DIFFERENTIAL PROTECTION

Differential protection is a very reliable method of protecting generators, transformers, buses, and transmission lines from the effects of internal faults.

Figure: Differential Protection of a Generator
In a differential protection scheme in the above figure, currents on both sides of the equipment are compared. The figure shows the connection only for one phase, but a similar connection is usually used in each phase of the protected equipment. Under normal conditions, or for a fault outside of the protected zone, current I₁ is equal to current I₂ . Therefore the currents in the current transformers secondaries are also equal, i.e. i₁ = i₂ and no current flows through the current relay.
If a fault develops inside of the protected zone, currents I₁ and I₂ are no longer equal, therefore i₁ and i₂ are not equal and there is  a current flowing through the current relay.

Differential Protection of a Station Bus

The principle of the differential protection of a station bus is the same as for generators.

The sum of all currents entering and leaving the bus must be equal to zero under normal conditions or if the fault is outside of the protected zone. If there is a fault on the bus, there will be a net flow of current to the bus and the differential relay will operate.

Figure: Single Line Diagram of Bus Differential Protection

Percentage Differential Relays
The disadvantage of the current differential protection is that current transformers must be identical, otherwise there will be current flowing through the current relays for faults outside of the protected zone or even under normal conditions. Sensitivity to the differential current due to the current transformer errors is reduced by percentage differential relays.

Figure: Percentage Differential Relay

In percentage differential relays, the current from each current transformer flows through a restraint coil. The purpose of the restraint coil is to prevent undesired relay operation due to current transformer errors. The operating coil current | i₁ - i₂ |  required for tripping is a percentage of the average current through the restraint coils. It is given by

where k is the proportion of the operating coil current to the restraint coil current. For example if k = 0.1, the operating coil current must be more than 10% of the average restraint coil current in order for the relay to operate.

Differential Protection of Three Phase Transformers

Differential protection of three phase transformers must take into account the change in magnitude and phase angle of the transformed current.

Transformers Connected Y-Y or Delta-Delta
In these two connections, the primary and secondary currents are in phase, but their magnitudes are different. The difference in the current magnitude must be balanced out by the current transformer ratios.

Figure: Differential Protection for a Y-Y Connected Transformer
If the transformer ratio is

 
The secondary currents of the current transformers are

During normal operating conditions or when the fault is outside of the protection zone,

 Therefore, the ratios of the current transformers on the two sides of the power transformer must be 

Sometimes standard current transformers with the ratios that satisfy the above equation are not available. In that case auxiliary transformers between one of the current transformers and the relay are used.

 

Transformers Connected Y-D or D -Y.

The primary and secondary currents have different magnitudes and they also have 30° phase shift. Both, the magnitude and the phase shift must be balanced by appropriate ratio and connection of the current transformers. The phase shift on a Y-D bank is corrected by connecting the C.T.’s on the D in Y, and on the Y side in D .
Refer to the following drawing. The full load current on the 66 kV side is

The full load current on the 230 kV side is

The secondary currents in the current transformers on the 66 kV side then are

The magnitude of the currents coming out of the differential relay should be the same

 
From that, the current in the D arms of the D connected C.T.’s should be

 
Ideally, the CTR on the 230 kV side of the transformer should be

The closest to that is the ratio

 
which is the ratio that will be used.. Using this ratio, the secondary current of the current transformers on the 230 kV side is

The current through the operating coil of the differential relay is then

The average current through the current restraint coil is

 
From that, the current through the operating coil as a percentage of the restraint current under normal full load conditions is

The percentage differential relays have settings for the allowable percentage difference. Examples of the percentage values are 15%, 30%, 40%, etc. Any of these relays could accommodate the 0.46% operating coil current without operating.

Connection of Differential Relays to a D -Y Connected Transformer.

