301 MCQ Questions on Synchronous Machine

51. Hunting in a synchronous motor can’t be due to

(a) Windage friction
(b) Variable load
(c) Variable frequency
(d) Variable supply voltage

52. The duration of sudden short-circuit test on a synchronous motor is usually about

(a) One hour
(b) One minute
(c) One second
(d) None of the above

53. Drop in alternator frequency is corrected by

(a) Damper winding
(b) Increased prime mover output
(c) Automatic voltage regulator
(d) None of these [DMRC JE-2016]

54. If the excitation of an alternator operating in parallel with other alternators decreased, its

(a) Power factor becomes more leading
(b) Output kW will change
(c) Power factor becomes more lagging
(d) Power factor becomes unity

55. A field excitation of 20 A in a certain alternator results in an armature current of 400 A in short circuit and a terminal voltage of 2000 V in open circuit. The magnitude of internal voltage drop within the machine at a load current of 200 A is

(a) 1500 V
(b) 2000 V
(c) 1000 V
 (d) 100 V

56. A synchronous motor is connected to an infinite bus at 1.0 pu voltage and draws 1.0 pu current at unity power factor. The synchronous reactance is 1.0 pu and resistance is negligible. What is the load angle?

(a) 90°
(b) 45°
(c) 0°
(d) 60°

57. ZPF (Zero power factor) method finds ……… of an alternator.

(a) Voltage regulation
(b) Efficiency
(c) Armature resistance
(d) Synchronous impedance

58. By reversing …………. the direction of rotation of a synchronous motor can be reversed. [UPPCL JE – 2014]

(a) Supply phase sequence
(b) Current to the field winding
(c) Polarity of rotor poles
(d) Either “polarity of rotor poles” or “supply phase sequence”.

59. An alternator’s operating principle is quite similar to that of

(a) A.C. Generator.
(b) D.C. Generator.
(c) A.C. Inverter.
(d) A.C. Stablizer. [UPPCL JE – 2014]

60. What is the value of load angle at a point where power output of a salient pole synchronous generator is maximum? [UPPCL JE 2014]

(a) 0°
(b) 45°
(c) 120°
(d) It is less than 90° but not fixed.

61.  Match the characteristics with its relevant relation. [UPPCL JE – 2014]

List-I (Characteristics)

1. Open circuit characteristics
2. Internal characteristics
3. External characteristics
4. Load saturation curve

List-II (Relation)

1. V vs I_f
2. E_o vs I_f
3. E vs I_a
4. V vs I

Where, E_o = no load induced e.m.f. ; I_f = field current; I_a = armature current; E = generated e.m.f. on load; V = terminal voltage; I = load current.
(a) A-4, B-3, C-2, D-1
(b) A-2, B-1, C-4, D-3
(c) A-4, B-1, C-2, D-3
(d) A-2, B-3, C-4, D-1

62. When a synchronous motor is ………., it operates with leading power factor current. [UPPCL JE – 2014]

(a) Under-excited
(b) Critically excited
(c) Over-excited
(d) Heavily loaded

63. What will be the effect on a salient pole synchronous motor if its field current is switched off (provided the motor runs at no load)?

(a) It will stop.
(b) It continues to run at synchronous speed.
(c) It continues to run at a speed slightly more than the synchronous speed.
(d) It continues to run at a speed slightly less than the synchronous speed. [UPPCL JE 2014]

64. The following system is used in connections of various lamps and appliances in parallel. Identify the system. [UPPCL JE – 2014]

(a) Gain-in-system
(b) Loop-in-system
(c) Voltage-in-system
(d) Parallel-in-system

65. On which of the following parameters does the power factor of an alternator depend? [UPPCL JE – 2014]

(a) Load
(b) Speed of rotor
(c) Core losses
(d) Armature losses

66. What is the purpose of dampers in a large generator? [UPPCL JE – 2014]

(a) They increase stability.
(b) They reduce voltage fluctuations.
(c) They reduce frequency fluctuations.
(d) They decrease stability.

67. On which of the following does the back emf set up in the stator of a synchronous motor depend? [UPPCL JE 2014]

1. Rotor speed
2. Rotor excitation

III. Coupling angle
Choose the correct answer from the options given below.
(a) Only II
(b) Only II and III
(c) I, II and III
(d) Only I and III

68. For the following specifications of a synchronous motor, determine the ratio of no load speed to full load speed. [UPPCL JE – 2014]

(a) 2:3
(b) 1:1
(c) 3:5
(d) 2:7

69. Embedding the damper winding in the pole face serves which of the following purposes? [UPPCL JE – 2014]

(a) Eliminate hunting and provide adequate starting torque.
(b) Reduce windage losses.
(c) Eliminate losses on account of air friction.
(d) Reduce bearing friction.

70. Let us assume that two alternators running exactly in synchronism. Find the synchronizing power of the system. [UPPCL JE 2014]

(a) 0
(b) 1
(c) 0.8
(d) 0.5

71. A synchronous motor having 5 poles is running with a supply frequency of 40 Hz. What is the control operating speed of the motor?

(a) 100 rpm
(b) 1200 rpm
(c) 600 rpm
(d) 960 rpm

72. In a synchronous alternator which of the following coils will have emf closer to sine wave from? [LMRC 2015]

(a) Concentrated winding in full pitch coils.
(b) Distributed winding in full pitch coils.
(c) Distributed winding in short pitch coils.
(d) Concentrated winding in short pitch coil.

