Synchronous Motor MCQ Questions PDF

1. A synchronous motor is operating on no load at unity power factor. If the field current is increased, the power factor will become

(a) leading and the current will decrease.
(b) lagging and the current will increase.
(c) lagging and the current will decrease.
(d) leading and the current will increase. [I.E.S. E.E.-II, 1998]

2. A three-phase, salient pole synchronous motor is connected to an infinite bus. It is operated at no load at normal excitation. The field excitation of the motor is first reduced to zero and then increased in the reverse direction gradually. Then the armature current

(a) increases continuously.
(b) first increases and then decreases steeply.
(c) first decreases and then increases steeply.
(d) remains constant. [GATE E.E., 2011]

3. A 3-phase synchronous motor is operating at a given load. If an increase in excitation reduces the armature current, it can be concluded that the motor is

(a) operating at lagging power factor and absorbing reactive power from the mains.
(b) operating at leading power factor and delivering reactive power to the mains.
(c) either (a) or (b).
(d) none of these.

4. A synchronous motor installed at the end of a transmission line is operating at lagging power factor. With the fall in supply voltage, the power factor of the synchronous motor will

(a) go down.
(b) improve.
(c) remain unchanged.
(d) none of these.

5. A synchronous motor operates at 0.8 pf lagging. If the field current of the motor is continuously increased

(a) the power factor decreases upto a certain value of the field current and there-after it increases.
(b) the armature current increases upto a certain value of the field current and thereafter it decreases.
(c) the power factor increases upto a certain value of field current and thereafter it decreases.
(d) the armature current decreases upto a certain value of field current and there after it increases.
(e) both (c) and (d). [GATE E.E. 1993]

6. Stability of a synchronous motor with the increase in excitation.

(a) increases
(b) decreases
(c) remains unaffected

7. Armature reaction in a synchronous motor at rated voltage and zero power factor (lead) is

(a) magnetising.
(b) cross-magnetising.
(c) both magnetising and cross-magnetising.
(d) demagnetising. [A.M.I.E. Sec B. Elec. Machines Winter 1994]

8. The torque angle of a synchronous machine operating from a constant voltage bus is usually defined as the space angle between

(a) rotor mmf wave and stator mmf wave.
(b) rotor mmf wave and resultant flux density wave.
(c) stator mmf wave and resultant flux density wave.
(d) stator mmf wave and resultant mmf wave. [GATE E.E. 1992]

9. A synchronous motor will deliver maximum power when

(a) load angle is equal to internal angle θ.
(b) input power factor is unity.
(c) load angle is 45°.
(d) load angle is 0°. [A.M.I.E. Sec B Elec. Machines Winter 1996]

10. A synchronous motor with negligible armature resistance runs at a load angle of 20° at the rated frequency. If supply frequency is increased by 10%, keeping other parameters constant, the new load angle will be

(a) 16°
(b) 18°
(c) 20°
(d) 22° [U.P.S.C. I.E.S. E.E.-II, 1999]

11. The power developed by a synchronous motor for constant supply voltage and constant excitation will be maximum when load angle δ is

(a) 90°
(b) 0°
(c) slightly less than 90°.
(d) slightly more than 90°.

12. For a given developed power, a synchronous motor operating from a constant voltage and constant frequency supply, will draw the minimum and maximum armature currents, I_min and I_max respectively, corresponding to

(a) I_min at unity pf, but I_max at zero pf.
(b) I_max at unity pf, but I_min at zero pf.
(c) both I_min and I_max at unity pf.
(d) both I_min and I_max at zero pf.

13. An inverted V-curve of synchronous motor shows the variation of

(a) power factor and de excitation at constant load.
(b) supply voltage and field current at constant excitation.
(c) power factor and supply voltage during hunting.
(d) supply voltage and excitation current at constant load. [I.E.S. E.E.-II, 1992]

14. Synchronous capacitor is

(a) an ordinary static capacitor bank.
(b) an overexcited synchronous motor driving mechanical load.
(c) an overexcited synchronous motor without mechanical load.
(d) none of the above. [A.M.I.E. Sec B. Electrical Machines Winter 1993]

15. An induction motor and synchronous motor are connected to a common feeder line. To operate the feeder line at unity pf, the synchronous motor should be

(a) under-excited.
(b) over-excited.
(c) normally excited.
(d) disconnected from the common terminals. [I.E.S. E.E.-II 2000]

16. Synchronous condensers, when operated at power factor ranging from lagging through unity to leading for voltage control, are called the

(a) voltage boosters.
(b) synchronous reactors.
(c) mechanical synchronisers.
(d) none of the above.