Another problem that the differential relays used for transformer protection must overcome is the magnetizing inrush current.
The inrush current occurs when a transformer is being energized. Since during the energization of the transformer there is only current in and no current out, the inrush current appears to the differential relays as an internal fault. The inrush current has some characteristic properties. Its magnitude may be as high as sixteen times the full load current. It decays very slowly - from around ten cycles for small units to 1 minute for  large units. The harmonic content of the inrush current is different from normal load current and from fault currents. A typical waveform of inrush current has a large fundamental frequency component, a significant d.c. component, and 2nd and 3rd harmonic components. The 2nd harmonic component does not appear in the transformers under any other conditions except during energization. Desensitizing of the differential relay to the inrush current involves the use of the second harmonic component to restrain the relay from operating.
(a)

(b)

Figure: Harmonic Restraint Circuit: (a) connection to current transformer   (b) tripping circuit

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Multiple Choice Questions of Synchronous Motors

1. Synchronous motor can operate at

(A) Lagging power factor only

(B) Leading power factor only

(C) Unity power factor only

(D) Lagging, leading and unity power factor only.

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D

2. An unexcited single phase synchronous motor is

(A) reluctance motor

(B) repulsion motor

(C) universal motor

(D) AC series motor.

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A

3. The maximum power developed in the synchronous motor will depend on

(A) rotor excitation only

(B) maximum value of coupling angle

(C) supply voltage only

(D) rotor excitation supply voltage and maximum value of coupling angle.

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D

4. In case the field of a synchronous motor is under excited, the power factor will be

(A) leading

(B) lagging

(C) zero

(D) unity.

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B

5. A synchronous motor is switched on to supply with its field windings shorted on themselves. It will

(A) not start

(B) start and continue to run as an induction motor

(C) start as an induction motor and then run as synchronous motor

(D) bum immediately.

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C

6. When the excitation of an unloaded salient pole synchronous motor gets disconnected

(A) the motor will bum

(B) the motor will stop

(C) the motor will ran as a reluctance motor at the same speed

(D) the motor will run as a reluctance motor at a lower speed.

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B

7. The damping winding in a synchronous motor is generally used

(A) to provide starting torque only

(B) to reduce noise level

(C) to reduce eddy currents

(D) to prevent hunting and provide the starting torque.

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D

8. The back emf set up in the stator of a synchronous motor will depend on

(A) rotor speed only

(B) rotor excitation only

(C) rotor excitation and rotor speed

(D) coupling angle, rotor speed and excitation.

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B

9. A synchronous motor is a useful industrial machine on account of which of the following reasons ?

I. It improves the power factor of the complete installation

II. Its speed is constant at all loads, provided mains frequency remains constant

III. It can always be adjusted to operate at unity power factor for optimum efficiency and economy.

(A) I only

(B) II only

(C) III only

(D) I, II and III.

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D

10. Which of the following is an unexcited single phase synchronous motor ?

(A) A.C. series motor

(B) Universal motor

(C) Reluctance motor

(D) Repulsion motor.

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C

11. An over excited synchronous motor draws current at

(A) lagging power factor

(B) leading power factor

(C) unity power factor

(D) depends on the nature of load.

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B

12. With the increase in the excitation current of synchronous motor the power factor of the motor will

(A) improve

(B) decrease

(C) remain constant

(D) depend on other factors.

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A

13. The armature current of a synchronous motor has large values for

(A) low excitation only

(B) high excitation only

(C) both low and high excitation

(D) depends on other factors.

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C

14. A synchronous motor is switched on to supply with its field windings shorted on themselves. It will

(A) not start

(B) start and continue to run as an induction motor

(C) start as induction motor and then run as a synchronous motor.

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C

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

(A) lagging

(B) leading

(C) unity.

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A

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1.D ----- 2.A ----- 3.D ----- 4.B ----- 5.C ----- 6.B ----- 7.D ----- 8.B -----9.D -----10.C -----11.B ----- 12.A -----13.C -----14.C ----- 15.A

16. 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 some speed

(C) it runs as a reluctance motor at a lower speed.

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A

17. The armature current of the synchronous motor has large values for

(A) low excitation only

(B) high excitation only

(C) both high and low excitation.

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C

18. What is the ratio of no load speed to full load speed of a 200 kVA, 12 pole, 2200 V, 3 phase, 60 Hz synchronous motor ?

(A) 1

(B) 1.1

(C) 1.21

(D) infinite.

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A

19. If a synchronous motor drops too far behind, the power it takes from the supply also increases too much, and the armature tries to get accelerated, until it is in correct position. Sometimes, some motor overshoots the marks and then the process of acceleration-retardation continues. This phenomenon is known as

(A) synchronization

(B) hunting

(C) pulling out

(D) swinging.

