction of the field
(c) in any direction depending upon phase squence of supply
Ans: b

53. It is advisable to avoid line starting of induction motor and use starter because
(a) it will run in reverse direction
(b) it will pick up very high speed and may go out of step
(c) motor takes five to seven times its fullload current
(d) starting torque is very high
Ans: c

54. The speed characteristics of an induction motor closely resemble the speedload characteristics of which of the following machines
(a) D.C. series motor
(b) D.C. shunt motor
(c) universal motor
(d) none of the above
Ans: b

55. Which type of bearing is provided in small induction motors to support the rotor shaft ?
(a) Ball bearings
(b) Cast iron bearings
(c) Bush bearings
(d) None of the above
 Ans: a

56. 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
(a)     stall after sometime
(b)     stall immediately
(c) continue to run at lower speed without damage
(d) get heated and subsequently get damaged
Ans: d

57. 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 ?
(a) 730 r.p.m.
(b) 960 r.p.m.
(c) 1440 r.p.m.      
(d) 2880 r.p.m.
Ans: a

58. A 3-phase slip ring motor has
(a)     double cage rotor
(6)     wound rotor
(c) short-circuited rotor
(d) any of the above
Ans: b

59. The starting torque of a 3-phase squirrel cage induction motor is
(a) twice the full load torque
(b) 1.5 times the full load torque
(c) equal to full load torque
Ans: b


60. Short-circuit test on an induction motor cannot be used to determine
(a)     windage losses
(b)     copper losses
(c) transformation ratio
(d) power scale of circle diagram
Ans: a

61. In a three-phase induction motor
(a)     iron losses in stator will be negligible as compared to that in rotor
(6)     iron losses in motor will be neg¬ligible as compared to that in rotor
(c) iron losses in stator will be less than that in rotor
(d) iron losses in stator will be more than that in rotor
Ans: d

62. In case of 3-phase induction motors, plugging means
(a)     pulling the motor directly on line without a starter
(b)     locking of rotor due to harmonics
(c) starting the motor on load which is more than the rated load
(d) interchanging two supply phases for quick stopping
Ans: d

63. Which is of the following data is required to draw the circle diagram for an induction motor ?
(a)     Block rotor test only
(b)     No load test only
(c) Block rotor test and no-load test
(d) Block rotor test, no-load test and stator resistance test
Ans: d

64. In three-phase induction motors sometimes copper bars are placed deep in the rotor to
(a)     improve starting torque
(b)     reduce copper losses
(c) improve efficiency
(d) improve power factor
Ans: a

65. In a three-phase induction motor
(a) power factor at starting is high as compared to that while running
(b) power factor at starting is low as compared to that while running
(c) power factor at starting in the same as that while running
Ans: b

66. The voice of transformation ratio of an induction motor can be found by
(a)     open-circuit test only
(b)     short-circuit test only
(c) stator resistance test
(d) none of the above
Ans: b

67. The power scale of circle diagram of an induction motor can be found from
(a)     stator resistance test
(6)     no-load test only
(c) short-circuit test only
(d) noue of the above
Ans: c

68. The shape of the torque/slip curve of induction motor is
(a)     parabola
(b)     hyperbola
(c) rectangular parabola
(d) straigth line
Ans: c

69. A change of 4% of supply voltage to an induction motor will produce a change of appromimately
(a)     4% in the rotor torque
(b)     8% in the rotor torque
(c) 12% in the rotor torque
(d) 16% in the rotor torque
Ans: d

70. The stating torque of the slip ring induction motor can be increased by adding
(a)     external inductance to the rotor
(b)     external resistance to the rotor
(c) external capacitance to the rotor
(d) both resistance and inductance to rotor
Ans: b

71. 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
(a) 0.01
(b) 0.02
(c) 0.03
(d) 0.04
Ans: d

72. The complete circle diagram of induetion motor can be drawn with the help of data found from
(a)     noload test
(6)     blocked rotor test
(c) stator resistance test
(d) all of the above
Ans: d

73. In the squirrel-cage induction motor the rotor slots are usually given slight skew
(a)     to reduce the magnetic hum and locking tendency of the rotor
(b)     to increase the tensile strength of the rotor bars
(c) to ensure easy fabrication
(d) none of the above
Ans: a

74. The torque of a rotor in an induction motor under running condition is maximum
(a) at the unit value of slip
(b) at the zero value of slip
(c) at the value of the slip which makes rotor reactance per phase equal to the resistance per phase
(d) at the value of the slip which makes the rotor reactance half of the rotor
Ans: c

75. What will happen if the relative speed between the rotating flux of stator and rotor of the induction motor is zero ?
(a)     The slip of the motor will be 5%
(b)     The rotor will not run
(c) The rotor will run at very high speed
(d) The torque produced will be very large
Ans: b

76. The circle diagram for an induction motor cannot be used to determine
(a) efficiency
(b) power factor
(c) frequency
(d) output
Ans: a

