301 MCQ Questions on Synchronous Machine

1. A 3ɸ  synchronous motors has been provided with damper winding. It can be started as a

(a) Single phase synchronous motor.
(b) 3ɸ  squirrel cage induction motor.
(c) 1ɸ  induction motor.
(d) 3ɸ  alternator. [UPSSSC 2015]

2 Consider the following statements about a three-phase synchronous generator synchronized to an infinite bus when its mechanical input is increased gradually with field current held constant

1. The power factor of the current supplied becomes more lagging.
2. The power factor of the current supplied improves.
3 The power factor remains unity.
4 The load angle is increased.
Of these statements are correct:
(a) 1 only
(c) 2 and 4
(b) 2 only
(d) 3 and 4

3. Which of the following will change in a three-phase synchronous motor, as a consequence of excitation variations?

1. Pull-out torque
2. Torque angle
3. Magnitude and power factor of stator current
4. Output power

Select the correct answer using the codes given below:
(a) 1 and 3
(b) 2 and 4
(c) 2, 3 and 4
(d) 1, 2 and 3

4. A 3-phase alternator delivers power to a balanced 3-phase load of power factor 0.707 lagging. It is observed that the open-circuit emf phasor leads the corresponding terminal voltage phasor by 15. Neglecting the effect of harmonics, the angle between the axis of the main field mmf and the axis of the armature mmf will be

(a) 30° electrical
(b) 60° electrical
(c) 90° electrical
(d) 150° electrical

5. In an AC machine, the effect of distributing the turns in different slots, results in a further reduction of generated EMF by the factor k_d. This factor is called

(a) Distribution spread factor
(b) Coil pitch factor
(c) Winding factor
(d) Generation factor [RRB SSE – 2015]

6. A single-phase alternator has a synchronous Reactance of 2 ohms and negligible resistance. If it supplies 10 A to a purely capacitive load at 200V, then the generated emf will be

(a) 240 volts
(b) 220 volts
(c) 200 volts
(d) 180 volts

7. Suppose a synchronous generator connected to an infinite bus is supplying electrical power at unity PF to the bus. If its field current is now increased

(a) Both the active and reactive power output of the generator will remain unchanged.
(b) The active power supplied will remain unchanged but the machine will also supply lagging reactive power.
(c) The active power supplied will increase and the machine will draw leading reactive power.
(d) The active power supplied will decrease and the machine will supply leading reactive power.

8. If the load of a synchronous motor is increased while keeping the field excitation constant, then

(a) Power factor will increase and power angle will decrease.
(b) Power factor will decrease and power angle will increase.
(c) Both power factor and power angle will increase.
(d) Both power factor and power angle will decrease.

9. A synchronous motor operates as a synchronous condenser when it is

(a) Operated at unity power factor
(b) Under-excited
(c) Over-excited
(d) Connected in parallel with condensers

10. The field winding of a 3-phase synchronous motor is short-circuited directly. If a 3-phase balanced voltage is impressed across the stator terminals, then the rotor will

(a) Rotate at synchronous speed
(b) Rotate at a speed slightly less than synchronous speed
(c) Rotate at a speed very near to half of the synchronous machine
(d) Not rotate at all

11. In a synchronous machine, the electric circuits are armature circuit, field circuit and damper circuit and X_d is d-axis synchronous reactance; X’_d, is d-axis transient synchronous reactance and X”_d is d-axis sub-transient synchronous reactance. If there is no damper winding, then

(a) X’_d = X”_d
(b) X’’_d = X_d
(c) X’_d = X_d
(d) X_d = X’_d = X”_d

12. A synchronous motor is operating with normal excitation. With the increase in load, the armature current drawn from the supply mains increase due to:

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

13. In a 3-phase ac machine, the phase emf and output power for a phase spread of 60° are respectively E₁ and P₁ the phase emf and output power for a phase spread of 120° are respectively E₂ and P₂. In this context which one of the following sets of relations is correct?

(a) E₁ = E₂ and P₁ = P₂
(b) E₁ = 1.15E₂ and P₁ = 1.15P₂
(c) E₁ =1.15E₂ and P₁ = P₂
(d) E₁ = E₂ and P₁ = 1.15P₂

14. A synchronous generator with a synchronous reactance of 1.3 pu is connected to an infinite bus whose voltage is 1 pu, through an equivalent reactance of 0.2 pu. For maximum output of 1.2 pu, the alternator emf must be

(a) 1.5 pu
(b) 1.56 pu
(c) 1.8 pu
(d) 2.5 pu

15. A 3-phase synchronous motor with negligible losses is connected to the supply at rated frequency and constant terminal voltage V. The induced emf of the motor is designated as E. If the motor is now gradually loaded to its rated power, adjusting its excitation to obtain, say 0.8 leading power factor operation, then E-phasor would

(a) Be less than V-phasor and also lead V-phasor.
(b) Be greater than V-phasor and also lead V-phasor.
(c) Be less than V-phasor and also lag V-phasor.
(d) Be greater than V-phasor and also lag V-phasor.

