Synchronous Motor MCQ

1. The mmf produced by single phase winding is

(a) pulsating and rotating with constant speed.
(b) pulsating and stationary.
(c) constant in amplitude and stationary.
(d) constant in amplitude and rotating.

2. A polyphase field is

(a) pulsating and stationary.
(b) pulsating and rotating.
(c) constant in amplitude and rotating at synchronous speed.
(d) constant in amplitude and stationary in space.

3. When a balanced 3-phase distributed type armature winding is carrying 3-phase, balanced currents, the strength of the resultant rotating magnetic field is

(a) three times the amplitude of each constituent pulsating magnetic field.
(b) equal to the amplitude of each constituent pulsating magnetic field.
(c) half the amplitude of each constituent pulsating magnetic field.
(d) one and half times the amplitude of each constituent of pulsating magnetic field. [U.P.S.C. I.E.S. 1994]

4. Synchronous motors generally have

(a) salient pole rotor.
(b) smooth cylindrical rotor.
(c) either salient pole or smooth cylindrical rotor.
(d) none of the above.

5. Number of slip-rings in a 3-phase synchronous motor will be

(a) 0
(b) 1
(c) 2
(d) 3 or 4. [A.M.I.E. Sec B. Electrical Machines Summer 1996]

6. Electromagnetic force or torque developed in any physical system tends to

(a) increase both the field energy and co-energy at constant current.
(b) reduce both the field energy and co-energy at constant current.
(c) reduce reluctance.
(d) reduce inductance.
(e) reduce permanence.
(f) both (a) and (c).

7. An ideal synchronous motor has no starting torque because the

(a) rotor is made up of salient poles.
(b) relative velocity between the stator and the rotor mmfs is zero.
(c) relative velocity between the stator and rotor mmfs is not zero.
(d) rotor winding is highly reactive. [I.E.S. 1998]

8. In a synchronous motor

(a) the rotor mmf and stator mmf are stationary with respect to each other.
(b) rotor mmf rotates slightly faster in comparison to stator mmf.
(c) stator mmf rotates slightly faster than rotor mmf.
(d) none of the above.

9. A synchronous niacinne is revolving armature and stationary field type. Under steady running conditions, the air gap field

(a) rotates at synchronous speed with respect to stator.
(b) rotates at synchronous speed in the direction of rotation of rotor.
(c) remains stationary with respect to stator.
(d) remains stationary with respect to rotor.

10. In a synchronous machine if the axis of the field flux coincides with that of armature flux, the machine

(a) will operate smoothly without any vibration.
(b) is said to be floating.
(c) is operating as a synchronous generator.
(d) is operating as a synchronous motor.

11. In a 3-phase synchronous motor the magnitude of field flux

(a) remains constant at all loads.
(b) varies with load.
(c) varies with speed.
(d) varies with power factor.

12. In a 3-phase synchronous motor

(a) the field mmf leads the air gap flux and air-gap flux leads the armature mmf.
(b) the armature mmf leads the air-gap flux and air-gap flux leads the field mmf.
(c) the armature mmf leads the air-gap flux and air-gap flux lags behind the field mnif.
(d) none of the above.

13. What is the angle between the induced voltage and supply voltage of a synchronous motor under running condition?

(a) Zero.
(h) Greater than zero but less than or equal to 90⁰.
(c) Between 90° and 180°.
(d) > 180^o. [U.P.S.C. I.E.S. 2007]

14. The flux per pole in a synchronous motor with the field circuit ON and the stator disconnected from the supply is found to be 25 m Wb. When the stator is connected to the rated supply with the field excitation unchanged, the flux per pole in the machine is found to be 20 m Wb while the motor is running on no load. Assuming no-load losses to be zero, the no-load current by the motor from the supply

(a) lags behind the supply voltage.
(b) leads the supply voltage.
(c) is in phase with the supply voltage. [GATE E.E. 2002; A.M.I.E. 2002]

15. The relative speed between the magnetic fields of stator and rotor under steady state operation is zero for

(a) a dc machine.
(b) an induction machine.
(c) a synchronous machine.
(d) all the above machines. [A.M.I.E. Electrical Science Summer 1994]

16. A 3-phase synchronous motor has

(a) high starting torque.
(b) no starting torque.
(c) low starting current.
(d) low starting torque. [I.E.S. E.E.-II, 2003]

17. A 10 pole, 25 Hz alternator is directly coupled to and is driven by 60 Hz synchronous motor. What is the number of poles for the synchronous motor?