77. Blocked rotor test on induction motors is used to find out
(a)     leakage reactance
(b)     power factor on short circuit
(c) short-circuit current under rated voltage
(d) all of the above
Ans: d

78. Lubricant used for ball bearing is usually
(a) graphite
(b) grease
(c) mineral oil
(d) molasses
Ans: b

79. An induction motor can run at synchronous speed when
(a)     it is run on load
(b)     it is run in reverse direction
(c) it is run on voltage higher than the rated voltage
(d) e.m.f. is injected in the rotor circuit
Ans: d

80. Which motor is preferred for use in mines where explosive gases exist ?
(a)     Air motor
(b)     Induction motor
(c) D.C. shunt motor
(d) Synchronous motor
Ans: a

81. The torque developed by a 3-phase induction motor least depends on
(a)     rotor current
(b)     rotor power factor
(c) rotor e.m.f.
(d) shaft diameter
Ans: d

82. In an induction motor if air-gap is increased
(a) the power factor will be low
(b) windage losses will be more
(c) bearing friction will reduce
(d) copper loss will reduce In an induction motor
Ans: a

83. In induction motor, percentage slip depends on
(a)     supply frequency
(b)     supply voltage
(c) copper losses in motor
(d) none of the above
Ans: c

84. When /?2 is tne rotor resistance, .X2 the
rotor reactance at supply frequency
and s the slip, then the condition for maximum torque under running condi-tions will be
(a) sR2X2 = 1 (b) sR2 = X2
(c) R2 = sX2 id) R2 = s2X2
Ans: c


85. In case of a double cage induction motor, the inner cage has
(a) high inductance arid low resistance
(b) low inductance and high resistance
(c) low inductance and low resistance
(d) high inductance and high resis¬tance
Ans: a

86. The low power factor of induction motor is due to
(a)     rotor leakage reactance
(b)     stator reactance
(c) the reactive lagging magnetizing current necessary to generate the magnetic flux
(d) all of the above
Ans: d

87. Insertion of reactance in the rotor circuit
(a)     reduces starting torque as well as maximum torque
(b)     increases starting torque as well as maximum torque
(c) increases starting torque but maxi-mum torque remains unchanged
(d) increases starting torque but maxi-mum torque decreases
Ans: a

88. Insertion of resistance in the rotcir of an induction motor to develop a given torque
(a)     decreases the rotor current
(b)     increases the rotor current
(c) rotor current becomes zero
(d) rotor current rernains same
Ans: d

89. For driving high inertia loods best type of induction motor suggested is
(a) slip ring type
(b) squirrel cage type
(c) any of the above
(d) none of the above
Ans: a

90. Temperature of the stator winding of a three phase induction motor is obtained by
(a)     resistance rise method
(b)     thermometer method
(c) embedded temperature method
(d) all above methods
Ans: d

91. The purpose of using short-circuit gear is
(a)     to short circuit the rotor at slip rings
(b)     to short circuit the starting resis¬tances in the starter
(c) to short circuit the stator phase of motor to form star
(d) none of the above
Ans: a

92. In a squirrel cage motor the induced e.m.f. is
(a) dependent on the shaft loading
(b) dependent on the number of slots
(c) slip times the stand still e.m.f. induced in the rotor
(d) none of the above
Ans: c

93. Less  maintenance  troubles  are  experienced in case of
(a) slip ring induction motor
(6) squirrel cage induction motor
(c) both (a) and (b)
(d) none of the above
Ans: b

94. A squirrel cage induction motor is not selected when
(a) initial cost is the main consideration
(b) maintenance cost is to be kept low
(c) higher starting torque is the main consideration
(d)    all above considerations are involved
Ans: c

95. Reduced voltage starter can be used with
(a) slip ring motor only but not with squirrel cage induction motor
(b) squirrel cage induction motor only but not with slip ring motor
(c) squirrel cage as well as slip ring induction motor
(d) none of the above
Ans: c

96. Slip ring motor is preferred over squirrel cage induction motor where
(a)     high starting torque is required
(b)     load torque is heavy
(c) heavy pull out torque is required
(d) all of the above
Ans: a

97. In a star-delta starter of an induction motor
(a) resistance is inserted in the stator
(b) reduced voltage is applied to the stator
(c) resistance is inserted in the rotor  
(d) applied voltage per1 stator phase is 57.7% of the line voltage
Ans: d

98. The torque of an induction motor is
(a) directly proportional to slip
(b) inversely proportional to slip
(c) proportional to the square of the slip
(d) none of the above
Ans: a

99. The rotor of an induction motor runs at
(a) synchronous speed
(b) below synchronous speed
(c) above synchronous speed
(d)     any of the above
Ans: b

100. The starting torque of a three phase induction motor can be increased by
(a) increasing slip
(b) increasing current
(c) both (a) and (b)
(d) none of the above
Ans: c

101. Insertion of resistance in the stator of an induction motor
(a) increases the load torque
(b) decreases the starting torque
(c) increases the starting torque
(d) none of the above
Ans: b