73. Damping winding in a synchronous motor

(a) Improves power factor of the motor.
(b) Increases hunting of the motor.
(c) Reduces windage losses.
(d) Increases starting torque. [NMRC JE-2017]

74. Synchronous Generator is a source of

(a) Real Power
(b) Reactive power
(c) Apparent power
(d) Both real and reactive power

75. The ratio of starting torque to running torque in a synchronous motor is

(a) Zero
(c) Two
(b) One
(d) Infinity

76. For very low speed and high power applications, the best suited ac motor is

(a) Slip-ring induction motor.
(b) Squirrel cage induction motor.
(c) AC commutator motor.
(d) Synchronous motor.

77. The generator develops an electric power by

(a) Conversion of heat.
(b) Conversion of light.
(c) Electro-magnetic induction.
(d) Conversion of chemical energy.

78. When a synchronous motor is started, the field winding is initially

(a) Excited by a d.c. sources.
(b) Short circuited.
(c) Open circuited.
(d) None of these.

79. A 10 pole, 25 Hz alternator is directly coupled to and is driven by a 60 Hz synchronous motor. The number of poles of a synchronous motor

(a) 48 poles
(b) 12 poles
(c) 24 poles
(d) all of the above

80. To eliminate the 5th harmonic of a short pitched coil should have a short pitching angle of

(a) 36°
(b) 18°
(c) 15°
(d) 12°

81. The direction of induced e.m.f. can be found with the help of

(a) Fleming’s right hand rule
(b) Kirchhoff’s voltage law
(c) Lenz’s law
(d) Laplace’s law [UPPCL JE 2007]

82. A 50 Hz alternator will run at the highest speed if it is wound for

(a) 8 poles
(b) 6 poles
(c) 4 poles
(d) 2 poles [UPPCL JE 2007]

83. Which one of the following methods gives voltage regulation higher than the actual value in an alternator ? [UPPCL JE – 2007]

(a) ZPF method
(b) mmf method
(c) emf method
(d) ASA method

84. The most common type of prime mover used for low speed alternators is

(a) Steam turbine
(b) Petrol engine
(c) Hydraulic turbine
(d) Diesel engine [UPPCL JE 2007]

85. When speed of an alternator is changed from 3600 rpm to 1800 rpm, the generated emf will be

(a) One-half
(b) Twice
(c) One-fourth
(d) Four times [UPPCL JE – 2007]

86. The power factor of an alternator is determined by its

(a) Prime mover
(b) Speed
(c) Excitation
(d) Load [UPPCL JE 2007]

87. Change in excitation of synchronous motor result

(a) Change in motor speed.
(b) Change in power factor.
(c) Both (a) and (b).
(d) None of the above.

88. From no-load to full-load, synchronous motors give

(a) Constant speed.
(b) Variable speed.
(c) Gradually increasing speed.
(d) Gradually decreasing speed. [UPPCL JE 2016]

89. Alternators are rated at

(a) kW
(b) kVA
(c) kVA or kW
(d) kWh [UPPCL JE 2016]

90. To reduce the peripheral speed of an alternator, diameter of the rotor is

(a) Increased
(b) Decreased
(c) Increased or decreased
(d) Kept same [UPPCL JE 2016]

91. The maximum power developed by a synchronous motor depends on……. and…….

(a) Supply voltage, Maximum value of load angle
(b) Supply voltage, Excitation current
(c) Excitation current, Maximum value of load angle
(d) Output voltage, Supply frequency [UPPCL AE – 2016]

92. Which of the following statement is false? [UPPCL AE – 2016]

(a) Silica gel is used in transformers
(b) Transformer is a constant flux machine
(c) Induction motor can be self starting
(d) Synchronous motor is self starting

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

(a) Speed
(c) Flux density
(b) Horse power rating
(d) All of these

94. Synchronous capacitor is

(a) An ordinary static capacitor bank.
(b) An over excited synchronous motor driving mechanical load.
(c) An over excited motor running without mechanical load.
(d) None of the above. [Uttrakhand – 2013]

95. What will be the rotation speed of a 3-phase, 4-pole, 50 Hz synchronous motor, if the frequency, number of poles and load torque is halved? [UPPCL JE – 2018]

(a) 3000 RPM
(b) 750 RPM
(c) 6000 RPM
(d) 1500 RPM

96. Quadrature axis synchronous reactance is the ratio of

(a) V_min to I_max
(b) V_min to I_min
 (c) V_max to I_max
(d) V_max to I_min [UPPCL JE 2018]

97. Stator and rotor fields rotate simultaneously in which of the following motors?

(a) Universal motor
(b) Synchronous motor
(c) D.C. motor
(d) Reluctance motor [UPPCL JE 2018]

98. In which of the following conditions will a 3-phase synchronous machine works as a capacitor? [UPPCL JE – 2018]

(a) Over Excited
(b) Critically Excited
(c) Under Excited
(d) None of these

99. What is the role of alternator in a steam power station? [UPPCL JE – 2018]

(a) It converts electrical energy into mechanical energy.
(b) It converts heat energy to mechanical energy.
(c) It converts heat energy to electrical energy.
(d) It converts mechanical energy into electrical energy.

100. What is the Synchronising power of a synchronous machine? [UPPCL JE – 2018]

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