(a) 48
(b) 12
(c) 24
(d) 16 [I.E.S. E.E.-II, 2007]

18. The coupling magnetic field reaction with system(s) is essential for extracting ………. energy from an electrical system.

(a) mechanical
(b) electrical
(c) both electrical and mechanical

19. The armature current of a synchronous motor on no load

(a) leads the applied voltage by 90°.
(b) lags behind the applied voltage by 90°.
(c) is in phase will applied voltage.
(d) zero.

20. The synchronous motor meets increase in load by taking more armature current as

(a) the rotor pole falls back relative to the stator pole causing an increase in motor current.
(b) the back emf decreases causing an increase in motor current.
(c) rotating field is strengthened causing an increase in motor current.
(d) none of the above.

21. A synchronous motor is operating with normal excitation. With the increase in load the armature current drawn from the supply main increases due to

(a) increase in back emf.
(b) fall in motor speed.
(c) increase in resultant voltage across the armature.
(d) increase in power factor.

22. The resultant voltage acting across the armature circuit of a synchronous motor (E_r) is the ……….. of induced emf in the armature circuit (E_b) and supply voltage V.

(a) arithmetic sum.
(b) arithmetic difference.
(c) phasor difference.
(d) phasor sum.

23. The magnitude of emf induced in the armature of a synchronous motor (E_b) depends upon

(a) supply voltage.
(b) rotor excitation.
(c) rotor speed.
(d) flux density.
(e) all of the above.

24. The phase of emf induced in the armature of a synchronous motor

(E_b) depends upon
(a) rotor speed.
(b) load.
(c) both load and speed.
(d) none of the above.

25. In a synchronous motor, the synchronizing power comes into action when

(a) rotor speed either exceeds or falls below the synchronous speed.
(b) rotor speed is equal to synchronous speed.
(c) rotor speed falls below the synchronous speed.
(d) rotor speed exceeds the synchronous speed.

26. Synchronizing power of a synchronous motor varies

(a) directly as synchronous reactance, X_s.
(b) directly as the square of the synchronous reactance, X_s².
(c) inversely as the synchronous reactance, 1/X_s.
(d) none of the above.

27. The coupling angle or load angle of a synchronous motor is defined as the space angle between the

(a) rotor and stator poles of the same polarity.
(b) rotor and stator poles of opposite polarity.
(c) rotor and stator teeth.
(d) none of the above.

28. The torque angle δ is defined as the space angle between

(a) stator field axis and resultant field axis.
(b) rotor field axis and resultant field axis.
(c) stator field axis and rotor field axis.
(d) stator field axis and mutual field axis.

29. The value of load angle for a synchronous motor depends mainly upon its

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

30. The total steady state torque to drive the synchronous motor and the load at synchronous speed is known as …….. torque.

(a) reluctance
(b) pull-out
(c) synchronous
(d) average

31. A synchronous motor may fail to pull into synchronism owing to

(a) excessive load.
(b) low excitation.
(c) high friction.
(d) any of these.

32. Which of the following is not the advantage of a synchronous motor ?

(a) High operation efficiency.
(b) Operation over a wide range of power factor—from lagging to leading.
(c) It can always be adjusted to operate at unity power factor for optimum efficiency and economy.
(d) Its rotor has 2 slip-rings.
(e) Operation at constant speed from no-load to full load.

33. Which of the following is not the disadvantage of a synchronous motor ?

(a) Its torque is less sensitive to variations in supply voltage.
(b) DC excitation is required.
(c) It has tendency to hunt.
(d) Its starting under load is not possible.
(e) It is unsuitable for line shaft drive in a small workshop.

34. Which of the following synchronous motors has cost comparable to that of an induction motor ?

(a) High kW output high speed.
(b) High kW output low speed.
(c) Low kW output low speed.
(d) Low kW output high speed.

35. A 3-phase synchronous motor is widely used for

(a) power factor improvement.
(b) control of voltage at the end of a transmission line.
(c) high torque loads.
(d) fluctuating loads.
(e) both (a) and (b).

36. Synchronous motors are not used

(a) in power houses and substations in parallel to bus-bars for pf correction.
(b) in factories having a large number of induction motors and other power apparatus operating at lagging power factor, for pf improvement.
(c) for driving electric trains, cranes, machine tools and line shaft drives.
(d) in rubber mills, textile mills, cement factories, mining industries and other big industries for power applications.
(e) for driving continuously operating and constant peed equipment such as centrifugal pumps, fans, blowers, ammonia and air compressors, motor generator sets etc.

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