Latest Polyphase Induction Motors Objective type Questions for freshers

50 TOP Single Phase Induction Motors Interview Questions and Answers pdf downoad

Real Time Single Phase INDUCTION MOTOR Interview Questions with Answers PDF

=> Also Read Induction Motors MCQs

1. Please Name some Single-phase induction motors?
(i) split-phase type
(ii) capacitor type
(iii) shaded-pole type

2. Please name two types of Repulsion motors?
(i) Repulsion-start induction-run motor
(ii) Repulsion-induction motor

3. What are different types of single phase Synchronous motors?
(i) Reluctance motor
(ii) Hysteresis motor

Single Phase Induction motor Questions and Answers

4. How a single phase motor is similar to 3 phase motor?
A single phase induction motor is very similar to a 3-phase squirrel cage induction motor. It has
(i) a squirrel-cage rotor identical to a 3-phase motor
(ii) a single-phase winding on the stator

5. What is major difference between a 3-phase induction motor and a single-phase induction motor?
Unlike a 3-phase induction motor, a single-phase induction motor is not self- starting but requires some starting means.

6. Why single phase induction motor does not self starts?
The single-phase stator winding produces a magnetic field that pulsates in strength in a sinusoidal manner. The field polarity reverses after each half cycle but the
field does not rotate. Consequently, the alternating flux cannot produce rotation in a stationary squirrel-cage rotor.

7. What is basic method to start single phase induction motor and why it is not preferred?
If the rotor of a single-phase motor is rotated in one direction by some mechanical means, it will continue to run in the direction of rotation. As a matter of fact, the rotor quickly accelerates until it reaches a speed slightly below the synchronous speed. Once the motor is running at this speed, it will continue to rotate even though single-phase current is flowing through the stator winding. This method of starting is generally not convenient for large motors. Nor can it be employed fur a
motor located at some inaccessible spot.

8. Explain why single phase induction motor does not start in the light of Double-Field Revolving Theory?
The double-field revolving theory is proposed to explain this dilemma of no torque at start and yet torque once rotated. This theory is based on the fact that an alternating sinusoidal flux can be represented by two revolving fluxes, each equal to one-half of the maximum value of alternating flux (i.e., m/2) and each rotating at synchronous speed in opposite directions.
Therefore, an alternating 􀃗eld can be replaced by two relating fields of half its amplitude rotating in opposite directions at synchronous speed. Note that the resultant vector of two revolving 􀃘ux vectors is a stationary vector that oscillates in length with time along X-axis.

9. What is the position of rotor at standstill?
Consider the case that the rotor is stationary and the stator winding is connected to a single-phase supply. The alternating 􀃘ux produced by the stator winding can be presented as the sum of two rotating 􀃘uxes 1 and 2, each equal to one half of the maximum value of alternating 􀃘ux and each rotating at synchronous speed (Ns = 120 f/P) in opposite directions.
At standstill, these two torques are equal and opposite and the net torque developed is zero. Therefore, single-phase induction motor is not self-starting.


10. What is the value of slip at standstill?
Note that each rotating field tends to drive the rotor in the direction in which the field rotates. Thus the point of zero slip for one field corresponds to 200% slip for the other . The value of 100% slip (standstill condition) is the same for both the fields.

11. Explain motor action from standstill to running condition?
At standstill, s = 1 so that impedances of the two circuits are equal. Therefore, rotor currents are equal i.e., I2f = I2b. However, when the rotor rotates, the impedances of the two rotor circuits are unequal and the rotor current I2b is higher (and also at a lower power factor) than the rotor current I2f. Their m.m.f.s, which oppose the stator m.m.f.s, will result in a reduction of the backward rotating 􀃘ux. Consequently, as speed increases, the forward flux increases, increasing the driving torque while the backward 􀃘ux decreases, reducing the opposing torque. The motor-quickly accelerates to the 􀃗nal speed.

12. How to make Single-Phase Induction Motor Self-Starting?
The single-phase induction motor is not self- starting and it is undesirable to resort to mechanical spinning of the shaft or pulling a belt to start it. To make a singlephase induction motor self-starting, we should somehow produce a revolving stator magnetic 􀃗eld. This may be achieved by converting a single-phase supply into two-phase supply through the use of an additional winding. When the motor attains suf􀃗cient speed, the starting means (i.e., additional winding) may be removed depending upon the type of the motor.

13. Name the types of single phase motors with respect to the method employed to make them self-starting?
(i) Split-phase motors-started by two phase motor action through the use of an auxiliary or starting winding.
(ii) Capacitor motors-started by two-phase motor action through the use of an auxiliary winding and a capacitor.
(iii) Shaded-pole motors-started by the motion of the magnetic 􀃗eld produced by means of a shading coil around a portion of the pole structure.

14. How to produce Rotating Magnetic Field From 2-Phase Supply?
As with a 3-phase supply, a 2-phase balanced supply also produces a rotating magnetic 􀃗eld of constant magnitude. With the exception of the shaded-pole motor, all single-phase induction motors are started as 2-phase machine. Once so started, the motor will continue to run on single-phase supply.

