In this article, we shared the polyphase induction motor multiple choice questions and answers for electrical and electronics engineers.
Polyphase Induction Motor Multiple Choice Questions
The below list provides all the objective questions related to the electrical polyphase induction motor.
Answers are available at the bottom of the article.
1. The reversing of the 3-phase A.C. motor is achieved by
| A) | an auto-transformer |
| B) | star-delta starter |
| C) | interchanging any two of the supply lines |
| D) | D.O.L. starter |
2. The starting torque of a three phase induction motor can be increased by
| A) | none of these answers |
| B) | both increasing slip and increasing current |
| C) | increasing slip |
| D) | increasing current |
3. Rotor rheostat control method of speed control is used for
| A) | none of these answers |
| B) | squirrel-cage induction motors only |
| C) | slip ring induction motors only |
| D) | both squirrel-cage induction motors only and slip ring induction motors only |
4. The stator of 5 H.P. induction motor is provided with
| A) | open slots with parallel teeth |
| B) | open slots with tapered teeth |
| C) | closed slots with parallel teeth |
| D) | semiclosed slots with parallel teeth |
5. In a 3-phase induction motor, iron loss
| A) | in rotor is negligible as compared to that in stator |
| B) | in rotor is equal to iron loss is stator |
| C) | in stator does not occur |
| D) | in rotor is much more than the iron loss in stator |
6. Speed variations of a squirrel cage induction motor are essentially similar to those of
| A) | D.C. shunt motor |
| B) | differential compound motor |
| C) | synchronous motor |
| D) | D.C. series motor |
7. The efficiency of an induction motor can be expected to be nearly
| A) | 80 to 90% |
| B) | 0.99 |
| C) | 95 to 98% |
| D) | 60 to 90% |
8. The maximum torque in an induction motor depends on
| A) | square of supply voltage |
| B) | all of these answers |
| C) | rotor inductive reactance |
| D) | frequency |
9. The starting torque of a3-phase squirrel cage induction motor is
| A) | twice the full load torque |
| B) | 1.5 times the full load torque |
| C) | none of these answers |
| D) | equal to full load torque |
10. If stator voltage of a squirrel cage induction motor is reduced to one-half its normal value, its starting current is reduced to____ percent of its full-voltage value.
| A) | 15 |
| B) | 75 |
| C) | 50 |
| D) | 60 |
11. The speed characteristics of an induction motor closely resemble the speedload characteristics of which of the following machines
| A) | D.C. shunt motor |
| B) | D.C. series motor |
| C) | none of these answers |
| D) | universal motor |
12. The low power factor of induction motor is due to
| A) | the reactive lagging magnetizing current necessary to generate the magnetic flux |
| B) | stator reactance |
| C) | all of these answers |
| D) | rotor leakage reactance |
13. The rotor of an induction motor runs at
| A) | above synchronous speed |
| B) | any of these answers |
| C) | below synchronous speed |
| D) | synchronous speed |
14. Under which method of starting an induction motor is expected to take the largest starting current?
| A) | Stator-rotor starting |
| B) | Star-delta starting |
| C) | Auto-transformer starting |
| D) | Direct online starting |
15. If the stator voltage of a squirrel cage induction motor is reduced to 50 percent of its rated value, the torque developed is reduced by____ percent of its full load value.
| A) | 90 |
| B) | 50 |
| C) | 75 |
| D) | 25 |
16. Out of the following methods of starting a 3-phase induction motor, which one requires six stator terminals?
| A) | Auto-transformer |
| B) | Rotor rheostat |
| C) | Direct-on-line |
| D) | Star-delta |
17. The ‘crawling’ in an induction motor is caused by
| A) | improper design of machine |
| B) | high loads |
| C) | low voltage supply |
| D) | harmonics developed in the motor |
18. The torque developed by a 3-phase induction motor least depends on
| A) | rotor current |
| B) | rotor e.m.f. |
| C) | rotor power factor |
| D) | shaft diameter |
19. In case single phasing occurs in delta connected motor
| A) | two phases will be seriously overloaded and there will be current in the third phase |
| B) | there will be no current in one phase |
| C) | there will be no current in two phases |
| D) | one phase will be seriously overloaded and two others will be slightly overloaded |
20. The stator frame in an induction motor is used
| A) | as a return path for the flux |
| B) | to hold the armature stampings/ stator |
| C) | to ventilate the armature |
| D) | to protect the whole machine |
21. An induction motor is
| A) | self-starting with zero torque |
| B) | non-self starting |
| C) | self-starting with high torque |
| D) | self-starting with low torque |
22. The torque of a rotor in an induction motor under running conditions is maximum
| A) | at the unit value of slip |
| B) | at the value of the slip which makes rotor reactance per phase equal to the resistance per phase |
| C) | at the value of the slip which makes the rotor reactance half of the rotor resistance |
| D) | at the zero value of slip |
23. In a 3-phase induction motor which of the following statements, regarding the frequency of induced rotor e.m.f. is incorrect?
