1. If load (or torque) angle of a 4-pole synchronous motor is 6° electrical, its value in mechanical degrees is
(a) 2
(b) 3
(c) 4
(d) 6
Ans: b
2. For V-curves for a synchronous motor the graph is drawn between
(a) field current and armature current
(b) terminal voltage and load factor
(c) power factor and field current
(d) armature current and power factor
Ans: a
3. The back e.m.f. of a synchronous motor depends on
(a) speed
(b) load
(c) load angle
(d) all of the above
Ans: c
4. A synchronous motor can operate at
(a) lagging power factor only
(6) leading power factor only
(c) unity power factor only
(d) lagging, leading and unity power factors
Ans: d
5. In a synchronous motor which loss varies with load ?
(a) Windage loss
(b) Bearing friction loss
(c) Copper loss
(d) Core loss
Ans: c
6. A synchronous motor can be made self starting by providing
(a) damper winding on rotor poles
(b) damper winding on stator
(c) damper winding on stator as well as rotor poles
(d) none of the above
Ans: d
7. The oscillations in a synchronous motor can be damped out by
(a) maintaining constant excitation
(b) running the motor on leading power factors
(c) providing damper bars in the rotor pole faces
(d) oscillations cannot be damped
Ans: c
8. The shaft of synchronous motor is made of
(a) mild steel
(b) chrome steel
(c) alnico
(d) stainless steel
Ans: a
9. When the field of a synchronous motor is under-excited, the power factor will be
(a) leading
(b) lagging
(c) unity
(d) zero
Ans: b
10. The speed regulation of a synchronous motor is always
(a) 1% (b) 0.5%
(c) positive
(d) zero
Ans: d
11. The percentage slip in case of a synchronous motor is
(a) 1% (b) 100% (c) 0.5%
(d) zero
Ans: d
12. The operating speed of a synchronous motor can be changed to new fixed value by
(a) changing the load
(b) changing the supply voltage
(c) changing frequency
(d) using brakes
Ans: c
13. A synchronous motor will always stop when
(a) supply voltage fluctuates
(b) load in motor varies
(c) excitation winding gets disconnected
(d) supply voltage frequency changes
Ans: c
14. riunting in a synchronous motor takes place
(a) when supply voltage fluctuates
(b) when load varies
(c) when power factor is unity
(d) motor is under loaded
Ans: b
15. When load on an over-excited or under excited synchronous*motor is increased, rate of change of its armature current as compared with that of power factor is
(a) more
(b) less
(c) equal
(d) twice
Ans: b
16. The rotor copper losses, in a synchronous motor, are met by
(a) d.c. source
(b) armature input
(c) motor input
(d) supply lines
Ans: a
17. The maximum power developed in a synchronous motor occurs at a coupling angle of
(a) 30°
(b) 60°
(c) 90° (d) 180°
Ans: c
18. When the stator windings are connected in such a fashion that the number of poles are made half, the speed of
the rotor of a synchronous motor
(a) remains same as the original value
(b) decreases to half the original value
(c) tends to becomes zero
(d) increases to two times the original value
Ans: d
19. In which of the following motors the stator and rotor magnetic field rotate at the same speed ?
(a) Universal motor
(b) Synchronous motor
(c) Induction motor
(d) Reluctance motor
Ans: b
20. Synchronsizingpower of a synchronous machine is
(a) direcly proportional to the synchronous reactance
(6) inversely proportional to the synchronous reactance
(a) equal to the synchronous reactance
(d) none of the above
Ans: b
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