15. What is the major cause of noisy operation of the single phase induction motor?
If the two windings arc displaced 90° electrical but produce fields that are not equal and that are not 90° apart in time, the resultant field is still rotating but is not constant in magnitude. One effect of this non uniform rotating field is the production of a torque that is non-uniform and that, therefore, causes noisy operation of the motor. Since 2-phase operation ceases once the motor is started, the operation of the motor then becomes smooth.

16. Describe the principal of operation of Split-Phase Induction Motor?
The stator of a split-phase induction motor is provided with an auxiliary or starting winding S in addition to the main or running winding M. The starting winding is located 90° electrical from the main winding and operates only during the brief period when the motor starts up. The two windings are so resigned that the starting winding S has a high resistance and relatively small reactance while the main winding M has relatively low resistance and large reactance as shown in the schematic
connections in Fig. Consequently, the currents flowing in the two windings have reasonable phase difference c (25° to 30°) as shown in the phasor diagram in Fig.

When the two stator windings are energized from a single-phase supply, the main winding carries current Im while the starting winding carries current Is. Since main winding is made highly inductive while the starting winding highly resistive, the currents Im and Is have a reasonable phase angle a (25° to 30°) between them as shown in Fig. Consequently, a weak revolving field approximating to that of a 2-phase machine is produced which starts the motor. where k is a constant whose magnitude depends upon the design of the motor.
When the motor reaches about 75% of synchronous speed, the centrifugal switch opens the circuit of the starting winding. The motor then operates as a singlephase induction motor and continues to accelerate till it reaches the normal speed. The normal speed of the motor is below the synchronous speed and depends upon the load on the motor.

17. What is starting torque and starting current of split phase induction motor?
The spinning torque is 15 to 2 times the full-loud torque mid (lie starting current is 6 to 8 times the full-load current.

18. Why split-phase induction motors are most popular single- phase motors in the market?
Due to their low cost, split-phase induction motors are most popular single- phase motors in the market.

19. Why built-in-thermal relay is necessary in split phase induction motor and what is its purpose?
Since the starting winding is made of 􀃗ne wire, the current density is high and the winding heats up quickly. If the starting period exceeds 5 seconds, the winding may burn out unless the motor is protected by built-in-thermal relay. This motor is, therefore, suitable where starting periods are not frequent

20. What is speed variation of split phase induction motor from no load to full load?
An important characteristic of these motors is that they are essentially constant-speed motors. The speed variation is 2-5% from no-load to full- load.

21. Usually where split phase induction motors are used?
These motors are suitable where a moderate starting torque is required and where starting periods are infrequent e.g., to drive:
(a) fans
(b) washing machines
(c) oil burners
(d) small machine tools etc.
The power rating of such motors generally lies between 60 W and 250 W.

22. Where do we require single-phase induction motors?
Single-phase induction motors are required where (i) 3 phase supply is not available (ii) efficiency is of lesser importance (iii) Rating is less than one H.P. (iv) Equipment is portable

23. Why is the power factor of a single-phase induction motor low?
It is due to the large magnetizing current which ranges from 60% to 70% of full-load current. As a result, even at no-load, these motors reach temperatures close to the full-load temperature.

24. What is the function of centrifugal starting switch in a single-phase induction motor?
The centrifugal switch is connected in series with the starting winding. The primary function of the centrifugal switch is to produce rotating flux in conjunction with main winding at the time of starting. When the motor has started and reaches nearly 75% of synchronous speed, it produces its own rotating field from the cross field effect. The starting winding now has no function to perform and is removed from the circuit by a centrifugally operated switch.

25. What happens when the centrifugal starting switch fails to open?
If the starting switch fails to open when needed, then the starting winding will overheat and burn out and motor will not start next time.

26.  What happens when the centrifugal switch fails to close when needed?
If the centrifugal starting switch fails to close, the motor will overheat the main winding without any failure of the main winding.

27.  Why are resistance split-phase inductions motors most popular?
These motors are most popular due to their low cost. They are used where moderate starting torque is required and where the starting periods are not frequent. They drive fans, pumps, washing machines, small machine tools etc. They have power ranting between 60 watts and 250 watts.

28. What is the draw back of the resistance split-phase induction motor?
The starting winding has a relatively small number of turns of fine wire and its resistance is higher than that of the main winding. Therefore the current density is high and the winding heats up quickly. If the starting period lasts for more than 5 seconds, the winding begins to smoke and may burn out unless the motor is protected by a built-in-thermal relay.

29.  Why is the starting torque of a resistance split-phase induction motor not high?
The starting torque is given as, Ts = K Im Is Sin Ф Where K = constant whose magnitude depends upon the design of the motor (i) The angle between Is and Im is small (approximately 25 degree) in a resistance split-phase induction motor, so the starting torque is small. (ii) Since currents Is and Im are not equal in magnitude, the rotating magnetic field is not uniform and the starting torque produced is small.