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B

20. The maximum value of torque that a synchronous motor, can develop without losing its synchronism, is known as

(A) breaking torque

(B) synchronizing torque

(C) pull out torque

(D) slip torque.

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B

21. In a synchronous motor if the back emf generated in the armature at no load is approximately equal to the applied voltage, then

(A) the torque generated is maximum

(B) the excitation is said to be zero percent

(C) the excitation is said to be 100%

(D) the motor is said to be fully loaded.

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C

23. A 3 phase, 400 V, 50 Hz salient pole synchronous motor is fed from an infinite bus and is running at no load. Now if the field current of the motor is reduced to zero

(A) the motor will stop

(B) the motor will run

(C) the motor will run at synchronous speed

(D) the motor will run at less than synchronous speed.

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C

24. The purpose of embedding the damper winding in the pole face is to

(A) eliminate hunting and provide adequate starting torque

(B) reduce windage losses

(C) eliminate losses on account of air friction

(D) reduce bearing friction.

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A

25. A synchronous motor is switched on to supply with its field windings shorted on themselves. It will

(A) not start

(B) start but continue to run as an induction motor

(C) start as an induction motor and then run as a synchronous motor.

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B

26. In case of a synchronous motor we have

I. Load

II. Speed

III. DC excitation.

The magnitude of stator back emf depends on

(A) I only

(B) I and II only

(C) III only

(D) I, II and III.

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C

27. Which of the following motors is non-self starling ?

(A) squirrel cage induction motor

(B) wound rotor induction motor

(C) synchronous motor

(D) DC series motor.

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C

28. The back emf in the stator of a synchronous motor depends on

(A) speed of rotor

(B) rotor excitation

(C) number of poles

(D) flux density.

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B

29. Which motor can conveniently operate on lagging as well as leading power factor ?

(A) squirrel cage induction motor

(B) wound rotor induction motor

(C) synchronous motor

(D) any of the above.

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C

30. A synchronous motor working on leading power factor and not driving any mechanical, is known

(A) synchronous induction motor

(B) spinning motor

(C) synchronous condenser

(D) none of the above.

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C

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16.A ----- 17.C ----- 18.A ----- 19.B ----- 20.B ----- 21.C ----- 22.B ----- 23.C -----24.A -----25.B ----- 26.C ----- 27.C -----28.B -----29.C ----- 30.C

31. The constant speed of a synchronous motor can be changed to new fixed value by

(A) changing the applied voltage

(B) interchanging any two phases

(C) changing the load

(D) changing the frequency of supply.

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D

32. A 3 phase, 400 V, 50 Hz synchronous motor is operating at zero power factor lagging with respect to the excitation voltage. The armature reaction mmf. produced by the armature current will be

(A) demagnetizing

(B) magnetizing

(C) cross-magnetizing

(D) none of the above.

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B

33. In a synchronous motor, the torque angle is

(A) the angle between the rotating stator flux and rotor poles

(B) the angle between magnetizing current and back emf

(C) the angle between the supply voltage and the back emf

(D) none of the above.

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A

34. A 3 phase, 400 V, 50 Hz, 4 pole synchronous motor has a load angle of 10° electrical. The equivalent mechanical degrees will be 35.

(A) 10°

(B)5√2 degrees

(C) 5 degrees

(D) 1 degree.

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C

35. A 3 phase, 400 V, 50 Hz synchronous motor has fixed excitation. The load on the motor is doubled. The torque angle, φ_i will become nearly

(A) φ_r /2

(B) φ_r

(C)2 φ_r

(D) √ 2 φ_r

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C

36. The hunting in a synchronous motor takes place when

(A) friction in bearings is more

(B) air gap is less

(C) load is variable

(D) load is constant.

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C

37. V curves for a synchronous motor represent relation between

(A) field current and speed

(B) field current and power factor

(C) power factor and speed

(D) armature current and field current.

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D

38. The breakdown. torque of a synchronous motor varies as

(A)1 /(applied voltage )

(B) 1/(applied voltage )²

(C) applied voltage

(D) (applied voltage)².

