1. Synchronous motors are generally not self-starting because (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 Ans: b
2. In case one phase of a three-phase synchronous motor is short-circuited the motor will
(a) not start
(b) run at 2/3 of synchronous speed
(c) run with excessive vibrations
(d) take less than the rated load Ans: a
3. A pony motor is basically a
(a) small induction motor
(b) D.C. series motor
(c) D.C. shunt motor
(d) double winding A.C./D.C. motor Ans: a
4. A synchronous motor can develop synchronous torque
(a) when under loaded
(b) while over-excited
(c) only at synchronous speed
(d) below or above synchronous speed Ans: c
5. A synchronous motor can be started by
(a) pony motor
(b) D.C. compound motor
(c) providing damper winding
(d) any of the above
Ans: d
6. A three-phase synchronous motor will have
(a) no slip-rings
(b) one slip-ring
(c) two slip-rings
(d) three slip-rings
Ans: c
7. Under which of the following conditions hunting of synchronous motor is likely to occur ?
(a) Periodic variation of load
(b) Over-excitation
(c) Over-loading for long periods
(d) Small and constant load
Ans: a
8. When the excitation of an unloaded salient pole synchronous motor suddenly gets disconnected (a) the motor stops
(b) it runs as a reluctance motor at the same speed
(c) it runs as a reluctance motor at a lower speed
(d) none of the above
Ans: a
9. When V is the applied voltage, then the breakdown torque of a synchronous motor varies as (a) V
(b) V312
(c) V2
(d) 1/V
Ans: a
10. The power developed by a synchronous motor will be maximum when the load angle is
(a) zero
(b) 45°
(c) 90°
(d) 120°
Ans: c
11. A synchronous motor can be used as a synchronous capacitor when it is
(a) under-loaded
(b) over-loaded
(c) under-excited
(d) over-excited Ans: d
12. A synchronous motor is running on a load with normal excitation. Now if the load on the motor is increased
(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 increaseAns: d
13. Mostly, synchronous motors are of
(a) alternator type machines
(6) induction type machines
(c) salient pole type machines
(d) smooth cylindrical type machinesAns: c
14. The synchronous motor is not inherently self-starting because
(a) the force required to accelerate the rotor to the synchronous speed in an instant isabsent
(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 currentsAns: a
15. As the load is applied to a synchronous motor, the motor takes more armature current because
(a) the increased load has to take more current
(b) the rotor by shifting its phase backward causes motor to take more current
(c) the back e.m.f. decreases causing an increase in motor current
(d) the rotor strengthens the rotating field casuing more motor current Ans: b
16. Synchronous motor always runs at
(a) the synchronous speed
(b) less than synchronous speed
(c) more than synchronous speed
(d) none of the above
Ans: a
17. An over-excited synchronous motor takes
(a) leading current (b) lagging current
(c) both (a) and (b)
(d) none of the above
Ans: a
18. The working of a synchronous motor is similar to
(a) gear train arrangement
(b) transmission of mechancial power by shaft
(c) distribution transformer
(d) turbine
(e) none of the above
Ans: b
19. The minimum armature current of the synchronous motor corresponds to operation at
(a) zero power factor leading
(b) unity power factor
(c) 0.707 power factor lagging
(d) 0.707 power factor leading
Ans: b
20. In a synchronous motor, the magnitude of stator back e.m.f. £& depends on
(a) d.c. excitation only
(b) speed of the motor
(c) load on the motor
(d) both the speed and rotor flux Ans: a
21. 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
22. 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
23. 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
24. 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 factorsAns: d
25. In a synchronous motor which loss varies with load ?
(a) Windage loss
(b) Bearing friction loss
(c) Copper loss
(d) Core loss
Ans: c
26. 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
27. 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
28. The shaft of synchronous motor is made of
(a) mild steel
(b) chrome steel
(c) alnico
(d) stainless steel
Ans: a
29. 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
30. The speed regulation of a synchronous motor is always
(a) 1%
(b) 0.5%
(c) positive
(d) zero
Ans: d
31. The percentage slip in case of a synchronous motor is
(a) 1%
(b) 100%
(c) 0.5%
(d) zeroAns: d
32. 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
33. 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 changesAns: c
34. 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
35. 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
36. The rotor copper losses, in a synchronous motor, are met by
(a) d.c. source
(b) armature input
(c) motor input
(d) supply linesAns: a
37. The maximum power developed in a synchronous motor occurs at a coupling angle of
(a) 30°
(b) 60°
(c) 90°
(d) 180°
Ans: c
38. 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
39. 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
40. 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
41. Synchronous motors are
(a) not-self starting
(b) self-starting
(c) essentially self-starting
(d) none of the above
Ans: a
42. The standard full-load power factor ratings for synchronous motors are
(a) zero or 0.8 leading
(b) unity or 0.8 lagging
(c) unity or 0.8 leading
(d) unity or zero
Ans: c
43. A synchronous motor running with normal excitation adjusts to load increases essentially by increase in (a) back e.m.f.
(b) armature current
(c) power factor(d) torque angle
Ans: b
44. A synchronous motor has better power factor as compared to that of an equivalent induction motor. This is mainly because
(a) synchronous motor has no slip
(b) stator supply is not required to produce magnetic field
(c) mechanical load on the rotor remains constant
(d) synchronous motor has large airgap Ans: b
45. A synchronous motor working at leading power factor can be used as
(a) voltage booster
(b) phase advancer
(c) noise generator
(d) mechanical synchronizer Ans: b
46. Slip rings are usually made of
(a) carbon or graphite
(b) brass or steel
(c) silver or gold
(d) copper or aluminium
Ans: b
47. An over excited synchronous motor is used for
(a) fluctuating loads
(b) variable speed loads
(c) low torque loads
(d) power factor corrections
Ans: d
48. When the voltage applied to a synchronous motor is increased, which of the following will reduce ?
(a) Stator flux
(b) Pull in torque
(c) Both (a) and (b)
(d) None of the above
Ans: d
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