30. Why is the starting torque of a capacitor start induction motor high?
The capacitor C in the starting winding is so chosen that Is leads Im by 75 degree. Since the starting torque is directly proportional to Sin Ф, and it is quite high in capacitor-start induction motor.

31. Why do we use capacitor-start induction motors in applications requiring high starting torque in preference to repulsion induction motors?
Capacitors are easily available, cheaper and reliable. Repulsion-induction motors posses a special commutator and brushes that require maintenance. Most manufacturers have stopped making them.

32. What is the principle of operation of shaded-pole induction motor?
A shaded-pole motor is basically a small single-phase squirrel cage motor in which the starting winding is composed of short-circuited copper ring (called shading coil) surrounding one-third of each pole. The effect of the shading coil is to cause a flux to sweep across the pole faces, from unshaded to shaded portion of the pole, producing a weak rotating magnetic field. As a result, the rotor is set in motion due to induction principle.

33. Which type of torque is developed in single phase motors?
Pulsating torque is produced.

34.  If a single phase motor is driven in any direction by any means, it starts running in that direction. Explain why?
Actually a pulsating torque has two components which are equal in magnitude and rotate in opposite direction with synchronous speed at unity slip. Now if the motor rotates in any direction, the slip decreases and the torque component in this direction increases than the other component and hence motor runs in that direction.

35. What is a fractional H.P. motor?
A small motor having H.P. less than unit is called fractional H.P. motor.

36. How the starting winding produce rotation in a single phase resistance start induction motor?
The starting winding is highly resistive and the main winding is inductive. So the phase difference between the two currents becomes nearly 90 degree and hence the motor start as two phase motor.

37.  How the starting winding is made resistive?
It consists of only few turns of smaller diameter.

38. How the speed of rotation of a split phase induction motor is reversed?
The terminal connections of the starting windings are reversed with respect to main running windings.

39. What will happen if the centrifugal switch fails to open the starting winding?
Excessive heat will be produced due to high resistance of the starting winding due to which stator temperature will rise and eventually both windings will burn.

40. How speed control is made in single phase motors? 
It is usually controlled by applying a variable voltage from tapped transformers, variacs, potentio meters, and tapped reactors.

41. Is there any relation between the capacitances of two capacitors used in two value capacitor motor? 
Starting capacitor has about 10 – 15 times high capacity than the value of running capacitor.

42. What is size of shaded-pole motor? 
These are usually built in small fractional H.P, not exceed 1/4 H.P.

43. Why shaded-pole single phase induction motor does not need any special starting technique like capacitors and auxiliary winding etc. 
Because it is inherently self started motor. The construction of the poles is such that they give a sweep to the magnetic flux and motor starts rotating.

44. How can a universal motor be reversed? 
By reversing either the field leads or armature leads but not both.

45. What are applications of Stepper motors? 
(i) Paper feed motors in typewriters and printers
(ii) Positioning of print heads
(iii) Pens in XY-plotters
(iv) Recording heads in computer disc drives etc.

46. Why do we use capacitor-start induction motors in applications requiring high starting torque in preference to repulsion induction motors? 
Capacitors are easily available, cheaper and reliable. Repulsion-induction motors posses a special commutator and brushes that require maintenance. Most manufacturers have stopped making them.

47. If a single phase motor is driven in any direction by any means, it starts running in that direction. Explain why? 
Actually a pulsating torque has two components which are equal in magnitude and rotate in opposite direction with synchronous speed at unity slip. Now if the motor rotates in any direction, the slip decreases and the torque component in this direction increases than the other component and hence motor runs in that direction.

48. What is a fractional H.P. motor? 
A small motor having H.P. less than unit is called fractional H.P. motor.

49. Which type of rotor is used in single phase motors? 
Squirrel cage type

50. How the starting winding produce rotation in a single phase resistance start induction motor? 
The starting winding is highly resistive and the main winding is inductive. So the phase difference between the two currents becomes nearly 90 degree and hence the motor start as two phase motor. 

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1. What is he meant of induction motor ?
Ans. An induction motor derives its name from the current in the rotor conductor is induced by the motion of rotor conductor relative to the magnetic field developed by the stator currents.

2. What are the two type of induction motor and which is usually preferred ?
A. I- Squirrel cage induction motor
    II-Wound rotor/slip-ring motor
  Squirrel cage induction motor is generally preferred.

3. Why the induction motor is called asynchronous motor ?
A. Because its run at a speed other than the synchronous speed of rotating magnate field developed by the starter current.

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4. Why the air gap b/w the stator core and rotor of an induction motor is made very small ?
A. the air gap is made as small as possible so as to produced required flux with a minimum exciting current and give leakage reluctance as small as possible. this result in improved power factor.

5. Why the no. of poles of the stator and of the rotor of an ac motor should be equal ?
A. The flux set up by the stator winding is a rotating flux with a definite no. of pole ; operation is only possible if the no. of poles are same in both.