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C

39. Hunting in a synchronous motor cannot be due to

(A) variable frequency

(B) variable load

(C) variable supply voltage

(D) windage friction.

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D

40. 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 at a reluctance motor at a lower speed.

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A

41. Which synchronous motor will be smallest in size ?

(A) 5 HP, 500 rpm

(B) 5 HP, 375 rpm

(C) 10 HP, 500 rpm

(D) 10 HP, 375 rpm.

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B

42. A synchronous machine has its field winding on the stator and armature winding on the rotor. Under steady running conditions, the air-gap field

(A) rotates at synchronous speed with respect to stator

(B) rotates at synchronous speed with direction of rotation of the rotor

(C) remains stationary with respect to stator

(D) remains stationary with respect to rotor.

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C

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

(A) unity

(B) lagging

(C) leading

(D) more than unity.

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B

44. The name plate of an induction motor reads 3 phase. 400 V, 50 Hz. 0.8 of lagging, 1440 rpm. On similar lines the name plate of a synchronous motor should read

(A) 3 phase, 400 V, 50 Hz, 0.8 pf lagging, 1500 rpm

(B) 3 phase, 400 V. 50 Hz, 0.8 pf leading, 1500 rpm

(C) 3 phase, 400 V, 50/60 Hz, 0.8 pf lagging, 1500 rpm

(D) 3 phase. 400 V, 50/60 Hz, 0.8 pf leading, 1500 rpm.

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B

45. In which coil the emf generated will be more, for given flux distribution and number of turns

(A) Full pitch coil

(B) Short pitch coil

(C) Long pitch coil

(D) Equal emf will be generated in all cases.

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A

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31.D ----- 32.B ----- 33.A ----- 34.C ----- 35.C ----- 36.C ----- 37.D ----- 38.C ----- 39.D ----- 40.A -----41.B ----- 42.C -----43.B -----44.B ----- 45.A

46. In a synchronous motor which loss does not vary with load ?

(A) Copper losses

(B) Hysteresis losses

(C) Windage losses

(D) None of the above.

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C

47. In a 3-phase. 400 V, 50 Hz salient pole synchronous motor, the maximum power is obtained when the load angle is

(A) 45°

(B) less than 90°

(C) 90°

(D) more than 90°.

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B

48. A high starting torque synchronous motor has

(A) simplex rotor

(B) phase wound damper

(C) five slip rings

(D) all of the above.

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D

49. In a three phase synchronous motor, the magnitude of field flux

(A) varies with speed

(B) varies with load

(C) remains constant at all loads

(D) varies with power factor.

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C

50. The parameter connected with the operation of a synchronous motor are

I. Speed

II. Power factor

III. Armature current.

When the excitation of the motor is varied, which parameters vary along with it

(A) I only

(B) II only

(C) II and III only

(D) I, II and III.

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C

51. A 3 phase, 400 V, 50 Hz salient pole synchronous motor is running on no load. If there is break in the excitation winding of the motor

(A) the motor will stop

(B) the winding will get overheated

(C) the motor will run as reluctance motor at the rated rpm

(D) the motor will run as reluctance motor at lower rpm.

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A

53. A synchronous motor is said to be 'floating' when it operates

(A) on no load and without losses

(B) on constantly varying load

(C) on pulsating load

(D) on high load and variable supply voltage.

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A

54. The negative phase sequences in a three phase synchronous motor exist when

(A) motor is overloaded

(B) motor is under loaded

(C) unbalanced voltage is supplied

(D) motor armature is hot.

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C

55. The field winding of a synchronous motor is shorted. A variable voltage is now supplied to the stator. The result will be

(A) stator winding will bum

(B) motor will not run

(C) motor will rotate at synchronous speed on light load

(D) motor will rotate at a speed which is less than the synchronous speed.

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D

56. In a three phase synchronous motor, the poles

(A) are along the resultant air-gap flux wave φ _r

(B) arc opposite to φ _r

(C) lag φ _r

(D) lead φ _r

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C

57. A 3 phase synchronous motor is running clockwise. In case the direction

of its field current is reversed

(A) the motor will continue to run in the same direction

(B) the motor will run in the reverse direction

(C) the motor will stop

(D) the winding of the motor will burn.

