AC Machines (Synchronous and Induction) MCQ's

1) If the number of turns, area and current of an ac circuit is doubled then the new inductance will be

     a. Equal to the former

     b. Twice of the former

     c. Four times the former

     d. Half of the former

2) The magnitude of various voltage drops that occur in an alternator, depends on

     a. power factor of the load

     b. load current

     c. power factor x load current

     d. power factor x (load current)2.

3) Salient poles are generally used on

     a. high speed prime movers only

     b. medium speed prime movers only

     c. low speed prime movers only

     d. low and medium speed prime movers.

4) The frequency of voltage generated in an alternator depends on

     a. number of poles

     b.  rotative speed

     c. number of poles and rotative speed

     d.  number of poles, rotative speed and type of winding.

5) The number of electrical degrees passed through in one revolution of a six pole synchronous alternator is

      a. 360

      b. 720

      c. 1080

      d. 2160 .

6) Synchronous motor can operate at

      a. Lagging power factor only 

      b. Leading power factor only

      c. Unity power factor only

      d. Lagging, leading and unity power factor.

7) An unexcited single phase synchronous motor is

      a. reluctance motor

      b. repulsion motor

      c. universal motor

      d. AC series motor.

8) The maximum power developed in the synchronous motor will depend on

      a. rotor excitation only

      b. maximum value of coupling angle

      c.  supply voltage only

      d. rotor excitation supply voltage and maximum value of coupling angle.

9) In case the field of a synchronous motor is under excited, the power factor will be

      a. leading 

      b. lagging

      c. zero

      d. unity.

10) A synchronous motor is switched on to supply with its field windings shorted on themselves. It will

      a. not start

      b. start and continue to run as an induction motor

      c. bum immediately.

      d. start as an induction motor and then run as synchronous motor

11) The damping winding in a synchronous motor is generally used

       a. to provide starting torque only

       b. to reduce noise level

       c. to reduce eddy currents

       d. to prevent hunting and provide the starting torque.

12) The back emf set up in the stator of a synchronous motor will depend on

       a. rotor speed only

       b. rotor excitation only

       c. rotor excitation and rotor speed

       d. coupling angle, rotor speed and excitation.

13) An over excited synchronous motor draws current at

       a. lagging power factor

       b. leading power factor

       c. unity power factor

       d. depends on the nature of load.

14) With the increase in the excitation current of synchronous motor the power factor of the motor will

        a. improve

        b. decrease

        c. remain constant

        d. depend on other factors.

15) An over excited synchronous motor operating on no load condition is called as

        a. Synchronous capacitor

        b. Synchronous condense

        c. Both (a) and (b)

        d. None of these

16) The plot of power factor against the field current of synchronous motor is known as

        a. V-curves

        b. Inverted V-curves

        c. Both (a) and (b)

        d. None of these

17) In a synchronous motor the torque produced will be maximum when load angle is equal to

         a. 0° electrical

         b. 90° electrical

         c. 180° electrical

         d. 360° electrical

18) Synchronous motor works on the principle of

         a. Mutual induction

         b. Electromagnetic induction

         c. Magnetic locking

         d. None of these

19) Slip ring induction motor has

        a. Low starting torque 

        b. Medium starting torque

        c. High starting torque

        d. None of these

20) The frequency of rotor current in an induction motor is

       a. slip times the frequency of stator current

       b. slip times the frequency of supply

       c. 1/slip times the frequency of stator current

       d. One by slip times the frequency of supply

21) Slip of an induction motor increases with
       a. increase in current and decrease in torque
       b. increase in current and torque
       c. decrease in current and torque
       d. decrease in current and increase in torque

22) Slip ring induction motor are employed only for

        a. Speed control

        b. High starting torque

        c. Both (a) and (b)

        d. None of these

23) In induction motor, greater the number of poles

        a. Lesser the speed

        b. Greater the speed

        c. Lesser the frequency

        d. All of these

24) At standstill the slip is,
         a. 0
         b. 1
         c. infinite