| A) | It is directly dependent on slip |
| B) | It is maximum at/standstill |
| C) | It is zero at synchronous speed |
| D) | Its speed varies inversely as slip |
24. In an induction motor, no-load the slip is generally
| A) | 0.01 |
| B) | 0.04 |
| C) | 0.02 |
| D) | less than 1% |
25. Each of the following statements regarding stator flux of an induction motor is true except
| A) | it revolves around the stator at synchronous speed |
| B) | it is constant in magnitude |
| C) | it induces e.m.f. in the rotor bars |
| D) | its magnitude depends on the motor load |
26. Stepless speed control of induction motor is possible by which of the following methods?
| A) | Cascade operation |
| B) | e.m.f. injection in the rotor circuit |
| C) | None of these answers |
| D) | Changing the number of poles |
27. In three-phase squirrel-cage induction motors
| A) | rotor conductor ends are shortcircuited through slip rings |
| B) | rotor conductors are kept open |
| C) | rotor conductors are connected to the insulation |
| D) | rotor conductors are short-circuited through end rings |
28. The purpose of using short-circuit gear is
| A) | to short circuit the stator phase of the motor to form star |
| B) | none of these answers |
| C) | to short circuit the rotor at slip rings |
| D) | to short circuit the starting resistances in the starter |
29. Temperature of the stator winding of a three-phase induction motor is obtained by
| A) | resistance rise method |
| B) | all of these answers |
| C) | embedded temperature method |
| D) | thermometer method |
30. It is advisable to avoid line starting of induction motor and use starter because
| A) | it will run in the reverse direction |
| B) | starting torque is very high |
| C) | it will pick up very high speed and may go out of step |
| D) | motor takes five to seven times its full-load current |
31. A 3-phase induction motor stator delta connected is carrying a full load and one of its fuses blows out. Then the motor
| A) | will continue running burning its two phases |
| B) | will stop and carry heavy current causing permanent damage to its winding |
| C) | will continue running without any harm to the winding |
| D) | will continue running burning its one phase |
32. The primary winding of a Schrage motor is located
| A) | in lower part of rotor |
| B) | partly in stator and partly in rotor |
| C) | in upper part of rotor |
| D) | in stator |
33. Short-circuit test on an induction motor cannot be used to determine
| A) | power scale of circle diagram |
| B) | transformation ratio |
| C) | copper losses |
| D) | windage losses |
34. In squirrel cage induction motors, the rotor slots are usually given slight skew in order to
| A) | reduce magnetic hum |
| B) | reduce accumulation of dirt and dust |
| C) | reduce eddy currents |
| D) | reduce windage losses |
35. The torque developed in an induction motor is nearly proportional to____ (where V = applied voltage)
| A) | 1/V2 |
| B) | 1/V |
| C) | V2 |
| D) | V |
36. Which of the following motors requires the most complicated speed control arrangement?
| A) | D.C. shunt motor |
| B) | Rotor supplied three-phase commutator |
| C) | Three-phase squirrel-cage induction motor |
| D) | Stator supplied three-phase commutator |
37. In medium-sized induction motors, the slip is generally around
| A) | 0.004 |
| B) | 0.0004 |
| C) | 0.14 |
| D) | 0.04 |
38. In a 3-phase induction motor reactance under running conditions is less than its standstill value because of a decrease in
| A) | mutual flux linking the stator and rotor conductor |
| B) | stator magnetic flux |
| C) | rotor inductance |
| D) | frequency of rotor e.m.f. |
39. When an induction motor is switched on the rotor frequency is
| A) | 60 Hz |
| B) | same as slip frequency |
| C) | zero |
| D) | same as supply frequency |
40. It is advisable to avoid line-starting of induction motor and use starter because
| A) | it will run in reverse direction |
| B) | motor takes five to seven times its full load current |
| C) | it will pick-up very high speed and may go out of step |
| D) | starting torque is very high |
41. The noise and tooth pulsation losses can be minimized by using
| A) | small number of open slots in stator |
| B) | small number of narrow slots in stator |
| C) | large number of narrow slots in stator |
| D) | large number of open slots in stator |
42. In a three-phase induction motor, the relative speed of stator flux with respect to____ is zero.
| A) | rotor flux |
| B) | space |
| C) | rotor |
| D) | stator winding |
43. Which of the following methods is easily applicable to control the speed of the squirrel-cage induction motor?
| A) | By injecting e.m.f. in the rotor circuit |
| B) | Rotor rheostat control |
| C) | By changing the number of stator poles |
| D) | By operating two motors in cascade |
44. Which of the following A.C. motors is used for industrial application?
| A) | D.C. series motor |
| B) | Commutator motor |
| C) | Synchronous motor |
| D) | 3-phase induction motor |
45. In an induction motor, the rotor reactance per phase is proportional to____ (where s is the slip)
| A) | 1/s2 |
| B) | s |
| C) | s2 |
| D) | 1/s |
46. What will happen if the relative speed between the rotating flux of the stator and rotor of the induction motor is zero?