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A

58. The speed regulation of a 3 phase synchronous motor is

(A) 5% 

(B) 1%

(C) 0.4% 

(D) zero.

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D

59. When E is the supply voltage and R is the rotor resistance per phase,

the mechanical power develop by a synchronous motor per phase is given

by

(A) V²R 

(B) V²/2R

(C) V²/4R 

(D) 2V/3R

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C

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

when

(A) rotor speed is equal to synchronous speed

(B) rotor speed is less than synchronous speed

(C) rotor speed is more than synchronous speed

(D) rotor speed is either less or more than synchronous speed.

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D

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46.C ----- 47.B ----- 48.D ----- 49.C ----- 50.C ----- 51.A ----- 52.D ----- 53.A ----- 54.C ----- 55.D ----- 56.C ----- 57.A ----- 58.D ----- 59.C ----- 60.D

61. The size of a synchronous motor decreases with the increase in

(A) speed 

(B) horse power rating

(C) flux density 

(D) all of the above.

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C

62. The flux density used in synchronous motor is around

(A) 0.05 to 0.46 wb/m2

(B) 0.5 to 0.6 wb/m2

(C) 5 to 6 wb/m2

(D) 50 to 60 wb/m2.

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B

63. In a synchronous motor if the saturation is neglected, then the short

circuit ratio (SCR) will be related to the synchronous reactance (xd) as

(A) SCR 

(B) SCR = (xd)2

(C) SCR = 1/ (xd)2 

(D) SCR = 1/ (xd).

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D

64. In synchronous motor out of the following losses, which one will have

the highest proportion ?

(A) Stator copper losses

(B) Friction and windage losses

(C) Eddy current losses

(D) Iron losses.

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D

65. Which of the following losses is not dissipated by the stator core

surface in a synchronous motor ?

(A) Copper losses in the slot portion of the conductors

(B) Eddy current losses in the conductors

(C) Iron losses in the stator

(D) Windage losses.

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D

66. The speed of a synchronous motor

(A) reduces as load increases

(B) increases as load increases

(C) adjusts itself to new equilibrium speed whenever load changes

(D) always remains constant.

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D

67. 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.

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C

68. When a synchronous motor is connected to an infinite bus, while

operating on leading power factor .

(A) the excitation voltage will be less than the supply voltage

(B) the excitation voltage will be more than the supply voltage

(C) the excitation voltage will be equal to the supply voltage

(D) the excitation voltage will be independent of the supply voltage.

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B

69. In a synchronous motor

(A) total number of rotor slots = total number of stator slots

(B) total number of rotor slots is more than total number of stator slots

(C) total number of rotor slots is less than the total number of stator slots.

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C

70. In a synchronous motor during hunting if the rotor speed becomes more than the synchronous speed

(A) negative phase sequence currents are generated

(B) harmonics are developed in the armature circuit

(C) damper bars develop induction generator torque

(D) field excitation increases.

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C

71. If a synchronous motor fails to pull into synchronism after applying dc field current, the probable clause may be

(A) high core losses

(B) low field current

(C) high field current

(D) low short circuit ratio.

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B

72. In case of a 3 phase synchronous motor, maximum speed variation is

(A) 10%

(B) 5%

(C) 3%

(D) zero.

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D

73. The synchronous motors are not self-starting because

(A) stator is not used

(B) starting winding is not provided

(C) the direction of instantaneous torque on the rotor reverses after half cycle

(D) there is no slip.

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C

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61.C ----- 62.B ----- 63.D ----- 64.D ----- 65.D ----- 66.D ----- 67.C ----- 68.B ----- 69.C ----- 70.C ----- 71.B ----- 72.D -----73.C ----- 74.D ----- 75.D

77. In a synchronous motor hunting can be minimized

(A) by using damper bars

(B) by using a flywheel

(C) by designing the motor for adequate synchronizing power

(D) by any of the above methods.

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D

78. A three phase 400 V, 50 Hz synchronous motor is working at 50 percent load. In case an increase in the field current of the motor causes a reduction in the armature current, it can be concluded that

(A) the motor is delivering reactive power to the mains

(B) the motor is absorbing reactive power to the mains

(C) the motor is neither absorbing nor delivering reactive power.