6. Why the exciting current of induction motor is so high as compared to the power transformer?
Unlike that of a power transformer, the magnetic circuit of the induction motor has an air-gap. Therefore, the exciting current of induction motor (3O to 4O% of full-load current) is much higher than that of the power transformer. Consequently, the exact equivalent circuit must be used for accurate results.

7. How transformation ratio of induction motor is different as compared to power transformer?
In a transformer, the windings are concentrated whereas in an induction motor, the windings are distributed. This affects the transformation ratio.

8. Why it is required to start 3-Phase Induction Motors? 
The induction motor is fundamentally a transformer in which the stator is the primary and the rotor is short-circuited secondary. At starting,
the voltage induced in the induction motor rotor is maximum ( s = 1).
Since the rotor impedance is low, the rotor current is excessively large. This large rotor current is reflected in the stator because of
transformer action. This results in high starting current (4 to 1O times the full-load current) in the stator at low power factor and consequently the value of starting torque is low. Because of the short duration, this value of large current does not harm the motor if the motor accelerates normally.

9. What is the effect of staring of induction motor on connected line?
Large starting current will produce large line-voltage drop. This will adversely affect the operation of other electrical equipment connected
to the same lines. Therefore, it is desirable and necessary to reduce the magnitude of stator current at starting and several methods are
available for this purpose.


10. Please describe the Methods of Starting 3-Phase Induction Motors? 
The common methods used to start induction motors are:
(i) Direct-on-line starting
(ii)    Stator resistance starting
(iii) Autotransformer starting
(iv)    Star-delta starting
(v) Rotor resistance starting

11. How slip ring motors are started? 
Slip ring motors are invariably started by rotor resistance starting.

12. What is Direct-on-line starting of induction motor? 
This method of starting in just what the name implies the motor is started by connecting it directly to 3-phase supply. The impedance of
the motor at standstill is relatively low and when it is directly connected to the supply system, the starting current will be high (4 to
1O times the full-load current) and at a low power factor. Consequently, this method of starting is suitable for relatively small (up to 7.5 kW)
machines.

13. Does starting torque is higher than full load torque? 
No, starting current is as large as five times the full-load current but starting torque is just equal to the full-load torque. Therefore,
starting current is very high and the starting torque is comparatively low. If this large starting current flows for a long time, it may
overheat the motor and damage the insulation.

14. What is Stator resistance starting method? 
In this method, external resistances are connected in series with each phase of stator winding during starting. This causes voltage drop across
the resistances so that voltage available across motor terminals is reduced and hence the starting current. The starting resistances are gradually cut out in steps (two or more steps) from the stator circuit as the motor picks up speed. When the motor attains rated speed, the resistances are completely cut out and full line voltage is applied to the rotor.

15. Why Stator resistance starting is not recommended?
This method suffers from two drawbacks. First, the reduced voltage applied to the motor during the starting period lowers the starting torque and hence increases the accelerating time. Secondly, a lot of power is wasted in the starting resistances. Therefore, this method is used for starting small motors only.

16. What are the advantages of Autotransformer starting?
Autotransformer starting has several advantages like low power loss, low starting current and less radiated heat. For large machines (over 25
H.P.), this method of starting is often used. This method can be used for both star and delta connected motors.

17. What are the advantages of slip-ring induction motors over the squirrel cage motors?
(i) High starting torque with low starting current.
(ii) Smooth acceleration under heavy loads.
(iii) No abnormal heating during starting.
(iv) Good running characteristics after external rotor resistances are cut out.
(v) Adjustable speed

18. Is there any disadvantage of slip-ring motors?/*
(i) The initial and maintenance costs are greater than those of squirrel cage motors.
(ii) The speed regulation is poor when run with resistance in the rotor circuit

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1. How does the Induction motor work? (OR) Why does the Rotor rotate?
When the 3 phase stator windings are fed by 3 phase supply, a magnetic flux of constant magnitude which is rotating at synchronous speed is set up.
The flux passes through the air-gap and sweeps past the rotor surface  thus it cuts the rotor conductors.
Due to the relative speed between the rotating flux and the stationary rotor conductors, an emf is induced in the stationary rotor conductors as per the Faraday's laws of electromagnetic induction.
The frequency of the induced emf is the same as the supply frequency.
Its magnitude is proportional to the relative velocity between the flux and the conductors.
Its direction will be as per Fleming's right hand rule.
Since the rotor conductors form a closed circuit the rotor current is produced.
This current's direction will oppose the very cause producing it ( as per Len's law)
Here the cause is the relative velocity between the rotating flux of the stator and the stationary rotor conductors.
Hence, in order to reduce the relative speed, the rotor begins to rotate in the same direction as that of the rotating magnetic flux and tries to catch up with the rotating flux. Thus the rotor of induction motor starts to rotate.

