51. Transformer efficiency mainly depends on
A) Voltage
B) Frequency
C) Load power factor
D) Cooling system
Answer: C
52. A 100 kVA transformer has full-load copper loss of 1 kW and iron loss of 1.5 kW. The efficiency at full load, 0.8 pf lag is
A) 96.6%
B) 97%
C) 97.5%
D) 98%
Answer: A
53. The magnetizing current of a transformer is small because
A) Core has high permeability
B) Leakage flux is small
C) Primary resistance is high
D) Voltage is small
Answer: A
54. The copper loss in a transformer varies with
A) Load current
B) Square of load current
C) Voltage
D) Frequency
Answer: B
55. For maximum efficiency of a transformer, the variable losses should be equal to
A) Constant losses
B) Half of constant losses
C) Twice the constant losses
D) Zero
Answer: A
56. The open-circuit test is conducted on
A) LV side with HV open
B) HV side with LV open
C) LV side with HV shorted
D) HV side with LV shorted
Answer: A
57. The short-circuit test is conducted on
A) LV side with HV open
B) HV side with LV open
C) LV side with HV shorted
D) HV side with LV shorted
Answer: D
58. The equivalent resistance of a transformer referred to primary is
A)
B)
C)
D)
Answer: A
59. A transformer rated 230/115 V has turns ratio of
A) 2:1
B) 1:2
C) 1:1
D) 3:1
Answer: A
60. The main cause of noise in a transformer is
A) Cooling fan
B) Vibration due to magnetostriction
C) Winding vibration
D) Poor insulation
Answer: B
61. In an ideal transformer, the flux leakage is
A) Zero
B) 10%
C) 1%
D) 5%
Answer: A
62. If load on secondary increases, the primary current
A) Increases
B) Decreases
C) Remains same
D) Becomes zero
Answer: A
63. The iron loss in transformer depends on
A) Voltage and frequency
B) Load current
C) Temperature
D) Power factor
Answer: A
64. The function of tap changer is to
A) Change current
B) Regulate voltage
C) Increase efficiency
D) Change frequency
Answer: B
65. If both primary and secondary of transformer are short-circuited
A) Core will burn
B) Large current will flow
C) Transformer will act as autotransformer
D) No current flows
Answer: B
66. The function of transformer bushing is
A) To provide insulation for current carrying conductors
B) To hold core
C) To prevent leakage of oil
D) To increase efficiency
Answer: A
67. In a distribution transformer, which loss is constant?
A) Copper loss
B) Core loss
C) Stray loss
D) Load loss
Answer: B
68. In a power transformer, efficiency is maximum at
A) Full-load
B) Half-load
C) ¾ load
D) All loads
Answer: C
69. The main purpose of Buchholz relay is to
A) Protect transformer from short circuit
B) Detect internal faults
C) Measure temperature
D) Prevent overloading
Answer: B
70. The dielectric strength of transformer oil should be more than
A) 10 kV
B) 20 kV
C) 30 kV
D) 50 kV
Answer: C
71. The leakage reactance of transformer depends on
A) Winding configuration
B) Frequency
C) Core area
D) Flux density
Answer: A
72. The efficiency of distribution transformer is generally
A) 50–60%
B) 60–70%
C) 90–98%
D) 30–50%
Answer: C
73. The core loss of transformer at rated voltage and frequency is 200 W. At half voltage and frequency, core loss will be approximately
A) 50 W
B) 100 W
C) 200 W
D) 400 W
Answer: B
74. The resistance of transformer winding increases with
A) Temperature
B) Load
C) Voltage
D) Flux
Answer: A
75. The voltage regulation of transformer depends upon
A) Load current and power factor
B) Frequency only
C) Supply voltage
D) Core loss
Answer: A
76. The efficiency of an ideal transformer is
A) 0%
B) 50%
C) 75%
D) 100%
Answer: D
77. In a current transformer, the secondary winding always connected to
A) Open circuit
B) Ammeter
C) Voltmeter
D) Wattmeter
Answer: B
78. The purpose of varnish coating on transformer core is to
A) Prevent rusting
B) Reduce eddy current loss
C) Reduce hysteresis loss
D) Improve mechanical strength
Answer: B
79. If secondary load of transformer is disconnected, primary current will
A) Remain full-load
B) Reduce to magnetizing current
C) Increase
D) Become zero
Answer: B
80. The load current in secondary is transferred to primary due to
A) Mutual induction
B) Self-induction
C) Eddy currents
D) Electrostatic effect
Answer: A
81. The temperature rise in transformer is mainly due to
A) Copper and iron losses
B) Core loss only
C) Copper loss only
D) Hysteresis loss
Answer: A
82. Laminations in a transformer core are insulated from each other by
A) Air
B) Varnish coating
C) Paint
D) Oil
Answer: B
83. The total copper loss in transformer is proportional to
A)
B)
C)
D)
Answer: B
84. Transformer rating is usually expressed in
A) kW
B) kVAR
C) kVA
D) HP
Answer: C
85. The induced emf per turn in transformer is same in both windings because
A) Same flux links both windings
B) Turns are same
C) Resistances are equal
D) Leakage flux is same
Answer: A
86. The efficiency of a transformer can be increased by
A) Increasing copper loss
B) Decreasing iron and copper losses
C) Using small core
D) Decreasing flux density
Answer: B
87. The frequency of emf induced in secondary is
A) Double of primary
B) Same as primary
C) Half of primary
D) Depends on turns ratio
Answer: B
88. In transformer design, the window area is determined by
A) Output and current density
B) Core area
C) Frequency
D) Voltage
Answer: A
89. The type of cooling used in large power transformers is
A) Air natural
B) Air blast
C) Oil natural or forced
D) Water cooling
Answer: C
90. The core loss remains constant because
A) Voltage and frequency are constant
B) Load varies
C) Core flux varies
D) Current varies
Answer: A
91. The short-circuit test is used to find
A) Iron loss
B) Copper loss and equivalent impedance
C) Efficiency
D) Regulation only
Answer: B
92. The voltage drop in transformer at lagging power factor is due to
A) Resistance only
B) Reactance only
C) Both resistance and reactance
D) Core loss
Answer: C
93. The all-day efficiency of transformer depends upon
A) Load power factor
B) Load fluctuations
C) Frequency
D) Voltage
Answer: B
94. In power transformers, the main objective of design is to have
A) Maximum voltage regulation
B) Minimum cost
C) Maximum efficiency
D) Minimum flux
Answer: C
95. A transformer designed for 50 Hz can be used for
A) 25 Hz
B) 100 Hz
C) DC
D) Both 25 and 100 Hz
Answer: B
96. The energy transfer in transformer takes place through
A) Electrostatic field
B) Electromagnetic induction
C) Conduction
D) Convection
Answer: B
97. The no-load current in a transformer is not exactly 90° lagging because of
A) Copper loss
B) Core loss component
C) Load current
D) Voltage drop
Answer: B
98. The air gap in transformer is kept
A) Large
B) Very small
C) Medium
D) Equal to armature
Answer: B
99. The secondary induced emf in a transformer depends on
A) Turns ratio and flux
B) Supply current
C) Power factor
D) Core losses
Answer: A
100. When a transformer runs on no-load, most of the input power is consumed in
A) Winding copper losses
B) Core losses
C) Load
D) Eddy currents only
Answer: B
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