McqMate
These multiple-choice questions (MCQs) are designed to enhance your knowledge and understanding in the following areas: Electrical Engineering .
| 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 |
| Answer» D. its power factor goes on increasing upto full load and then it falls again | |
| 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 |
| D. | none |
| Answer» B. in the same direction as the direction of the field | |
| 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 |
| Answer» C. motor takes five to seven times its fullload current | |
| 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 |
| Answer» B. D.C. shunt motor | |
| 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 |
| Answer» A. Ball bearings | |
| 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 |
| Answer» D. get heated and subsequently get damaged | |
| 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. |
| Answer» A. 730 r.p.m. | |
| 58. |
A 3-phase slip ring motor has |
| A. | double cage rotor |
| B. | wound rotor |
| C. | short-circuited rotor |
| D. | any of the above |
| Answer» B. wound rotor | |
| 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 |
| Answer» B. 1.5 times the full load torque | |
| 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 |
| Answer» A. windage losses | |
| 61. |
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 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 |
| Answer» D. iron losses in stator will be more than that in rotor | |
| 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 |
| Answer» D. interchanging two supply phases for quick stopping | |
| 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 |
| Answer» D. Block rotor test, no-load test and stator resistance test | |
| 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 |
| Answer» A. improve starting torque | |
| 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 |
| Answer» B. power factor at starting is low as compared to that while running | |
| 66. |
The vafcie 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 |
| Answer» B. short-circuit test only | |
| 67. |
The power scale of circle diagram of an induction motor can be found from |
| A. | stator resistance test |
| B. | no-load test only |
| C. | short-circuit test only |
| D. | noue of the above |
| Answer» C. short-circuit test only | |
| 68. |
The shape of the torque/slip curve of induction motor is |
| A. | parabola |
| B. | hyperbola |
| C. | rectangular parabola |
| D. | straigth line |
| Answer» C. rectangular parabola | |
| 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 |
| Answer» D. 16% in the rotor torque | |
| 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 |
| Answer» B. external resistance to the rotor | |
| 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 |
| Answer» D. 0.04 | |
| 72. |
The complete circle diagram of induetion motor can be drawn with the help of data found from |
| A. | noload test |
| B. | blocked rotor test |
| C. | stator resistance test |
| D. | all of the above |
| Answer» D. all of the above | |
| 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 |
| Answer» A. to reduce the magnetic hum and locking tendency of the rotor | |
| 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 |
| Answer» C. at the value of the slip which makes rotor reactance per phase equal to the resistance per phase | |
| 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 |
| Answer» B. The rotor will not run | |
| 76. |
The circle diagram for an induction motor cannot be used to determine |
| A. | efficiency |
| B. | power factor |
| C. | frequency |
| D. | output |
| Answer» A. efficiency | |
| 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 |
| Answer» D. all of the above | |
| 78. |
Lubricant used for ball bearing is usually |
| A. | graphite |
| B. | grease |
| C. | mineral oil |
| D. | molasses |
| Answer» B. grease | |
| 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 |
| Answer» D. e.m.f. is injected in the rotor circuit | |
| 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 |
| Answer» A. Air motor | |
| 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 |
| Answer» D. shaft diameter | |
| 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 |
| Answer» A. the power factor will be low | |
| 83. |
In induction motor, percentage slip depends on |
| A. | supply frequency |
| B. | supply voltage |
| C. | copper losses in motor |
| D. | none of the above |
| Answer» C. copper losses in motor | |
| 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 i |
| D. | R2 = s2X2 |
| Answer» C. R2 = sX2 i | |
| 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 |
| Answer» A. high inductance arid low resistance | |
| 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 |
| Answer» D. all of the above | |
| 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 |
| Answer» A. reduces starting torque as well as maximum torque | |
| 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 |
| Answer» D. rotor current rernains same | |
| 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 |
| Answer» A. slip ring type | |
| 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 |
| Answer» D. all above methods | |
| 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 |
| Answer» A. to short circuit the rotor at slip rings | |
| 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 |
| Answer» C. slip times the stand still e.m.f. induced in the rotor | |
| 93. |
Less maintenance troubles are experienced in case of |
| A. | slip ring induction motor |
| B. | squirrel cage induction motor |
| C. | both (a) and (b) |
| D. | none of the above |
| Answer» B. squirrel cage induction motor | |
| 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 |
| Answer» C. higher starting torque is the main consideration | |
| 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 |
| Answer» C. squirrel cage as well as slip ring induction motor | |
| 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 |
| Answer» A. high starting torque is required | |
| 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 |
| Answer» D. applied voltage per1 stator phase is 57.7% of the line voltage | |
| 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 |
| Answer» A. directly proportional to slip | |
| 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 |
| Answer» B. below synchronous speed | |
| 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 |
| Answer» C. both (a) and (b) | |
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