2. What is the general working principle of Induction motor?
The conversion of electrical power into mechanical power takes place in the rotating part of an electric motor.
In DC motors the electrical power is conducted directly to the armature through brushes and commutator.
Thus the DC motor can be called as conduction motor. But in case of AC motors, the rotor receives electric power, not by conduction but by induction.
This is exactly in the same way as the secondary of two winding transformer receives its power from the primary.
That is why such motors are known as induction motors.
Thus an induction motor is also known as rotating transformer ( ie, one in which primary winding is stationary and the secondary is free to rotate)

Induction Motors Interview Questions

3. What is the advantage of skewed stator slots in the rotor of Induction motors?
In the induction motor design, the rotor slots are purposely made with a slight skew arrangement. It will not be parallel to the shaft.
This is for the purpose of reducing magnetic locking or reducing magnetic attraction between stator and rotor teeth.
In addition to that this arrangement will help to reduce the magnetic hum and noise.

4. What is meant by cogging in the Induction motor? How to prevent the cogging?
When the number of teeth in stator and rotor are equal, the stator and rotor teeth have a tendency to align themselves exactly opposite to each other, since this corresponds to minimum reluctance position. In such case the rotor may refuse to accelerate. This phenomenon is called as magnetic locking or cogging.
This problem can be prevented by proper choice of stator and rotor slots and also by skewing the rotor slots by one slot pitch.

5. What are the various methods of measuring slip?
1. By actual measurement of rotor speed
2. By measurement of rotor frequency
3. Stroboscopic method

6. What are the various methods of speed control in three phase induction motors?
(i) Control from stator side
1. By changing the supply frequency
2. By changing the number of stator poles
3. By changing the supply voltage
(ii) Control from rotor side
1. By inserting resistance in rotor circuit
2. By various ways of cascade connection
3. By injecting EMFs in the rotor circuit.

7. What is meant by crawing in the induction motor?
In induction motors, particularly squirrel cage type induction motors, sometimes exhibit a tendency to run stably at speeds as low as one-seventh of their synchronous speed Ns. This phenomenon is known as crawling of an induction motor and the speed is called as crawling speed.


8. What is an Induction Motor?
An induction motor (IM) is a type of asynchronous AC motor where power is supplied to the rotating device by means of electromagnetic induction.

9. What is an Electric Motor?
An Electric Motor converts electrical power to mechanical power in its rotor.

10. How to supply power to rotor?
In a DC motor this power is supplied to the armature directly from a DC source, while in an AC motor this power is induced in the rotating device.

11. Why an Induction Motor sometimes called Rotating transformer?
An induction motor is sometimes called a rotating transformer because the stator (stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side.

12. Who invented Induction Motor?
Nikola Tesla.

13. What is the basic difference between Synchronous motor and an Induction Motor?
The basic difference between an induction motor and a synchronous AC motor is that in the latter a current is supplied onto the rotor. This then creates a magnetic field which, through magnetic interaction, links to the rotating magnetic field in the stator which in turn causes the rotor to turn. It is called synchronous because at steady state the speed of the rotor is the same as the speed of the rotating magnetic field in the stator.

14. Why stator windings are arranged around the rotor?
the induction motor does not have any direct supply onto the rotor; instead, a secondary current is induced in the rotor. To achieve this, stator windings are arranged around the rotor so that when energised with a polyphase supply they create a rotating magnetic field pattern which sweeps past the rotor. This changing magnetic field pattern can induce currents in the rotor conductors. These currents interact with the rotating magnetic field created by the stator and the rotor will turn.

15. Why the speed of the physical rotor and the speed of the rotating magnetic field in the stator must be different?
the speed of the physical rotor and the speed of the rotating magnetic field in the stator must be different, or else the magnetic field will not be moving relative to the rotor conductors and no currents will be induced.

16. What is the SLIP?
This difference between the speed of the rotor and speed of the rotating magnetic field in the stator is called slip. It is unitless and is the ratio between the relative speed of the magnetic field as seen by the rotor to the speed of the rotating field. Due to this an induction motor is sometimes referred to as an asynchronous machine.

17. How many types of Induction Motor ?
Based on type of phase supply
Three phase induction motor (self starting in nature)
Single phase induction motor (not self starting)
Other
Squirrel-cage induction motor
Slip ring induction motor

18. What is the relationship between the supply frequency number of poles and sycnchronous speed?
f = Pns/2

19. What is a Synchronous Speed?
\mbox{Synchronous speed, }n_s = \frac{120f}{P}\quad[\mbox{rev/min}]

20. What is a Rotor Speed?
Rotor speed, nr = ns(1-s)

21. How to calculate the Slip?
Slip is calculated using:

s = [ns-nr]/ns

22. What is a Stator?
The stator consists of wound 'poles' that carry the supply current that induces a magnetic field in the conductor. The number of 'poles' can vary between motor types but the poles are always in pairs (i.e. 2, 4, 6, etc.).