| A) | The rotor will not run |
| B) | The rotor will run at very high speed |
| C) | The slip of the motor will be 5% |
| D) | The torque produced will be very large |
47. For starting a Schrage motor, a 3-phase supply is connected to
| A) | stator as in an ordinary induction motor |
| B) | regulating windings |
| C) | rotor via slip rings |
| D) | secondary winding via brushes |
48. In the circle diagram for the induction motor, the diameter of the circle represents
| A) | line voltage |
| B) | running torque |
| C) | slip |
| D) | rotor current |
49. If any two phases for an induction motor are interchanged
| A) | the motor will burn |
| B) | the motor will not run |
| C) | the motor will run in reverse direction |
| D) | the motor will run at reduced speed |
50. What is the disadvantage of starting an induction motor with a star-delta starter?
| A) | The starting torque increases and the motor runs with jerks |
| B) | During starting high losses are produced |
| C) | The starting torque is one third of the torque in case of the delta connection |
| D) | None of these answers |
51. Which of the following methods of speed control is not affected through stator side ?
| A) | Change of number of poles |
| B) | Change of supply voltage frequency |
| C) | Change of supply voltage |
| D) | Change of rotor resistance |
52. A squirrel cage induction motor running on no-load is loaded, which of the following statements is incorrect?
| A) | Motor speed decreases |
| B) | Stator flux keeps rotating synchronously |
| C) | Stator flux cuts the rator bars more rapidly |
| D) | Torque developed by the rotor increases |
| E) | Current in the rotor bars decreases |
53. A 3-phase slip-ring induction motor is always started with
| A) | a starting winding |
| B) | no external resistance in rotor circuit |
| C) | squirrel cage winding |
| D) | full external resistance in rotor circuit |
54. During starting if an induction motor hums, the probable cause could be
| A) | any of these answers |
| B) | inter-turn short circuit on rotor |
| C) | unequal phase resistance |
| D) | open circuit |
55. In the case of a 3-phase wound-rotor induction motor, an increase in rotor resistance affects the motor performance in the following way
| A) | the maximum torque is reduced |
| B) | the motor efficiency increases |
| C) | the motor efficiency decreases |
| D) | starting current decreases |
| E) | starting current increases |
56. In case the air gap in an induction motor is increased
| A) | speed of motor will increase |
| B) | the power factor will decrease |
| C) | the magnetising current of the rotor will decrease |
| D) | the windage losses will increase |
57. In the case of the induction motors the torque is
| A) | inversely proportional to (√slip) |
| B) | inversely proportional to slip |
| C) | directly proportional to slip |
| D) | directly proportional to (slip)2 |
58. A 3-phase 440 V, 50 Hz induction motor has 4% slip. The frequency of rotor e.m.f. will be
| A) | 0.2 Hz |
| B) | 200 Hz |
| C) | 50 Hz |
| D) | 2 Hz |
59. If an induction motor with a 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 |
60. Which is of the following data is required to draw the circle diagram for an induction motor?
| A) | No load test only |
| B) | Block rotor test, no-load test and stator resistance test |
| C) | Block rotor test and no-load test |
| D) | Block rotor test only |
61. The principle of operation of a 3-phase induction motor is most similar to that of
| A) | synchronous motor |
| B) | capacitor-start induction run motor |
| C) | transformer with a shorted secondary |
| D) | repulsion-start induction motor |
62. The two important parts of a 3-phase induction motor are
| A) | rotor and stator |
| B) | rotor and armature |
| C) | slip ring and brushes |
| D) | stator and field |
63. The injected e.m.f. in the rotor of the induction motor must have
| A) | the same frequency as the slip frequency |
| B) | the same phase as the rotor e.m.f. |
| C) | high value for the satisfactory speed control |
| D) | zero frequency |
64. 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) | 1440 r.p.m. |
| B) | 960 r.p.m. |
| C) | 730 r.p.m. |
| D) | 2880 r.p.m. |
65. Slip of an induction motor is negative when
| A) | rotor speed is less than the synchronous speed of the field and are in the same direction |
| B) | magnetic field and rotor rotate in opposite direction |
| C) | none of these answers |
| D) | rotor speed is more than the synchronous speed of the field and are in the same direction |
66. If s is the slip and f is the supply frequency, the frequency of rotor current is given by
| A) | s.f |
| B) | s2f |
| C) | s.f/2 |
| D) | f/s |
67. The crawling in the induction motor is caused by
| A) | none of these answers |
| B) | high loads |
| C) | improper design of the machine |
| D) | low voltage supply |
| E) | harmonics developed in the motor |
68. What is the advantage of the double squirrel-cage rotor as compared to the round bar cage rotor?
| A) | The slip of the motor is larger |
| B) | The starting current of the motor is lower |
| C) | The efficiency of the motor is higher |
| D) | The power factor of the motor is higher |
69. The shaft of an induction motor is made of
| A) | high-speed steel |
| B) | carbon steel |
| C) | stainless steel |
| D) | cast iron |
70. Whenever any polyphase induction motor is loaded
| A) | induced e.m.f. in the rotor increases and its frequency also increases |
| B) | induced e.m.f. in the rotor remains constant |
| C) | induced e.m.f. decreases and frequency increases |
| D) | induced e.m.f. in the rotor increases and its frequency falls |
71. An induction motor with large number of slots has
| A) | low over-load capacity |
| B) | none of these answers |
| C) | either low over-load capacity or high over-load capacity |
| D) | high over-load capacity |
72. Which of the following motors has the highest power to weight ratio?
| A) | Synchronous motor |
| B) | Induction motor |
| C) | Universal motor |
| D) | Capacitor motor |
73. The frame of an induction motor is usually made of
| A) | cast iron |
| B) | bronze |
| C) | silicon steel |
| D) | aluminium |
74. 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) | none of these answers |
| C) | in the same direction as the direction of the field |
| D) | in any direction depending upon phase sequence of supply |
75. 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 without any harm to the winding |
| C) | will continue running burning its two phase |
| D) | will stop and carry heavy current causing permanent damage to its winding |
76. The value of average flux density in air gap in an induction motor, should be small
| A) | to get good power factor |
| B) | for minimum cost |
| C) | to achieve good efficiency |
| D) | to get poor power factor |
77. Imbalance in the shaft of the induction motor occurs due to
| A) | air gap is not uniform |
| B) | slip rings |
| C) | overheating of the winding |
| D) | rugged construction |
78. The principle of operation of a three-phase induction motor closely resembles to that of which of the following?
| A) | D.C. series motor |
| B) | Synchronous motor |
| C) | Transformer with open-circuited secondary |
| D) | Transformer with short-circuit secondary |
79. A 3-phase induction motor is running at a load of the rated torque. What happens when one of the outer mains is interrupted while the motor is running?
