Speed Control of Induction Motor MCQ

1. The speed of a slip-ring induction motor cannot be controlled by

(a) rotor resistance control.
(b) pole changing method.
(c) concatenation or cascade operation.
(d) rotor slip power control.

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2. Consequent pole technique employed for speed control of induction motors involves changing of the

(a) slip.
(b) supply frequency.
(c) number of poles.
(d) any of these.

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3. For the applications requiring speed ratio other than 2 : 1, the speed control can be affected by varying the number of stator poles employing

(a) multiple stator winding.
(b) pole amplitude modulation technique.
(c) consequent pole technique.
(d) any of the above.

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4. Rotor resistance speed control used for 3-phase slip-ring induction motors has the drawbacks of

(a) reduced starting torque.
(b) lower efficiency and poor speed regulation.
(c) both (a) and (b).
(d) none of these.

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5. In rotor resistance control method, with the increase in speed.

(a) torque increases
(b) torque decreases
(c) slip increases
(d) losses increase

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6. Consider the following statements regarding speed control of induction motors by means of external rotor resistors :

  1. Regulation in speed is accompanied by reduced efficiency.
  2. With a large resistance in the rotor circuit, the speed would vary considerably with variation in torque.
  3. The method is very complicated. The disadvantages of such a method of speed control would include

Among the above, correct statements are

(a) 1 and 2.
(b) 2 and 3.
(c) 1 and 3.
(d) 1, 2 and 3. [I.E.S. E.E.-II, 1997]

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7. A 3-phase induction motor is driving a constant torque load at rated voltage and frequency. If both voltage and frequency are halved, following statements relate to new condition if stator resistance, leakage reactance and core loss are ignored.

P. The difference between synchronous . speed and actual speed remains the same.
Q. The air gap flux remains same.
R. The stator current remains same.
S. The pu slip remains same.

Among the above, correct statements are
(a) All.
(b) P, Q and R.
(c) Q, R and S.
(d) P and S. [GATE E.E. 2003]

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8. The absolute speed of magnetic field in space of a 3-phase rotor fed induction motor is

(a) synchronous speed Ns.
(b) rotor speed Ns.
(c) (Ns — Nr)
(d) (Ns + Ns). [I.E.S. E.E.-II, 1996]

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9. Line voltage control is used only with small squirrel cage motors driving fans and blowers as

(a) it reduces pull-out torque.
(b) the range of speed control is limited.
(c) both (a) and (b).
(d) none of these.

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10. A wound rotor induction motor runs with a slip of 0.03 when developing full-load torque. Its rotor resistance is 0.25 Ω per phase. If an external resistance of 0.5 Ω per phase is connected across slip-rings, what is the slip for full load torque?

(a) 0.03.
(b) 0.06.
(c) 0.09.
(d) 0.1. [U.P.S.C. I.E.S. E.E.-II, 2009]

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11. The injected emf in the rotor of induction motor must have

(a) the same frequency as the stator slip frequency.
(b) the same phase as the rotor emf.
(c) a high value for satisfactory speed control.
(d) the same phase as the rotor emf and a high value for satisfactory speed control. [U.P.S.C. I.E.S. E.E.-II, 2003]

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12. In rotor resistance control of induction motor, the hardness of speed torque characteristic

(a) increases.
(b) decreases.
(c) remains same. [A.M.I.E. Sec B. Winter 1996]

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13. The method that gives continuous speed control of a slip-ring induction motor is

(a) rotor resistance control.
(b) secondary foreign voltage control.
(c) concatenation or cascade operation.
(d) line voltage control.

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14. In case of voltage injection method of speed control, the injected emf should be of

(a) supply frequency (f)
(b) slip frequency (sf)
(c) (1— s) f
(d) (2 — s)f [A.M.I.E. Sec B. Winter 1995]

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15. When will a slip-ring induction motor run at super synchronous speed?

(a) If a voltage is injected in the rotor circuit in phase opposition to the rotor induced emf.
(b) If an emf is injected in the rotor circuit in phase with the rotor induced mot
(c) If motor is coupled with active load.
(d) If motor is coupled with passive load. [U.P.S.C. I.E.S. 2009]

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16. A voltage source inverter is used to control the speed of a three-phase, 50 Hz, squirrel cage induction motor. Its slip for rated torque is 4%. The flux is maintained at rated value. If the stator resistance and rotational losses are neglected, then the frequency of the impressed voltage to obtain twice the rated torque at starting should be

(a) 10 Hz
(b) 5 Hz
(c) 4 Hz
(d) 2 Hz

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17. Speeds higher than synchronous speed can be had by

(a) line voltage control.
(b) rotor slip power control.
(c) rotor resistance control.
(d) frequency control.

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18. Slip-changing method of speed control can be used in case of

(a) slip-ring induction motors only.
(b) squirrel cage induction motors only.
(c) squirrel cage as well as slip-ring induction motors.
(d) none of the above.

