Synchronous Motor MCQ Questions

1. Electromagnetic torque is present in rotating machines when

(a) stator winding carries current.
(b) rotor winding carries current.
(c) both stator and rotor windings carry current.
(d) none of the above.

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2. In a synchronous motor electromagnetic power varies

(a) directly as applied voltage.
(b) directly as square of the applied voltage.
(c) inversely as applied voltage.
(d) none of the above.

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3. The interaction torque in a synchronous motor depends upon

(a) stator field strength alone.
(b) rotor field strength alone.
(c) torque angle alone
(d) all of the above i.e. on stator field and rotor field strengths and torque angle.

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4. The breakdown torque of a synchronous motor varies as

(a) applied voltage V
(b) V2
(c) 1/V
(d) √V

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5. Higher the applied voltage

(a) higher will be the stator flux.
(b) higher will be the pull-in torque.
(c) lower will be the stator flux.
(d) lower will be pull-in torque.
(e) both (a) and (b).
(f) both (a) and (d).

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6. The operation of a 3-phase synchronous motor operating on constant excitation across infinite bus will not be stable if power angle δ

(a) exceeds intemal angle θ.
(b) is less than θ.
(c) exceeds θ/2.
(d) is less than θ/2.

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7. A 3-phase synchronous motor will operate on lagging power factor till

(a) open-circuit voltage per phase Eo exceeds V sinθ.
(b) Eo < V sinθ
(c) Eo< V
(d) Eo sinθ < V

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8. A 3-phase synchronous motor connected to an infinite bus-bar, with constant excitation, is driving a certain load and operating at leading power factor. If the shaft load is reduced,

(a) the load angle will increase.
(b) the load angle will decrease.
(c) power factor will increase.
(d) power factor will decrease.
(e) both load angle and power factor will decrease.

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9. A synchronous motor is supplying a load at unity pf. If the load on the motor is increased keeping its excitation and terminal voltage

constant, the power factor
(a) will remain the same.
(c) will become lagging.
(b) will become leading.
(d) none of the above. [A.M.I.E. Sec B. 1994]

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10. When does a synchronous motor operate with leading power factor current?

(a) While it is underexcited.
(b) While it is critically excited.
(c) While it is overexcited.
(d) While it is heavily loaded. [I.E.S. E.E.-II, 2007]

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11. A threes-phase synchronous motor connected to ac mains is running at full load and unity power factor. If its shaft load is reduced by half, with field current held constant, its new power factor will be

(a) unity.
(b) leading.
(c) lagging.
(d) dependent on machine parameters. [GATE E E. 2007]

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12. A 3-phase synchronous motor connected to an infinite bus is operating at half-full load with normal excitation. When the load on the synchronous motor is suddenly increased

(a) its speed will first decrease and then become synchronous.
(b) its speed will first increase and then become synchronous.
(c) its speed will fluctuate around synchronous speed and then become synchronous.
(d) its speed will remain unchanged.. [I.E.S. E.E-II, 1994]

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13. A synchronous generator connected to an infinite bus is over-excited. Considering only the reactive power, from the point of view of the system, the machine acts as

(a) capacitor.
(b) inductor.
(c) resistor.
(d) none of the above. [GATE E.E. 1998]

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14. A synchronous machine connected to a power system grid bus-bar is operating as a generator. To make the machine operate as a motor, the

(a) direction of rotation is to be reversed.
(b) phase-sequence is to be changed.
(c) field excitation is to be decreased.
(d) mechanical input is to be less than the losses at the shaft. [I.E.S. E.E.-II, 1993]

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15. A salient pole synchronous motor is running on no load. If its excitation is made off, it will

(a) continue running at synchronous speed.
(b) continue running but at a speed slightly less than synchronous one. (c) stop.
(d) none of these.

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16. What is the effect of the field failure of salient pole synchronous motor connected with infinite bus?

(a) Reduce motor torque and speed.
(b) Not change motor torque and speed.
(c) Stop the motor.
(d) Reduce motor torque but motor will continue to run at synchronous speed. [IES. E.E.-II, 2009]

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17. A salient pole synchronous motor continues running even if its excitation current is reduced to zero. This is due to

(a) rotating magnetic field of stator.
(b) rotating magnetic field of rotor.
(c) magnetization of rotor salient poles by stator magnetic field.
(d) interlocking action between stator and rotor rotating magnetic fields.

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18. When the excitation of normally operating unloaded salient-pole synchronous motor suddenly gets disconnected, it continues to run as a

(a) Scharge motor.
(b) Spherical motor.
(c) Switched-reluctance motor.
(d) Variable-reluctance motor. [I.E.S. E.E.-II, 2002]

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19. A smooth cylindrical synchronous motor will always stop when

(a) excitation winding gets disconnected.
(b) load varies.
(c) supply voltage fluctuates.
(d) supply frequency varies.

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20. The speed of a synchronous motor can be varied by varying its

(a) excitation.
(b) supply voltage.
(c) supply frequency.
(d) load.

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21. For a 3-phase, 4-pole, 50 Hz synchronous motor the frequency, pole number and the load torque are all halved. The motor speed will be

(a) 375 rpm.
(b) 75 rpm.
(c) 1,500 rpm.
(d) 3,000 rpm. [A.M.I.E. Electrical Science Winter 1993]

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22. A synchronous motor with comparatively large air gap gives

(a) higher stability limit.
(b) high synchronizing power making the machine less sensitive to load variations.
(c) both (a) and (b).
(d) none of these.

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23. In a 3-phase synchronous motor, the magnitude of back emf set up in the stator depends on

(a) rotor excitation.
(b) supply voltage.
(c) rotor speed.
(d) load on motor.

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24. The magnitude of counter or back emf in a synchronous motor

(a) is always less than supply voltage.
(b) is always more than supply voltage.
(c) is always equal to supply voltage.
(d) may be either equal to or less than or more than supply voltage.

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25. In a synchronous motor, the excitation voltage on no load is approximately equal to the applied voltage. This will happen

(a) when developed torque is maximum.
(b) with zero excitation.
(c) with 100% excitation.
(d) none of the above.

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26. Which motor can conveniently operate at lagging as well as leading power factor ?

(a) Squirrel cage induction motor.
(b) Wound rotor induction motor.
(c) Synchronous motor.
(d) DC shunt motor. [A.M.I.E. Electrical Science Summer 1995]

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27. At constant load, the magnitude of armature current drawn by a synchronous motor is large for

(a) low excitation.
(b) high excitation.
(c) both low and high values of field excitation.
(d) 100% excitation.

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28. Variation in dc excitation of a synchronous motor causes variation in

(a) speed of motor.
(b) power factor.
(c) armature current.
(d) both armature current and power factor.

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29. A synchronous motor, connected to an infinite bus, is working at a leading power factor. Its excitation emf Ef and terminal voltage Vt are related as under

(a) Ef > Vt and Ef lags Vt
(b) Ef < Vt and Ef lags Vt
(c) Ef > Vt and Ef leads Vt
(d) Ef< Vt, and Ef leads Vt [A.M.I.E. Sec B. Elec. Machines Summer 2002]

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30. If the field of a synchronous motor is under-excited, the power factor will be

(a) lagging.
(b) leading.
(b) unity.
(d) more than unity.

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