| A) | The motor keeps on running and the current drawn does not change |
| B) | The motor stops immediately |
| C) | The motor stops after a few seconds |
| D) | The motor keeps on running but draws more current |
80. In an induction motor the pulsation losses and noise can be reduced by using
| A) | less number of narrow slots |
| B) | large number of narrow slots |
| C) | large number of semi-open slots |
| D) | none of these answers |
81. For which motor the speed can be controlled from rotor side ?
| A) | Slip-ring induction motor |
| B) | Squirrel-cage induction motor |
| C) | None of these answers |
| D) | Both Squirrel-cage induction motor and Slip-ring induction motor |
82. 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) | reducing supply voltage |
| D) | changing winding resistance |
83. Which of the following statement about a double squirrel cage induction motor is incorrect?
| A) | It is particularly useful where frequent starting under heavy loads is required |
| B) | Its upper cage is of low resistance and lower cage of high resistance |
| C) | It has better speed regulation than squirrel cage motor |
| D) | It maintains high efficiency during normal operating conditions |
84. A wound rotor induction motor is usually not selected when
| A) | external voltage is to be fed into rotor |
| B) | variable speed operation is desired |
| C) | high starting torque is the main consideration |
| D) | cost is the main consideration |
85. Low voltage at motor terminals is due to
| A) | inadequate motor wiring |
| B) | any of these answers |
| C) | none of these answers |
| D) | poorly regulated power supply |
86. The value of transformation ratio of an induction motor can be found by
| A) | stator resistance test |
| B) | open-circuit test only |
| C) | none of these answers |
| D) | short-circuit test only |
87. The efficiency of an induction motor is ____than that of a transformer.
| A) | lower |
| B) | none of these answers |
| C) | either lower or higher |
| D) | higher |
88. The pull-out torque for a normal squirrel cage induction motor usually occurs at a percentage slip of about
| A) | 10 to 15 percent |
| B) | 20 to 25 percent |
| C) | 50 to 60 percent |
| D) | 35 to 45 percent |
89. When a Scharge motor runs exactly at synchronous speed, then
| A) | secondary voltage E2 comes into phase with primary back e.m.f. E1 |
| B) | the speed of the motor field relative to that of regulating winding becomes zero |
| C) | it behaves, in a sense, like a synchronous motor with compensating winding functioning as D.C. excite. |
| D) | voltage injected from the compensation winding into secondary becomes equal and opposite to the secondary induced voltage |
90. In Ns is the synchronous speed and s the slip then the actual running speed of an induction motor will be
| A) | Ns |
| B) | s.Ns |
| C) | (1 – s)Ns |
| D) | (Ns – 1) |
91. The speed of a 3-phase wound motor can be increased by
| A) | short-circuiting slip-rings |
| B) | decreasing external resistance in the rotor circuit |
| C) | increasing applied voltage |
| D) | decreasing applied voltage |
92. If the rotor circuit of a squirrel cage induction motor is open, the rotor will
| A) | run at very low speed |
| B) | run at very high speed |
| C) | not run |
| D) | make noise |
93. The direction of rotation of a 3-phase induction motor can be reversed by
| A) | interchanging any two phases |
| B) | reducing load |
| C) | reducing frequency |
| D) | supplying low voltage |
94. In an induction motor, percentage slip depends on
| A) | copper losses in motor |
| B) | supply frequency |
| C) | none of these answers |
| D) | supply voltage |
95. 3-phase slip ring motor has
| A) | short-circuited rotor |
| B) | double cage rotor |
| C) | wound rotor |
| D) | any of these answers |
96. Lubricant used for ball bearing is usually
| A) | graphite |
| B) | molasses |
| C) | mineral oil |
| D) | grease |
97. When a load on a squirrel-cage induction motor is increased from no-load to full load, which of the following statement is incorrect?
| A) | The magnetizing component of the stator exiting current is increased |
| B) | Stator power factor is improved |
| C) | Slip increases |
| D) | Torque developed by the motor increases till it equals the applied torque |
98. Regarding single-phasing of a 3-phase induction motor under running conditions which of the following statements is incorrect?