17. The power factor of a synchronous motor

(a) improves with increase in excitation and may even become leading at higher excitation.
(b) decreases with increase in excitation.
(c) is independent of excitation.
(d) increases with loading for a given excitation. [I.E.S. E.E.-II, 2002]

18. A 3-phase induction motor draws 1,000 kVA at a pf of 0.8 lag. A synchronous condenser is connected in parallel to draw an additional 750 kVA at a power factor of 0.6 lead. The pf of the total load supplied by the mains is

(a) unity.
(b) 0.707 lead.
(c) 0.6 lag.
(d) zero. [I.E.S. E.E.-II 2003]

19. Synchronous condenser means

(a) a synchronous motor with capacitor connected across terminals to improve pf.
(b) a synchronous motor operating at full load with leading pf.
(c) an overexcited synchronous motor partially supplying mechanical load, and also improving pf of the system to which it is connected.
(d) an overexcited synchronous motor operating at no load with leading pf used in large power stations for improvement of pf. [I.E.S. E.E.-II 2001]

20. Which of the following devices can be used as a phase advancer ?

(a) 3-phase induction motor squirrel cage type.
(b) 3-phase induction motor slip-ring type.
(c) Synchronous motor working at leading power factor.
(d) Synchronous motor working at lagging power factor. [I.E.S. 1996]

21. The phenomenon of oscillation of the rotor of a synchronous motor about its equilibrium position corresponding to new load on sudden throwing off or increasing of load is called the

(a) swinging.
(b) crawling.
(c) hunting.
(d) none of these.

22. A 3-phase synchronous motor hunts due to

(a) fluctuating load.
(b) fluctuating supply voltage.
(c) excessive field current.
(d) faulty connections.
(e) either fluctuating load or fluctuating supply voltage.

23. In a synchronous motor hunting can be reduced to minimum possible by

(a) providing damper winding in the rotor pole faces.
(b) using a flywheel.
(c) designing the motor for adequate synchronizing power.
(d) any of the above methods.

24. The damper windings also called the squirrel cage windings or damper grids

(a) are provided in a synchronous motor to make it self-starting and to prevent hunting.
(b) consists of short-circuited copper bars embedded in the field pole faces of a synchronous motor.
(c) are provided on the stator of a synchronous motor for improving the power factor.
(d) both (a) and (b).

25. In a synchronous machine, damper windings are used to

(a) help in starting as a motor.
(b) run it as an induction motor.
(c) help in starting as a motor and to reduce hunting.
(d) increase efficiency. [I.E.S. E.E.-II 2000]

26. During hunting of synchronous motor

(a) negative phase sequence currents are generated.
(b) harmonics are developed in the armature circuit.
(c) damper bar develops torque.
(d) field excitation increases. [GATE E.E. 1996]

27. When a salient pole 3-phase synchronous motor is started by induction motor action and its field is connected across a field discharge resistance, starting torque is developed by

(a) reluctance torque due to saliency of the rotor.
(b) eddy current and hysteresis torque in pole faces.
(c) induction motor torque in field and damper windings.
(d) all of the above.

28. Which of the following methods is employed for starting a 3-phase synchronous motor ?

(a) Star-delta starter.
(b) Damper winding.
(c) Resistance starter in the stator circuit.
(d) Damper winding in conjunction with a star-delta starter or an auto-transformer starter.

29. While starting a 3-phase synchronous motor by induction motor action, very high emf is induced in the field winding. The damage to the insulation of field winding and slip-rings can be avoided by

(a) splitting the field winding in several sections.
(b) short circuiting the field winding through field discharge resistance. (c) either (a) or (b).
(d) none of these.

30. Synchronous motors are inherently not self-starting motors as

(a) the direction of instantaneous torque on the rotor reverses after each half cycle.
(b) there is no slip.
(c) the stator does not produce revolving magnetic field.
(d) it has no starting winding.

 

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