Synchronous Motors Objective Questions and Answers - ObjectiveBooks

# Practice Test: Question Set - 12

1. An important advantage of a synchronous motor over wound round induction motor is that
(A) Its power factor may be varied at will
(B) Its speed is independent of supply frequency
(C) Its speed may be controlled more easily
(D) None of the above

2. The mechanical displacement of the rotor with respect to the stator, in polyphase multi-polar synchronous motors running at full load, is of the order of
(A) Zero degree
(B) Two degrees
(C) Five degrees
(D) Ten degrees

3. A synchronous motor installed at the receiving end substation operates with such an excitation that it takes power at lagging power factor. Now if the applied voltage of the synchronous motor goes down, the power factor of the synchronous motor will
(A) Remain same
(B) Go down
(C) Improve
(D) None of the above

4. 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
(A) Will become more
(B) Will become less
(C) Will remain unchanged
(D) None of the above

5. If the field circuit of an unloaded salient-pole synchronous motor gets suddenly open-circuited, then
(A) It runs at a slower speed
(B) The motor stops
(C) It continues to run at the same speed
(D) It runs at a very high speed

6. The maximum speed variation in a 3phase synchronous motor is
(A) 10 percent
(B) 6 percent
(C) 4 percent
(D) Zero

7. The ratio of starting torque to running torque in a synchronous motor is
(A) Zero
(B) One
(C) Two
(D) Infinity

8. In a synchronous motor, the magnitude of stator back e.m.f. Eb depends on
(B) D.C. excitation only
(C) Both the speed and rotor flux
(D) None of the above

9. The torque angle, in a synchronous motor, is the angle between
(A) The supply voltage and the back e.m.f.
(B) Magnetizing current and back e.m.f.
(C) The rotating stator flux and rotor poles
(D) None of the above

10. In a synchronous motor, the armature current has large values for
(A) High excitation only
(B) Low excitation only
(C) Both high and low excitation
(D) None of the above

11. The size of a synchronous motor decreases with the increase in
(A) Flux density
(B) Horse power rating
(C) Speed
(D) All of the above

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