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19. Cascade method of speed control involves the use of two coupled induction motors. The necessary condition for speed control is that

(a) both the motors are of the wound rotor type having the same number of poles.
(b) both the motors are of the squirrel cage rotor type having different number of poles.
(c) one motor is of the slip-ring type but both the motors have the same number of poles.
(d) one motor is of the slip-ring type and the two motors have different number of poles. [U.P.S.C. I.E.S. E.E.-II, 1993]

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20. At sub-synchronous speeds, in Kramer system, the electrical power fed to the auxiliary commutator machine at slip frequency is

(a) dissipated as heat.
(b) converted into mechanical power and supplied to the driven shaft.
(c) converted into electrical energy at line frequency and re-turned back to the supply mains.
(d) none of the above.

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21. At sub-synchronous speed, in Scherbius system, the electrical power supplied to the auxiliary commutator machine at the slip frequency is

(a) dissipated as heat.
(b) converted into mechanical power and supplied to the driven shaft.
(c) converted into electrical energy at line frequency and re-turned back to the supply mains.
(d) none of the above.

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22. In a slip power recovery scheme for a 3-phase induction motor, if slip power is

(a) returned to supply, constant power drive is obtained.
(b) added to the main shaft, constant power drive is obtained.
(c) subtracted from the main shaft, constant torque drive is obtained.
(d) obtained from the supply, constant torque drive is obtained. [A.M.I.E. Sec B. Summer 2000]

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23. A three phase, wound rotor induction motor is to be operated with slip energy recovery in the constant torque mode, when it delivers an output power Po at slip s. Then theoretically, the maximum power that is availablefor recovery at the rotor terminals, is equal to

(a) Po
(b) Pos
(c) Po/(1— s)
(d) Pos/(1— s) [GATE E.E. 2000]

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24. The stator and the rotor of a 3-phase, 4-pole wound rotor induction motor are excited respectively from a 50 Hz and 30 Hz source of appropriate voltage. Neglecting all losses, what is/are the possible no-load speed/speeds at which the motor would run?

(a) 1,500 rpm and 900 rpm.
(b) 2,400 rpm and 600 rpm.
(c) 2,400 rpm only.
(d) 600 rpm only. [U.P.S.C. I.E.S. 2005]

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25. The motor that requires the most complicated arrangement for speed control is

(a) dc shunt motor.
(b) squirrel cage induction motor.
(c) stator supplied 3-phase commutator motor.
(d) rotor supplied 3-phase commutator motor.

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26. The factor(s) to be considered in selection of motor is/are

(a) voltage rating of the motor.
(b) kW output rating of the motor.
(c) speed and frame enclosure size.
(d) bearings and protection provided.
(e) all of the above.

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27. A 3-phase wound rotor induction motor, when started with load connected to its shaft, was found to start but settle down at about half synchronous speed. If the rotor winding as well as stator winding were star-connected, the cause of the malfunctioning could be attributed to

(a) one of the stator phase winding being short-circuited.
(b) one of the supply fuses being blown.
(c) one of the rotor phase being open-circuited.
(d) two of the rotor phases being open-circuited. [U.P.S.C. I.E.S. E.E.II, 1997]

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28. Armature short-circuits can be detected and identified by test(s).

(a) growler
(b) bar-to-bar
(c) voltage drop
(d) any of these

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29. Induction generators deliver power at

(a) leading power factor only
(b) lagging power factor only.
(c) leading as well as lagging power factor.
(d) unity power factor only.

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30. In a self excited induction generator, to keep the frequency of generated voltage constant with the increase in load, the speed of the induction machine should be

(a) increased.
(b) decreased.
(c) maintained less than the rated synchronous speed.
(d) maintained more than the rated synchronous speed. [U.P.S.C. I.E.S. E.E.-II, 1995]

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31. A 3-phase induction motor runs at super synchronous speed. For self excitation the machine

(a) draws real power from the mains.
(b) draws reactive power from the mains.
(c) feeds reactive power to the mains.
(d) generates emf at the expense of residual magnetism. [A.M.I.E. Sec B. Elec. Machines Winter 1994]

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32. Consider the following statements; If a 3-phase squirrel cage induction machine operates at a slip of – 0.05 (i.e. minus 0.05), then the machine will

  1. draw electrical power from the mains.
  2. draw mechanical power through the shaft.
  3. deliver electrical power to the mains.

Of these statements
(a) 1,2 and 3 are correct.
(b) 1 and 2 are correct.
(c) 2 and 3 are correct.
(d) 1 and 3 are correct. [U.P.S.C. I.E.S. E.E.-II, 1996]

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33. Which of the following is not the advantage of an induction generator ?

(a) It does not hunt or drop out of synchronism.
(b) It is simple and rugged in construction, cheaper in cost and needs little maintenance.
(c) It delivers only leading current.
(d) When short circuited it delivers little or no sustained power.

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34. Which of the following is not the disadvantage of an induction generator ?

(a) It cannot be operated independently.
(b) When short circuited it delivers little or no sustained power.
(c) It can deliver only leading current.
(d) none of the above.

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35. An 8-pole wound rotor induction motor operating at 60 Hz supply is driven at 1,800 rpm by a prime mover in the opposite direction of the revolving field. The rotor current frequency is

(a) 60 Hz.
(b) 120 Hz.
(c) 180 Hz.
(d) none of the above. [A.M.I.E. Sec B. Elec. Machines Winter 1993, Summer 1997]

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Speed Control of Induction Motor MCQ

 

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