| A) | It will stop and blow the remaining fuses |
| B) | It will continue to run without damage if it is carrying half load or less |
| C) | It will try to keep running though over-heating of part of its winding if it is carrying full-load or slight overload |
| D) | It will stop and promptly burnt out if it is heavily overloaded |
99. In an induction motor if air gap is increased
| A) | copper loss will reduce |
| B) | windage losses will be more |
| C) | bearing friction will reduce |
| D) | the power factor will be low |
100. If air gap of the induction motor is increased, its
| A) | magnetizing current will decrease |
| B) | power factor will increase |
| C) | magnetizing current will increase |
| D) | power factor will decrease |
101. 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) | continue to run at lower speed without damage |
| B) | stall immediately |
| C) | get heated and subsequently get damaged |
| D) | stall after sometime |
102. DOL starting of induction motors is usually restricted to
| A) | high horsepower motors |
| B) | low horsepower motors |
| C) | variable speed motors |
| D) | high speed motors |
103. The torque developed in the cage induction motor with the auto starter is
| A) | k/torque with direct switching |
| B) | k. torque with direct switching |
| C) | k2/ torque with direct switching |
| D) | k2. torque with direct switching |
104. Under average service conditions, motor bearings may be safely operated at temperatures up to
| A) | 95°C |
| B) | 120°C |
| C) | 75°C |
| D) | 105°C |
105. An induction motor can run at synchronous speed when
| A) | it is run in reverse direction |
| B) | e.m.f. is injected in the rotor circuit |
| C) | it is run on voltage higher than the rated voltage |
| D) | it is run on load |
106. In the squirrel-cage induction motor, the rotor slots are usually given slight skew
| A) | to ensure easy fabrication |
| B) | none of these answers |
| C) | to increase the tensile strength of the rotor bars |
| D) | to reduce the magnetic hum and locking tendency of the rotor |
107. The term `cogging` is associated with
| A) | three phase transformers |
| B) | compound generators |
| C) | induction motors |
| D) | D.C. series motors |
108. In moist surroundings which frame of the motor will be preferred?
| A) | Splash proof |
| B) | Drip proof |
| C) | Totally enclosed fan cooled |
| D) | Open. type |
109. The difference between the synchronous speed and the actual speed of an induction motor is known as
| A) | slip |
| B) | lag |
| C) | regulation |
| D) | back lash |
110. The shape of the torque/slip curve of the induction motor is
| A) | parabola |
| B) | hyperbola |
| C) | straight line |
| D) | rectangular parabola |
111. In a squirrel cage motor the induced e.m.f. is
| A) | slip times the stand still e.m.f. induced in the rotor |
| B) | none of these answers |
| C) | dependent on the shaft loading |
| D) | dependent on the number of slots |
112. When the equivalent circuit diagram of a double squirrel-cage induction motor is constructed the two cages can be considered
| A) | in parallel with stator |
| B) | in parallel |
| C) | in series-parallel |
| D) | in series |
113. An induction motor with 1000 r.p.m. speed will have
| A) | 8 poles |
| B) | 4 poles |
| C) | 2 poles |
| D) | 6 poles |
114. 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 maximum torque decreases |
| D) | increases starting torque but maximum torque remains unchanged |
115. Which of the following type of bearing is generally used to support the rotor of an induction motor?
| A) | Bush bearing |
| B) | Needle bearing |
| C) | Ball bearing |
| D) | Plummer block |
116. The upper bars in a double squirrel cage induction motor have
| A) | low resistance and low reactance |
| B) | high resistance and low reactance |
| C) | high resistance and high reactance |
| D) | low resistance and high reactance |
117. When a double squirrel cage motor is started, the current induced in the rotor
| A) | flows mostly through the lower winding |
| B) | is equally divided between the two windings |
| C) | is directly proportional to the impedance offered by each cage |
| D) | flows mostly through the upper winding |
118. An induction motor has a rated speed of 720 r.p.m. How many poles have their rotating magnetic field?
| A) | 4 poles |
| B) | 6 poles |
| C) | 8 poles |
| D) | 2 poles |
119. In an induction motor, the relationship between stator slots and rotor slots is that
| A) | stator slots are exact multiple of rotor slots |
| B) | stator slots are equal to rotor slots |
| C) | stator slots are not an exact multiple of rotor slots |
| D) | none of these answers |
120. The probable reason for an induction motor running too hot could be
| A) | any of these answers |
| B) | uneven air gap |
| C) | clogged ventilating ducts |
| D) | low voltage |
121. If there is an open circuit in the rotor of a squirrel cage induction motor
| A) | motor will not start |
| B) | motor will be noisy |
| C) | line fuses will blow |
| D) | rotor will overheat |
122. The magnetizing current drawn by induction motors and transformers is the cause of their____ power factor.
| A) | unity |
| B) | lagging |
| C) | zero |
| D) | leading |
123. Less maintenance troubles are experienced in the case of
| A) | slip ring induction motor and squirrel cage induction motor |
| B) | slip ring induction motor |
| C) | squirrel cage induction motor |
| D) | none of these answers |
124. What is the function of putting resistance in parallel to one phase of a 3-phase induction motor?
| A) | To attain a higher starting torque |
| B) | To achieve a smooth’ starting |
| C) | To reduce the starting current to a very low value |
| D) | To attain a higher maximum torque |
125. The starting torque of the slip ring induction motor can be increased by
| A) | adding resistance to the rotor |
| B) | adding resistance to the stator as well as the rotor |
| C) | none of these answers |
| D) | adding resistance to the stator |
126. The reactance per phase as compared to the resistance per phase of an induction motor is
| A) | quite high |
| B) | very small |
| C) | almost same |
| D) | slightly large |
127. Which of the following statements about the working of Scharge motor is incorrect?
| A) | When injected e.m.f. boosts the secondary induced e.m.f. the motor runs at super synchronous speed |
| B) | Its speed is varied by varying the amount of slip-frequency e.m.f. injected into the secondary circuit |
| C) | As the two brushes of a set are opened out, i.e., their distance from each other is increased, magnitude of the e.mt between them is decreased |
| D) | It is capable of speed variation from zero to approximately twice the synchronous speed |
