Induction Motor Question Bank

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

(a) efficiency
(b) power factor
(c) frequency
(d) output


Answer: (a) efficiency
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2. Blocked rotor test on induction motor 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.
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3. We can run an Induction motor at synchronous speed by

(a) reverseing the direction
(b) applying a higher voltage than the rated voltage
(c) injecting an emf in the rotor circuit.


Answer: (c) injecting an emf in the rotor circuit.
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4. 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
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5. 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


Answer: (a) the power factor will be low
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6. When R2 is the rotor resistance. X2, the rotor reactance at supply frequency and s the slip, then the condition for maximum torque under running conditions will be

(a) sR2X2 = 1
(b) sR2 = X2
(c) R2 = sX2


Answer: (c) R2 = sX2
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7. In case of a double cage induction motor, the outer cage has

(a) high inductance and low resistance
(b) low inductance and high resistance
(c) low inductance and low resistance
(d) high resistance and low inductance


Answer: (d) high resistance and low inductance
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8. 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
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9. 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 remains unchanged
(d) increases starting torque but maximum torque decreases


Answer: (a) reduces starting torque as well as maximum torque
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10. Insertion of resistance in the rotor 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 remains same


Answer: (d) rotor current remains same
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12. To driving high inertia loads, best type of induction motor is

(a) slip-ring type
(b) squirrel cage type
(c) any of the above
(d) none of the above


Answer: (a) slip-ring type
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13. 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
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14. 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
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15. Less maintenance troubles are experienced in case of

(a) slip ring induction motor
(b) squirrel cage induction motor
(c) both
(d) none of the above


Answer: (b) squirrel cage induction motor
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16. 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
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17. 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
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18. A star-delta starter applies ……..to the induction motor.

(a) reduced voltage
(b) 57.7% of the line voltage
(c) 37.7% of the line voltage


Answer: (b) 57.7% of the line voltage
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19. 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
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20. 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


Answer: (b) decreases the starting torque
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21. 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
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Induction Motor Question Bank

 

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