16. Consider the following statements regarding an electrical machine having cylindrical stator and salient-pole rotor:

1. Reluctance torque is produced when rotor winding is excited.
2. Reluctance torque is produced when stator winding is excited.
3. When both stator and rotor windings are excited, electromagnetic torque is produced.
4. When both stator and rotor windings are excited, electromagnetic as well as reluctance torques are produced.

Which of these statements are correct?
(a) 2 and 4
(b) 1 and 4
(c) 1 and 3
(d) 2 and 3

17. To calculate the full-load voltage regulation of a synchronous generator at any power factor using potier triangle method, it is required to have

(a) Open-circuit and short-circuit characteristics.
(b) Open-circuit and zero power factor characteristics.
(c) Open-circuit characteristics, field current at rated short-circuit current and armature resistance.
(d) Short-circuit characteristics, zero power factor characteristics and armature resistance.

18. An under-excited synchronous motor is operating at no load on an infinite bus of voltage 0.75 p.u and drawing a current of 0.8 p.u. at 0.8 pf lag. Its synchronous reactance is 0.5 p.u. What is the p.u. value of its excitation emf?

(a) 1.4
(b) 1.0
(c) 0.6
 (d) 0.4

19. The field current of a synchronous motor is increased while its load is constant How will its power angle and power factor change?

(a) Power angle decreases and power factor improves.
(b) Power angle remains same throughout but power factor improves.
(c) Power angle increases while its power factor gradually decreases.
(d) Power angle and power factor both increase.

20. Two 3-phase alternators are synchronized so that their no-load voltages E_n1 and E_n2 are equal. If the excitation of machine 1 is now increased, then

1. A local circulating current I_c is set up.
2. I_c magnetizing in nature for machine 2.
3. I_c tends to reduce E_n1
4. The terminal voltage does not change.

Which of the statements given above are correct?
(a) 2 and 3
(c) 1, 3 and 4
(b) 1, 2 and 4
(d) 1, 2 and 3

21. A 3-phase cylindrical rotor alternator is delivering rated power to a grid at its rated power factor. On account of some fault, steam supply to the turbine of the alternator is cut-off. The alternator would

(a) Stop.
(b) Continue running as an alternator at synchronous speed N, and deliver active power to the grid.
(c) Continue running as a synchronous motor at N, and deliver reactive power to the grid.
(d) Continue running as a synchronous motor at N, and draw reactive power from the grid.

22. Which of the following information is obtained, by Potier’s method using open circuit characteristic and zero power factor characteristics?

(a) Field current equivalent of armature reaction.
(b) Synchronous reactance.
(c) Leakage reactance.
(d) Leakage reactance and field current equivalent of armature reaction.

23. Alternator is operating on infinite bus. It will develop maximum synchronizing power when it is operating at

(a) No load.
(b) Full Load.
(c) Its steady state stability limit.
(d) Any load.

24. Short circuit ratio of a synchronous machine is the ratio of

(a) Field current required to produce rated voltage on open circuit and rated armature current.
(b) Field current required to produce rated armature current on short circuit to that required to produce rated voltage on open circuit.
(c) Field current required to produce rated voltage on open circuit to that required to produce rated armature current on short circuit.
(d) Open circuit voltage to short circuit armature current for the same field current.

25. A salient pole synchronous motor is running with normal excitation. If the excitation is reduced to zero

(a) It becomes an induction motor.
(b) It becomes a reluctance motor.
(c) It remains a synchronous motor.
(d) The motor would stop.

26. In a synchronous generator operating at zero pf lagging, the effect of armature reaction is

(a) Magnetizing.
(b) Demagnetizing.
(c) Cross-magnetizing.
(d) Both magnetizing and cross-magnetizing.

27. Consider the following reasons excited for providing damper bars on the pole faces of a synchronous motor:

1. Starting the motor as a squirrel cage induction motor.
2. To reduce the tendency of oscillation of the rotor with load changes.
3. To provide additional induction motor torque besides the main synchronous motor torque
4. To reduce the effects of slot harmonics causing noise and vibrations

Of these reasons:
(a) 1 alone is valid.
(b) 1 and 2 are valid.
(c) 1, 2 and 3 are valid.
(d) 2, 3 and 4 are valid.