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B

79. Inverted V-curves for a synchronous motor show

(A) Variation of power factor with dc field current when load on the motor remains constant

(B) Variation of field current and supply voltage when excitation remains constant

(C) Variation of power factor and supply voltage when motor is hunting

(D) none of the above.

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A

80. The armature current of the synchronous motor has large values for

(A) low excitation only

(B) high excitation only

(C) both low and high excitation.

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C

81. In which range the cost of a synchronous motor can be comparable to the cost of a induction motor ?

(A) Low HP high speed

(B) High HP low speed

(C) High HP high speed

(D) Low HP low speed.

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B

82. Insulation resistance test on synchronous motor can be conducted to measure which of the following resistances ?

(A) Stator winding to earthed frame

(B) Rotor winding to earthed shaft

(C) Phase to phase winding resistance

(D) All of the above.

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D

84. During short circuit test which of the following is short circuited ?

(A) Armature terminals

(B) One of the phase windings of stator

(C) AH phase windings of stator

(D) All phase windings of stator and armature.

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A

85. The duration of sudden short circuit test on a synchronous motor is usually

(A) about one second

(B) about one minute

(C) about one hour

(D) 40 minutes.

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A

86. The maximum torque which a synchronous will develop at rest for any angular positions of the rotor, at rated stator supply voltage and frequency, is known as

(A) Reluctance torque

(B) Synchronous torque

(C) Locked-rotor torque

(D) Pull up torque.

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C

87. The maximum constant load torque under which a synchronous motor will pull into synchronism at rated rotor supply voltage and frequency is known as

(A) Synchronous torque

(B) Pull in torque

(C) Pull up torque

(D) Pull out torque.

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B

88. The maximum sustained torque which a synchronous motor will develop at synchronous speed for 1 mm with rated frequency and rated field current is known as

(A) Pull out torque

(B) Reluctance torque

(C) Synchronous torque

(D) Average torque.

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A

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

(A) Synchronous torque

(B) Asynchronous torque

(C) Reluctance torque

(D) Pull-up torque.

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A

90. The space angle between the axis of the stator revolving magnetic field and the rotor-pole axis, both looked and running at synchronous speed, is known as

(A) angle of lead

(B) power angle

(C) power factor angle

(D) reactive power angle.

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B

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76.A ----- 77.D ----- 78.B ----- 79.A ----- 80.C ----- 81.B ----- 82.D ----- 83.A ----- 84.A ----- 85.A ----- 86.C ----- 87.B ----- 88.A ----- 89.A ----- 90.B

91. In a synchronous machine in case the axis of field flux is in line with the armature flux, then
(A) the machine is working as synchronous motor
(B) the machine is working as synchronous generator
(C) the machine is said to be floating
(D) the machine will vibrate violently.

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C

92. If other factors remain constant, the speed of a synchronous motor in its operating (and load) range is correctly described by which of the following ?
(A) The speed varies directly only in proportion to the number of its poles
(B) The speed is independent of the frequency of the voltage supply
(C) The speed depends on the frequency of the voltage supply and the number of its poles
(D) The speed depends on the magnitude of the voltage supply and the number of its poles.

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C

93. The induced emf in a synchronous motor working on leading pf will be
(A) equal to the supply voltage
(B) less than the supply voltage
(C) more than the supply voltage.

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C

94. A synchronous machine with low value of short-circuit ratio has
(A) good speed regulation
(B) good voltage regulation
(C) higher stability limit
(D) lower stability limit.

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D

95. While starting a synchronous motor by induction motor action, very high emf is induced in the field. This induced emf may damage the insulation of the field winding and of slip rings. The insulation damage can φ be prevented by
(A) short-circuiting the field winding by field discharge resistance
(B) splitting the field winding into several sections
(C) either of (A) or (B) above
(D) none of the above.

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C

96. Synchronous motors, arc generally of
(A) induction type machines
(B) cylindrical pole type machines
(C) salient pole type machines
(D) hysteresis type machines.

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C

97. In which of the following motor the stator and rotor magnetic fields rotate at the same speed
(A) Universal motor
(B) Reluctance motor
(C) Synchronous motor
(D) Induction motor.