23. How many types of Rotor are there?
There are two types of rotors
Squirrel Cage rotor.
and
Slip Ring rotor.

24. What techniques is used to produce a desired Speed?
The most commonly used technique is Pulse Width Modulation in which a DC signal is switched on and off very rapidly, producing a sequence of electrical pulses to the inductor windings.

25. What is the difference between DC motors and the Induction motors?
The induction motor has no brushes and is easy to control, many older DC motors are being replaced with induction motors and accompanying inverters in industrial applications.

26. How an Induction Motor is started? Why the Starter is Used?
When the motor is started, the slip is equal to 1 as the rotor speed is zero, so the induced emf in the rotor is large. As a result, a very high current flows through the rotor. This is similar to a transformer with the secondary coil short circuited, which causes the primary coil to draw a high current from the mains. Similarly, when an induction motor starts, a very high current is drawn by the stator, on the order of 5 to 9 times the full load current. This high current can damage the motor windings and because it causes heavy line voltage drop, other appliances connected to the same line may be affected by the voltage fluctuation. To avoid such effects, the starting current should be limited. A soft start starter is a device which limits the starting current by providing reduced voltage to the motor. Once the rotor speed increases, the full rated voltage is given to it.

27. What is a Rotor?
The rotor is the non-stationary part of a rotary electric motor or alternator, which rotates because the wires and magnetic field of the motor are arranged so that a torque is developed about the rotor's axis. In some designs, the rotor can act to serve as the motor's armature, across which the input voltage is supplied. The stationary part of an electric motor is the stator. A common problem is called cogging torque.

28. What is a Stator?
The stator is the stationary part of an electric generator or electric motor. The non-stationary part on an electric motor is the rotor.
Depending on the configuration of a spinning electromotive device the stator may act as the field magnet, interacting with the armature to create motion, or it may act as the armature, receiving its influence from moving field coils on the rotor.

29. What is Commutator?
A commutator is an electrical switch that periodically reverses the current direction in an electric motor or electrical generator. A commutator is a common feature of direct current rotating machines. By reversing the current direction in the moving coil of a motor's armature, a steady rotating force (torque) is produced. Similarly, in a generator, reversing of the coil's connection to the external circuit produces unidirectional current in the circuit. The first commutator-type direct current machine was built by Hippolyte Pixii in 1832,

30. What is an Armature?
An armature is one of the two principal electrical components of an electromechanical machine--a motor or generator. The other is the field winding, field magnet. The role of the "field" component is simply to create a magnetic field (magnetic flux) for the armature to interact with, so this component can comprise either permanent magnets, or electromagnets formed by a conducting coil. The armature, in contrast, must carry current so it is always a conductor or a conductive coil, oriented normal to both the field and to the direction of motion, torque (rotating machine), or force (linear machine). The armature's role is two-fold: (a) to carry current crossing the field, thus creating shaft torque (in a rotating machine) or force (in a linear machine), and (b) to generate an electromotive force ("EMF").

31. What is a Cogging Torque?
Cogging torque of electrical motors is the torque due to the interaction between the permanent magnets of the rotor and the stator slots of a Permanent Magnet (PM) machine. Also termed as detent or 'no-current' torque, it is an undesirable component for the operation of such a motor. It is especially prominent at lower speeds, with the symptom of jerkiness.

32. What are the advantages and disadvantages of Induction Motors?
Advantages:
It is simple and rugged in construction
It is relatively cheap
Induction motors require less maintenance
Induction motor has high efficiency and reasonably good power factor
3-phase induction machines are self starting

Disadvantages:
Induction motors are essentially a constant speed motor and its speed cannot be changed easily
Its starting torque is inferior to dc shunt motor

33. What is the condition for maximum torque in induction motor?
Starting torque will be maximum when the rotor resistance / phase is equal to standstill rotor reactance / phase

34. Slip ring induction motor advantages and disadvantages compared to squirrel cage motors?
Answer:
Advantages:
High starting torque with low starting current
Smooth acceleration under heavy loads
No abnormal heating during starting
Good running characteristics after external rotor resistances are cut out
Adjustable speed
Disadvantages:
The initial and maintenance costs are greater than those of squirrel cage motors
The speed regulation is poor when run with resistance in the rotor circuit

35. Methods to control speed of Wound Rotor Motors?
Answer: The speed of wound rotor motors are changed by changing the slip of the motor. This can be achieved by:
Varying the stator line voltage
Varying the resistance in the rotor circuit
Inserting and varying a foreign voltage source in the rotor circuit

36. Explain how Torque-Slip Characteristics vary when adding resistance to rotor circuit?
Answer: The addition of resistance to the rotor circuit does not change the value of maximum torque but it only changes the value of the slip at which the maximum torque occurs

37. Disadvantages of Star-Delta Starting of Induction motor?
Answer: In Star-Delta starting induction motor stator is connected in star connection for starting after picking up speed it is connected to delta connection. When induction motor is connected in star connection stator phase voltage reduced by 1/(31/2 ) times the line voltage. This also results in reduced starting torque (1/3 times compared to delta connection). 

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