Most Synchronous Motors

21. A three-phase synchronous motor will have

  1. no slip-rings
  2. one slip-ring
  3. two slip-rings
  4. three slip-rings

Correct answer: (C)
two slip-rings

22. An important advantage of a synchronous motor over wound round induction motor is that

  1. its power factor may be varied at will
  2. its speed is independent of supply frequency
  3. its speed may be controlled more easily
  4. none of the above

Correct answer: (A)
its power factor may be varied at will

23. An over excited synchronous motor is used for

  1. fluctuating loads
  2. variable speed loads
  3. low torque loads
  4. power factor corrections

Correct answer: (D)
power factor corrections

24. An over-excited synchronous motor takes

  1. leading current
  2. lagging current
  3. both (a) and (b)
  4. none of the above

Correct answer: (A)
leading current

25. Armature of a synchronous machine is

  1. of reducing number of slip rings on the rotor
  2. armature is associated with large power as compared to the field circuits
  3. of difficulty of providing high voltage insulation on rotor
  4. all of the above reasons

Correct answer: (D)
all of the above reasons

26. As the load is applied to a synchronous motor, the motor takes more armature current because

  1. the increased load has to take more current
  2. the rotor by shifting its phase backward causes motor to take more current
  3. the back e.m.f. decreases causing an increase in motor current
  4. the rotor strengthens the rotating field casuing more motor current

Correct answer: (B)
the rotor by shifting its phase backward causes motor to take more current

27. By which of the following methods the constant speed of a synchronous motor can be changed to new fixed value ?

  1. By changing the supply frequency
  2. By interchanging any two phases
  3. By changing the applied voltage
  4. By changing the load.

Correct answer: (A)
By changing the supply frequency

28. Change of D.C. excitation of a synchronous motor changes

  1. applied voltage of the motor
  2. motor speed
  3. power factor of power drawn by the motor
  4. any of the above

Correct answer: (C)
power factor of power drawn by the motor

29. Due to which of the following reasons a synchronous motor fails to pull into synchronism after applying D.C. field current ?

  1. High field current
  2. Low short circuit ratio
  3. High core losses
  4. Low field current

Correct answer: (D)
Low field current

30. Exciters of synchronous machines are

  1. d.c. shunt machines
  2. d.c. series machines
  3. d.c. compound machines
  4. any of the above

Correct answer: (A)
d.c. shunt machines

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