The auto-transformer efficiency compared to two-winding type is

 101. In a transformer, the magnetizing current is responsible for

A) Producing mutual flux
B) Producing leakage flux
C) Reducing eddy currents
D) Supplying load

Answer: A

102. The leakage reactance of transformer increases with
A) Tight coupling
B) Loose coupling
C) Reduced frequency
D) Reduced leakage flux

Answer: B

103. The open circuit test gives
A) Core losses
B) Copper losses
C) Leakage reactance
D) Total losses

Answer: A

104. In a 3-phase transformer, star–delta connection is preferred when
A) Step-up is required
B) Step-down is required
C) Load is balanced
D) Load is unbalanced

Answer: A

105. The vector group Dy11 means
A) 11° phase difference
B) 30° leading secondary voltage
C) 30° lagging secondary voltage
D) Delta–delta connection

Answer: C

106. The tertiary winding in a 3-phase transformer is used to
A) Reduce leakage reactance
B) Supply auxiliary load and stabilize system
C) Reduce core losses
D) Increase voltage

Answer: B

107. In a transformer, voltage regulation improves with
A) Lagging power factor
B) Leading power factor
C) Inductive load
D) Low frequency

Answer: B

108. The core loss at 25 Hz compared to 50 Hz will be approximately
A) Same
B) Half
C) Double
D) Four times

Answer: B

109. The magnetizing reactance of a transformer is
A) Low
B) High
C) Equal to leakage reactance
D) Zero

Answer: B

110. Eddy current loss in transformer core is proportional to
A) $f$
B) $f^2$
C) $f^{1/2}$
D) Independent of $f$

Answer: B

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111. The hysteresis loss in a transformer is proportional to
A) $f$
B) $f^2$
C) $f^{1.6}$
D) $f^{1.2}$

Answer: A

112. A transformer cannot change
A) Voltage
B) Current
C) Frequency
D) Power factor

Answer: C

113. The emf equation of transformer is
A) $E = 4.44 f N \phi_m$
B) $E = 2.22 f N \phi_m$
C) $E = 1.11 f N \phi_m$
D) $E = 0.707 f N \phi_m$

Answer: A

114. The harmonics in transformer no-load current are due to
A) Non-sinusoidal flux
B) Linear magnetization curve
C) Uniform air gap
D) Load variation

Answer: A

115. The efficiency of a transformer can be determined by
A) Open and short circuit tests
B) Load test only
C) Regulation test
D) Direct loading

Answer: A

116. A 200/400 V transformer used as step-down will
A) Work perfectly
B) Not work efficiently
C) Burn out
D) Work with reduced efficiency

Answer: D

117. The primary copper loss at half-load is
A) ¼ of full-load loss
B) ½ of full-load loss
C) Same as full-load
D) Double

Answer: A

118. The voltage ratio of transformer in no-load and full-load conditions
A) Increases
B) Decreases slightly
C) Remains constant
D) Increases with load

Answer: B

119. The transformer’s leakage reactance causes
A) Phase shift
B) Voltage drop under load
C) Power factor improvement
D) Reduced hysteresis loss

Answer: B

120. The ideal transformer has
A) No leakage flux and no losses
B) Core loss only
C) Copper loss only
D) Equal losses

Answer: A

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121. If the frequency of supply is doubled, keeping voltage constant, the core flux
A) Doubles
B) Halves
C) Remains same
D) Becomes zero

Answer: B

122. Transformer oil serves as
A) Insulator and coolant
B) Conductor
C) Only coolant
D) Only insulator

Answer: A

123. The winding resistance of a transformer is measured using
A) Megger
B) Voltmeter–ammeter method
C) Bridge method
D) Both B and C

Answer: D

124. A transformer designed for 50 Hz will have excessive losses if operated at
A) 60 Hz
B) 25 Hz
C) 100 Hz
D) 75 Hz

Answer: B

125. The core-type transformer is preferred for
A) High-voltage applications
B) Low-voltage and high-current applications
C) Both A and B
D) None

