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EEE :: Synchronous Motors

  1. A synchronous motor is running on a load with normal excitation. Now if the load on the motor is increased

  2. A.

     power factor as well as armature current will decrease

    B.

     power factor as well as armature current will increase

    C.

     power factor will increase but armature current will decrease

    D.

     power factor will decrease and armature current will increase

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  3. To limit the operating temperature an electrical machine should have proper

  4. A.

     voltage rating

    B.

     current rating

    C.

     power factor

    D.

     speed

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  5. In a synchronous motor, the magnitude of stator back e.m.f. £& depends on

  6. A.

     d.c. excitation only

    B.

     speed of the motor

    C.

     load on the motor

    D.

     both the speed and rotor flux

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  7. In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on

  8. A.

     load on the motor

    B.

     d.c. excitation only

    C.

     both the speed and rotor flux

    D.

     none of the above

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  9. If excitation of a synchronous motor running with a constant load is decreased from its normal value, ignoring effects of armature reaction, it leads to

  10. A.

     increase in both armature current and power factor angle

    B.

     increase in back e.m.f. but decrease in armature current

    C.

     increase in both armature current and power factor which is lagging

    D.

     increase in torque angle but decrease in back e.m.f.

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  11. If load (or torque) angle of a 4-pole synchronous motor is 6° electrical, its value in mechanical degrees is

  12. A.

     2

    B.

     3

    C.

     4

    D.

     6

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  13. In a synchronous motor running with fixed excitation, when the load is increased three times, its torque angle becomes approximately

  14. A.

     One-third

    B.

     Twice

    C.

     Thrice

    D.

     Six times

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  15. If in a synchronous motor, driving a given mechanical load and drawing current at a leading power factor from constant voltage supply its field excitation is increased, its power factor

  16. A.

     will become more

    B.

     will become less

    C.

     will remain unchanged

    D.

     none of the above.

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  17. Synchronous motors are generally not self-starting because

  18. A.

     the direction of rotation is not fixed

    B.

     the direction of instantaneous torque reverses after half cycle

    C.

     startes cannot be used on these machines

    D.

     starting winding is not provided on the machines

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  19. The synchronous motor is not inherently self-starting because

  20. A.

     the force required to accelerate the rotor to the synchronous speed in an instant is absent

    B.

     the starting device to accelerate the rotor to near synchronous speed is absent

    C.

     a rotating magnetic field does not have enough poles

    D.

     the rotating magnetic field is produced by only 50 Hz frequency currents

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