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C

98. Higher the applied voltage .................... will be the stator flux and ................... will be the pull in torque
(A) lower, greater
(B) greater, lower
(C) greater, greater
(D) lower, lower.

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C

Questions 99 to 101 refer to data given below:
 
An industrial plant has a load of 1500 kVA at an average power factor of 0.6 lagging.

99. Neglecting all losses, the kVA input to a synchronous condenser for an overall power factor of unity will be
(A) 300 kVA
(B) 400 kVA
(C) 600 kVA
(D) 1200 kVA.

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D

100. A 750 kVA synchronous condenser is used to correct the lagging power factor of the plant. The total kVA of the plant is
(A) 503
(B) 10006
(C) 2012
(D) 4024.

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B

101. The overall power factor is
(A) 0.895 lagging
(B) 0.75 lagging
(C) 0.75 leading
(D) unity.

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A

102. As the load is applied on a synchronous motor, its speed does not fall. The load is now supplied by
(A) decrease in back emf
(B) change in supply voltage
(C) the stator magnetic field
(D) rotor taking new angular position slightly back of its no load position.

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D

103. The maximum power developed in a synchronous motor depends on all of the following EXCEPT:
(A) Supply voltage
(B) Direction of rotation
(C) Rotor excitation
(D) Maximum value of coupling angle.

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B

104. In a synchronous motor, on a fixed excitation when the load is doubled, the torque angle φ_r will become
(A) 2 φ_r
(B) √2 φ_r
(C)φ_r/2
(D) φ_r/√2

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A

105. In a synchronous motor, the armature current has the highest value when excitation is
(A) zero
(B) low
(C) high
(D) high and low.

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D

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91.C ----- 92.C ----- 93.C ----- 94.D ----- 95.C ----- 96.C ----- 97.C ----- 98.C ----- 99.D -----100.B -----101.A ----- 102.D -----103.B -----104.A ----- 105.D

106. For a synchronous motor, the ratio starting torque/running torque is
(A) infinite
(B) 1.0
(C) 0.5
(D) 0.

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D

107. Synchronous motors for power factor correction operate at
(A) normal load with minimum excitation
(B) normal load with zero excitation
(C) no load and greatly over-excited fields
(D) no load and under-excited fields.

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C

108. The construction of a synchronous motor resembles which of the following machines?
(A) Slip ring induction motor
(B) DC shunt generator
(C) Single phase reluctance motor
(D) DC compound motor.

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B

109. The construction of a synchronous motor resembles which of the following machine ?
(A) An induction motor
(B) A rotor converter
(C) An alternator
(D) A series motor.

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C

110. In a synchronous motor, "hunting" may be due to variation in any of the following EXCEPT:
(A) Load
(B) Supply voltage
(C) Frequency
(D) Winding friction.

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D

111. A synchronous motor is switched on to supply with its field winding short-circuited, the motor will
(A) not start
(B) bun out
(C) start and run as induction motor
(D) start as induction motor and run as synchronous motor.

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D

112. In a synchronous motor, at no load, the armature current is
(A) in phase with the applied voltage
(B) leading the applied voltage by 90°
(C) lagging the applied voltage by 90°
(D) zero.

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B

113. In a synchronous motor, during hunting when the rotor speed exceeds the synchronous speed
(A) field excitation increases
(B) harmonics are developed
(C) negative phase sequence currents come into action
(D) damper bars develop induction generator torque.

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D

114. For a synchronous motor when V is the supply voltage, the breakdown torque will be proportional to
(A) V²
(B) V
(C) 1/ V
(D) 1/ V²

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A

115. When the field winding of an unloaded salient pole synchronous motor is open - circuited, the motor will
(A) burn with dense smoke
(B) stop
(C) run as induction motor
(D) function as static condenser

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B

116. In case one of the three phases of a synchronous motor is short-circuited, the motor will
(A) not start
(B) get overheated
(C) bum out
D) run normally.

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B

117. The fact that a synchronous motor with salient poles will operate, even if field current is reduced to zero, can be explained by
(A) magnetization of rotor poles by stator magnetic field
(B) rotating magnetic field of the rotor
(C) rotating magnetic field of the stator
(D) interlocking action between stator and rotor fields.