Answer: B

126. The shell-type transformer is preferred when
A) High voltage required
B) Low voltage and high current
C) High mechanical strength and compactness are needed
D) Light weight required

Answer: C

127. The phase shift between primary and secondary voltages in Dy1 group is
A) 0°
B) 30° leading
C) 30° lagging
D) 180°

Answer: B

128. Transformer voltage regulation is negative for
A) Lagging PF
B) Leading PF
C) Unity PF
D) Inductive load

Answer: B

129. The core loss in transformer is due to
A) Hysteresis and eddy current
B) Eddy current only
C) Magnetic saturation
D) Load current

Answer: A

130. The magnetizing component of no-load current is
A) In phase with voltage
B) 90° lagging
C) 90° leading
D) In phase with flux

Answer: B

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131. A transformer’s copper losses are 800 W at full load. At half load, they are
A) 800 W
B) 400 W
C) 200 W
D) 100 W

Answer: B

132. The all-day efficiency is important for
A) Power transformers
B) Distribution transformers
C) Instrument transformers
D) Auto-transformers

Answer: B

133. The turns ratio of a step-up transformer is
A) < 1
B) > 1
C) = 1
D) 0

Answer: B

134. Transformer is rated in kVA instead of kW because
A) Losses depend on voltage and current only
B) Power factor is variable
C) Both A and B
D) None

Answer: C

135. The power input at no-load is mainly due to
A) Copper loss
B) Core loss
C) Load
D) Magnetizing current

Answer: B

136. The air core transformer is used in
A) Power transmission
B) Radio-frequency circuits
C) Distribution systems
D) Welding

Answer: B

137. The induced emf in secondary winding is
A) Proportional to mutual flux
B) Proportional to leakage flux
C) Constant
D) Inversely proportional to flux

Answer: A

138. The open-circuit test is conducted at
A) Rated voltage and no-load
B) Rated current and no-load
C) Half voltage and short circuit
D) Rated current and short circuit

Answer: A

139. The short-circuit test is conducted at
A) Rated voltage
B) Reduced voltage
C) Rated frequency only
D) Increased frequency

Answer: B

140. Efficiency of transformer under unity power factor is
A) Maximum
B) Minimum
C) Zero
D) Same as lagging PF

Answer: A

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141. The term “per-unit impedance” means
A) Impedance expressed as fraction of base impedance
B) Resistance per turn
C) Impedance of one winding
D) None

Answer: A

142. The purpose of core clamping in transformer is
A) Mechanical stability
B) Reduce noise
C) Reduce vibration
D) All of the above

Answer: D

143. If both iron and copper losses are equal, efficiency is
A) Maximum
B) Minimum
C) Zero
D) Constant

Answer: A

144. If voltage applied to transformer increases by 10%, core losses increase by approximately
A) 10%
B) 21%
C) 44%
D) 100%

Answer: C

145. A transformer has efficiency of 98% at full-load and 0.8 pf lag. Its efficiency at half-load, 0.8 pf lag is
A) 97.8%
B) 97.6%
C) 97.9%
D) 98%

Answer: B

146. The equivalent impedance referred to secondary is
A) $Z_2 + k^2 Z_1$
B) $Z_2 + Z_1/k^2$
C) $Z_1 + Z_2/k^2$
D) $Z_1 + k^2 Z_2$

Answer: A

147. Transformer cooling method used in 132 kV power transformer is
A) Air natural
B) Oil natural air forced (ONAF)
C) Air blast
D) Water cooled

Answer: B

148. In three-phase transformer, circulating current occurs when
A) Voltage unbalance
B) Phase shift mismatch
C) Incorrect polarity
D) All of the above

Answer: D

149. Transformer impedance voltage is
A) Voltage drop at full-load
B) Voltage required to circulate rated current during short-circuit test
C) No-load voltage
D) Voltage regulation

Answer: B

150. The auto-transformer efficiency compared to two-winding type is
A) Higher
B) Lower
C) Same
D) Variable

Answer: A