128. Which type of bearing is provided in small induction motors to support the rotor shaft?
| A) | Bush bearings |
| B) | Cast iron bearings |
| C) | None of these answers |
| D) | Ball bearings |
129. A double squirrel-cage induction motor has
| A) | two series windings in stator |
| B) | two parallel windings in stator |
| C) | two rotors moving in oppsite direction |
| D) | two parallel windings in rotor |
130. The shaft of an induction motor is made of
| A) | flexible |
| B) | hollow |
| C) | stiff |
| D) | any of these answers |
131. In the case of 3-phase induction motors, plugging means
| A) | locking of rotor due to harmonics |
| B) | starting the motor on load which is more than the rated load |
| C) | interchanging two supply phases for quick stopping |
| D) | pulling the motor directly online without a starter |
132. Synchronous wattage of induction motor means
| A) | combined stator and rotor input in watts |
| B) | rotor input in watts |
| C) | stator input in watts |
| D) | shaft output in watts |
133. The ‘cogging’ of an induction motor can be avoided by
| A) | auto-transformer starter |
| B) | proper ventilation |
| C) | using DOL starter |
| D) | having number of rotor slots more or less than the number of stator slots (not equal) |
134. Which of the following features of induction motor helps in preventing clogging of motor?
| A) | High slip |
| B) | Large number of poles |
| C) | Skewed slots |
| D) | Use of better insulating materials |
135. When a stationary 3-phase induction motor is switched on with one phase disconnected
| A) | remaining intact fuses will be blown out due to heavy rush of current |
| B) | it is likely to burn out quickly unless immediately disconnected |
| C) | it will start but run very slowly |
| D) | it will make jerky start with loud growling noise |
136. The shaft, on which the rotor of an induction motor is mounted is made of
| A) | chrome vanadium steel |
| B) | aluminium |
| C) | high speed steel |
| D) | mild steel |
| E) | cast-iron |
137. Synchronous speed is defined as
| A) | the natural speed at which a magnetic field rotates |
| B) | the speed of an induction motor at no load |
| C) | the speed of the rotor of an induction motor |
| D) | the speed of a synchronous motor |
138. An induction motor is identical to
| A) | synchronous motor |
| B) | D.C. series motor |
| C) | asynchronous motor |
| D) | D.C. compound motor |
139. Two of the power supply terminals to a 3-phase induction motor get interchanged during reconnection after the maintenance of the motor. When put back into service, the motor will
| A) | rotate in the reverse direction to that prior to maintenance |
| B) | fail to rotate |
| C) | none of these answers |
| D) | get heated up and damaged |
| E) | rotate in the same direction as it was prior to maintenance |
140. The starting torque of the slip ring induction motor can be increased by adding
| A) | both resistance and inductance to the rotor |
| B) | external inductance to the rotor |
| C) | external resistance to the rotor |
| D) | external capacitance to the rotor |
141. If a three-phase squirrel-cage induction motor runs slow, which of the following could not be the reason for the same?
| A) | Overload |
| B) | Low voltage |
| C) | Shorted stator coils |
| D) | High frequency |
142. The advantage of a slip-ring induction motor over a squirrel cage induction motor is that
| A) | it can be started with the help of rotor resistance stater |
| B) | none of these answers |
| C) | it has higher power factor |
| D) | it has higher efficiency |
143. The rotor of an induction motor cannot run with synchronous speed because
| A) | air friction prevents it to do so |
| B) | Lenz’s law would be violated |
| C) | rotor torque would then become zero |
| D) | induction motor would then become synchronous motor |
144. A pump induction motor is switched on to a supply 25 percent lower than its rated voltage. The pump runs eventually
| A) | the pump will continue to run at lower speed without damage |
| B) | the pump will get heated and consequently get damaged |
| C) | none of these answers |
| D) | the pump will stall after sometimes |
145. Slip rings are usually made of
| A) | phospor bronze |
| B) | aluminium |
| C) | copper |
| D) | carbon |
146. The auto-starters (using three autotransformers) can be used to start a cage induction motor of the following type
| A) | star connected only |
| B) | none of these answers |
| C) | delta connected only |
| D) | star connected only and delta connected only |
147. What is the advantage of starting a slip-ring induction motor with the help of rotor resistances as compared to other methods?