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A

118. The negative phase sequence in a three phase synchronous motor exists when the motor is
(A) under loaded
(B) overloaded
(C) supplied with unbalanced voltage
(D) hot.

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C

119. The regulation of a synchronous motor is
(A) 0%
(B) 1%
(C) 50%
(D) 100%.

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A

120. In a synchronous motor, the angle between the rotor poles and stator poles is known as
(A) synchronizing angle
(B) torque angle
(C) angle of retardation
(D) power factor angle.

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B

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106.D ----- 107.C ----- 108.B ----- 109.C ----- 110.D ----- 111.D ----- 112.B ----- 113.D ----- 114.A ----- 115.B -----116.B ----- 117.A -----118.C -----119.A ----- 120.B

121. In a synchronous motor, under running conditions, the angle between the induced voltage and supply voltage will be
(A) zero
(B) between 0 and 90°
(C) between 90° and 180°
(D) more than 180°.

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D

122. The rotor of a synchronous motor can only run at synchronous speed of the stator magnetic field due to
(A) Faraday's law of electro-magnetic induction
(B) Lenz's law
(C) Magnetization of rotor poles by stator magnetic field
(D) Interlocking action between stator and rotor fields.

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D

123. An inverted V-curve of a synchronous motor is the variation of
(A) field current and power factor 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.

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A

124. Damper windings are provided on
(A) pole faces
(B) separate armature
(C) rotor shaft
(D) stator frame.

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A

125. Hunting of a synchronous motor may be due to
(A) pulsations in power supply
(B) reciprocating type of load
(C) pulsating torque of driven equipment
(D) any of the above.

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D

126. The V-curves of a synchronous motor show relationship between
(A) armature current and supply voltage
(B) dc field current and ac armature current
(C) excitation current and back emf
(D) none of the above.

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B

127. In a synchronous motor with field under excited, the power factor will be
(A) leading
(B) lagging
(C) unity
(D) none of the above.

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B

128. In a synchronous motor, maximum value of torque angle is
(A) Below 45 degrees electrical
(B) 45 degrees electrical
(C) 90 degrees electrical
(D) Above 90 degrees electrical.

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C

129. In a synchronous motor, net armature voltage is of E_b and V
(A) Vector sum
(B) Vector difference
(C) Arithmetic difference
(D) Arithmetic sum.

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B

130. Increasing load on a normally-excited synchronous motor, the power factor
(A) remain unchanged
(B) becomes increasing lagging
(C) becomes increasing leading
(D) none of the above.

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B

131. Synchronous motor speed is controlled by varying
(A) supply voltage only
(B) supply frequency only
(C) supply voltage and frequency both
(D) none of the above.

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C

132. Maximum electrical power input of a synchronous motor is
(A) (V_t E_f) / X_s
(B) V²_t / X_s
(C) E²_f/ X_s
(D) X_s/ (V_t E_f )
where the symbols have their usual meanings.

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A

133. In a synchronous motor armature reaction at rated voltage and zero power factor leading is
(A) Magnetizing
(B) Cross magnetizing
(D) Demagnetizing
(D) None of the above.

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C

134. A synchronous motor is operating with excitation adjusted for unity power factor current at constant load. On increasing the excitation, the power factor
(A) will lag
(B) will lead
(C) will become zero
(D) none of the above.

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B

135. A synchronous motor is operated from a bus voltage of 1.0 pu at 1.0 pu pf leading current. The synchronous reactance is 0.5 p.u. The excitation e.m.f of the motor is
(A) 0.5
(B) 1
(C) 1.5
(D) 3.

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C

136. 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.

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D

137. A synchronous motor operating at rated voltage draws 1.0 pu current at 1.0 power factor. The machine parameters are : synchronous reactance 1.0 pu ; armature resistance, negligible. Apart from supplying this rated power, if the motor has to supply an additions! leading reactive power of 0.8 pu, then the field current has to be increased by
(A) 42%
(B) 46%
(C) 52%
(D) 60%.

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B

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121.D ----- 122.D ----- 123.A ----- 124.A ----- 125.D ----- 126.B ----- 127.B ----- 128.C ----- 129.B -----130.B -----131.C ----- 132.A -----133.C -----134.B ----- 135.C ----- 136.D ----- 137.B