28. Open-circuit (OC) and short circuit (SC) tests on an AC generator are conducted under which of the following conditions?

1. OC test at nominal flux and SC test at nominal current
2. OC test at reduced flux and SC test at reduced current
3. OC test at zero armature current and SC test at reduced flux
4. OC test zero current and SC test at nominal flux

Select the correct answer using the codes given below:
(a) 1 and 3
(b) 2 and 3
(c) 1 and 4
(d) 2 and 4

29. During the slip-test for determining the direct and the quadrature axis synchronous reactances of an alternator, the voltage across the open field circuit terminals

(a) DC voltage
(b) AC voltage of supply frequency
(c) AC voltage of slip frequency
(d) A modulated voltage with an envelope of slip frequency

30. The fifth harmonic component of the induced voltage in a 3-phase ac generator coil, be entirely eliminated by using a winding pitch of

(a) 2/3
(b) 4/5
(c) 5/6
 (d) 6/7

31. For proper synchronisation of a large synchronous machine to a bus, the frequency of the incoming machine

(a) Should be exactly the same as that of the bus.
(b) Should be slightly higher than that of the bus.
(c) Should be slightly lower than that of the bus.
(d) Can be of any value.

32. An alternator coupled to which prime mover will usually have the highest rotating speed?

(a) Steam engine
(b) Reciprocating diesel engine
(c) Francis turbine
(d) Steam turbine

33. In an alternator the voltage generated per phase in proportional to

(a) Number of turns in coil
(b) Flux per pole
(c) Frequency of waveform
(d) All of the above

34. Turbo-alternators are generally used to run at

(a) 1500 r.p.m.
(c) 5000 r.p.m.
(b) 3000 r.p.m.
(d) 15000 r.p.m.

35. The dark and bright lamp method is used for

(a) Transfer of load
(b) Synchronizing
(c) Balancing of load
(d) Phase sequence

36. Which harmonic will be totally eliminated in an alternator by using a fractional pitch of 4/5?

(a) Third
(c) seventh
(b) Fifth
(d) Ninth

37. Damper winding on alternator results in all of the following except

(a) Increases instability of machine
(b) Elimination of harmonic effects
(c) Absorption of energy of oscillations when operating in parallel with other alternators
(d) Suppression of spontaneous hunting when supplying power to transmission line with high resistance to reactance ratio

38. In an alternator the armature reaction is mainly influenced by

(a) The power factor of the load
(b) Short-circuit ratio
(c) Speed of the alternator
(d) Total current drawn

39. In an alternator terminal voltage rise will be more

(a) When leading load is thrown off.
(b) When lagging load is thrown off.
(c) When unity power factor load is thrown off.
(d) None of the above.

40. The power drawn by the prime-mover of an alternator, under no-load conditions, goes to

(a) Meet copper losses both in armature and rotor windings
(b) Produce power in armature
(c) Meet no-load losses
(d) Produce e.m.f in armature winding

41. The advantage of salient poles in an alternator is

(a) Reduced windage loss
(b) Reduced bearing loads and noise
(c) Reduced noise
(d) Adaptability of low and medium speed operation

42. When two alternators are running in exact synchronism, the synchronizing power will be

(a) Unity
(b) Zero
(c) Sum of the output of two
(d) None of the above

43. A synchronous motor can be started by

(a) Pony motor
(b) D.C. compound motor
(c) Providing damper winding
(d) Any of the above

44. 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

45. 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

46. The back e.m.f. of a synchronous motor depends on

(a) Speed
(c) Load angle
(b) Load
(d) All of the above

47. Synchronizing power of a synchronous machine is

(a) Directly proportional to the synchronous reactance
(b) Inversely proportional to the synchronous reactance
(c) Equal to the synchronous reactance
(d) None of the above

48. An over-excited synchronous motor is used for

(a) Fluctuating loads
(b) Variable speed loads
(c) Low torque loads
(d) Power factor corrections

49. Stability of a synchronous machine

(a) Decreases with increase in its excitation
(b) Increases with increase in its excitation
(c) Remains unaffected with increase in excitation
(d) Any of the above

50. If one-phase of a 3-phase synchronous motor is short-circuited, motor

(a) Will refuse to start
(b) Will overheat in spots
(c) Will not come upto speed
(d) Will fail to pull into step