| A) | The starting current is reduced |
| B) | The starting torque increases due to the rotor resistances |
| C) | The starter has to be designed for only a very low current |
| D) | The starter can be built directly into the motor |
148. Slip rings for induction motors are made of
| A) | aluminium |
| B) | cobalt steel |
| C) | carbon |
| D) | phosphor bronze |
149. When a squirrel cage induction motor fails to start which of the following could not be the reason for the same?
| A) | Uneven air gap |
| B) | One or two phase open |
| C) | Overload |
| D) | Blown fuses |
150. Which of the following parameters for an induction motor varies as the square of the supply voltage
| A) | Maximum running torque |
| B) | Starting current |
| C) | Slip |
| D) | Synchronous speed |
151. Type of bearing used for the 25 H.P. motor is
| A) | roller bearing |
| B) | needle bearing |
| C) | bush bearing |
| D) | ball bearing |
152. If an induction motor is to be run on an unbalanced supply, then it should be run at
| A) | higher speeds |
| B) | lower loads |
| C) | low speeds |
| D) | higher loads |
153. A wound rotor induction motor can be distinguished from a squirrel cage induction motor by
| A) | presence of slip ring |
| B) | size of frame |
| C) | diameter of shaft |
| D) | direction or rotation |
154. A squirrel cage induction motor is not selected when
| A) | higher starting torque is the main consideration |
| B) | initial cost is the main consideration |
| C) | all of these answers |
| D) | maintenance cost is to be kept low |
155. Phase advancers are used with induction motors to
| A) | reduce copper losses |
| B) | reduce vibrations |
| C) | reduce noise |
| D) | improve power factor |
156. In a wound 3-phase induction motor, brushes should be connected to
| A) | equalizing coils |
| B) | external D.C. excitation |
| C) | power supply |
| D) | external star-connected resistors |
157. For a 50 H.P. motor, which type of rotor will be preferred?
| A) | Squirrel cage rotor using rectangular copper bars |
| B) | Wound rotor |
| C) | Squirrel cage rotor using round copper bars |
| D) | Die cast aluminium rotor |
158. In an induction motor
| A) | s = Ns – N/Ns |
| B) | s = N – Ns/N |
| C) | s = N – Ns/Ns |
| D) | s = Ns – N/N |
159. Which of the following statements is correct about an induction motor?
| A) | None of these answers |
| B) | It is self starting with high torque |
| C) | It is self starting with zero torque |
| D) | It is self starting with small torque as compared to rated torque |
160. 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) | 0.02 |
| B) | 0.01 |
| C) | 0.04 |
| D) | 0.03 |
161. Which motor is preferred for use in mines where explosive gases exist?
| A) | Induction motor |
| B) | Air motor |
| C) | D.C. shunt motor |
| D) | Synchronous motor |
162. The number of slip rings on a squirrel cage induction motor is
| A) | four |
| B) | none of these answers |
| C) | two |
| D) | three |
163. The overheating of an induction motor may be due to
| A) | any of these answers |
| B) | loss of ventilation |
| C) | overloading |
| D) | low supply voltage |
164. In a star-delta starter of an induction motor
| A) | applied voltage per stator phase is 57.7% of the line voltage |
| B) | resistance is inserted in the stator |
| C) | reduced voltage is applied to the stator |
| D) | resistance is inserted in the rotor circuit |
165. Slip ring motor is preferred over squirrel cage induction motor where
| A) | load torque is heavy |
| B) | high starting torque is required |
| C) | all of these answers |
| D) | heavy pull out torque is required |
166. Insertion of resistance in the rotor of an induction motor to develop a given torque
| A) | rotor current becomes zero |
| B) | decreases the rotor current |
| C) | increases the rotor current |
| D) | rotor current remains same |
167. If the rotor is open in a squirrel cage motor it
| A) | will not run |
| B) | will run at a very slow speed |
| C) | will make noise |
| D) | will run at a very high speed |
168. For driving high inertia body best type of induction motor suggested is
| A) | slip ring type |
| B) | any of these answers |
| C) | squirrel-cage type |
| D) | none of these answers |
169. In three-phase induction motors sometimes copper bars are placed deep in the rotor to
| A) | improve power factor |
| B) | improve starting torque |
| C) | reduce copper losses |
| D) | improve efficiency |
170. Which of the following is not determined by a circle diagram?
| A) | Power factor |
| B) | Frequency |
| C) | Output |
| D) | Efficiency |
171. The induction motors which are provided with open slots have
| A) | higher efficiency |
| B) | leading power factor |
| C) | more break down torque |
| D) | less break down torque |
172. A change of 4% of supply voltage to an induction motor will produce a change of approximately
| A) | 16% in the rotor torque |
| B) | 4% in the rotor torque |
| C) | 12% in the rotor torque |
| D) | 8% in the rotor torque |
173. The circle diagram for an induction motor cannot be used to determine
| A) | frequency |
| B) | output |
| C) | power factor |
| D) | efficiency |
174. Which of the following statements about the 3-phase induction motor is incorrect?
| A) | It is capable of operation under a wide range of power factors both lagging and leading |
| B) | It has no moving contact between stator and rotor |
| C) | Its no-load line current may be as large as 40% of the full load current |
| D) | It has no commutator and no slip rings |
| E) | It starts up from rest and has not be synchronized |
175. Star-delta starting of motors is not possible in case of
| A) | low horse power motors |
| B) | high speed motors |
| C) | variable speed motors |
| D) | single phase motors |
176. The power factor of an induction motor under no-load conditions is about
| A) | unity |
| B) | 0.4 lead |
| C) | 0.2 lag |
| D) | 0.4 lag |
177. The number of slip rings on a squirrel cage induction motor is usually
| A) | two |
| B) | four |
| C) | none of these answers |
| D) | three |
178. The synchronous speed of a 3-phase induction motor is given by the formula
| A) | Ns = 120 fp |
| B) | Ns = 120p/f |
| C) | Ns = fp/120 |
| D) | Ns = 120f/p |
179. The possible number of different speeds that can be obtained by connecting two motors in a cascade is
| A) | 2 |
| B) | 12 |
| C) | 6 |
| D) | 4 |
180. The most important factor in motor maintenance is to keep
| A) | it clean and free from foreign materials |
| B) | a regular check of its lubrication |
| C) | checking its insulation |
| D) | regular check on line voltage |
181. Which of the following statements is correct regarding a 3-phase motor?
| A) | Stator is externally connected rotor |
| B) | Three-phase supply is usually connected to its rotor |
| C) | It is cheap, robust and needs no skilled maintenance |
| D) | Its magnetic flux alternates at the supply frequency |
182. The starting torque of a squirrel-cage induction motor is
| A) | same as full-load torque |
| B) | negligible |
| C) | slightly more than full-load torque |
| D) | low |
183. In an induction motor if P is the power delivered to a rotor and s is the slip, then the power lost in the rotor as copper loss will be
| A) | s2P |
| B) | P/s |
| C) | P/s2 |
| D) | sP |
184. Reduced voltage starter can be used with
| A) | none of these answers |
| B) | squirrel cage induction motor wily but not with slip ring motor |
| C) | slip ring motor only but not with squirrel cage induction motor |
| D) | squirrel-cage as well as slip ring induction motor |
185. Which of the following component is usually fabricated out of silicon steel?
| A) | None of these answers |
| B) | Shaft |
| C) | Stator core |
| D) | Bearings |
186. 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
| A) | repulsion motor |
| B) | slip ring induction motor |
| C) | synchronous motor |
| D) | squirrel-cage |
187. The size of a high-speed motor as compared to low-speed motor for the same H.P. will be
| A) | any of these answers |
| B) | smaller |
| C) | bigger |
| D) | same |
188. The good power factor of an induction motor can be achieved if the average flux density in the air gap is
| A) | infinity |
| B) | large |
| C) | small |
| D) | absent |
189. The fractional slip of an induction motor is the ratio
| A) | rotor copper loss/stator copper loss |
| B) | stator copper loss/stator input |
| C) | rotor copper loss/rotor input |
| D) | rotor copper loss/rotor output |
190. What is the advantage of the slip ring induction motor over the squirrel cage induction motor?
| A) | It can be started with the help of rotor resistances |
| B) | It is suitable for higher speeds |
| C) | It has a higher power factor |
| D) | It can be started with the help of rotor resistances |
191. Insertion of resistance in the stator of an induction motor
| A) | none of these answers |
| B) | increases the starting torque |
| C) | decreases the starting torque |
| D) | increases the load torque |
192. In the case of a double cage induction motor, the inner cage has
| A) | low inductance and high resistance |
| B) | high inductance and high resistance |
| C) | low inductance and low resistance |
| D) | high inductance and low resistance |
193. Slip ring motor is recommended where
| A) | frequent starting, stopping, and reversing is required |
| B) | high starting torque is needed |
| C) | speed control is required |
| D) | all of these answers |
194. What is the disadvantage of the speed control of slip-ring induction motor with the help of resistances in the rotor circuit ?
| A) | None of these answers |
| B) | By using this method the speed can be easily controlled |
| C) | This method is associated with high losses |
| D) | With reductions in torque, the speed decreases considerably |
195. In a three-phase induction motor, the number of poles in the rotor winding is always
| A) | equal to number of poles in stator |
| B) | zero |
| C) | more than the number of poles in stator |
| D) | less than number of poles in stator |
196. 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 |
197. As the load on an induction motor goes on increasing
| A) | its power factor remains constant |
| B) | its power factor goes on increasing upto full load and then it falls again |
| C) | its power factor goes on decreasing |
| D) | its power factor goes on increasing even after full load |
198. In a three-phase induction motor
| A) | power factor at starting is low as compared to that while running |
| B) | none of these answers |
| C) | power factor at starting is high as compared to that while running |
| D) | power factor at starting in the same as that while running |
199. The complete circle diagram of the induction motor can be drawn with the help of data found from
| A) | stator resistance test |
| B) | blocked rotor test |
| C) | no-load test |
| D) | all of these answers |
200. Which of the following motors are used frequently?
| A) | D.C. shunt motor |
| B) | Three phase commutator motor |
| C) | A.C. induction motor |
| D) | Three phase induction motor |
201. Blocked rotor test on induction motors is used to find out
| A) | leakage reactance |
| B) | all of these answers |
| C) | power factor on short circuit |
| D) | short-circuit current under rated voltage |
202. When R2 is the rotor resistance, the X2 rotor reactance at supply frequency and s the slip, then the condition for maximum torque under running conditions will be
| A) | sR2 X 2 = 1 |
| B) | sR2 = X2 |
| C) | R2 = sX2 |
| D) | R2 = S2X2 |
203. The torque of an induction motor is
| A) | inversely proportional to slip |
| B) | proportional to the square of the slip |
| C) | directly proportional to slip |
| D) | none of these answers |
204. In a three-phase induction motor
| A) | iron losses in stator will be negligible as compared to that in rotor |
| B) | iron losses in motor will be negligible 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 |
205. If single phasing occurs when the motor is running, it should not be loaded beyond
| A) | 90 percent of rated load |
| B) | 5 percent of rated load |
| C) | 20 percent of rated load |
| D) | 50 percent of rated load |
206. If single-phasing occurs on the running position in an induction motor, the motor will
| A) | fail to carry load |
| B) | not start |
| C) | produce peculiar noise |
| D) | draw unbalanced and excessive currents |
207. The frame of an induction motor is made of
| A) | aluminium |
| B) | stainless steel |
| C) | closed grained cast iron |
| D) | carbon |
208. The speed of a three-phase cage-rotor induction motor depends on
| A) | input voltage |
| B) | number of poles only |
| C) | frequency of supply only |
| D) | none of these answers |
| E) | number of poles and frequency of the supply |
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