ELECTRICAL AND ELECTRONICS ENGINEERING : If a transformer is connected to DC supply

151. In a transformer, the leakage flux is minimized by

A) Increasing core area
B) Using tightly coupled windings
C) Reducing number of turns
D) Increasing air gap

152. The phase difference between exciting current and supply voltage in a transformer is approximately
A) 0°
B) 30°
C) 60°
D) 80°–85°

153. The resistance of transformer winding is measured to determine
A) Copper loss
B) Iron loss
C) Efficiency
D) Regulation

154. Which material is used for transformer core?
A) Cast iron
B) Silicon steel
C) Stainless steel
D) Mild steel

155. The efficiency of a transformer at half load and unity pf equals its efficiency at full load and pf = 0.8 lag. Then, full-load efficiency is
A) Maximum
B) Equal
C) Minimum
D) Independent of load

156. A 100 kVA transformer has 1 kW iron loss and 1.5 kW copper loss at full-load. Efficiency at ¾ load and unity pf is approximately
A) 97.7%
B) 98%
C) 96.8%
D) 99%

157. In a single-phase transformer, if secondary load current increases, the primary current
A) Increases
B) Decreases
C) Remains constant
D) Becomes zero

158. Transformer lamination thickness is usually of the order of
A) 0.1 mm
B) 0.35 mm
C) 1 mm
D) 2 mm

159. Transformer operates on
A) Mutual induction
B) Self-induction
C) Eddy current
D) Capacitive effect

160. The efficiency of transformer mainly depends on
A) Core design
B) Load and power factor
C) Voltage
D) Cooling medium

161. The core flux in a transformer is
A) Sinusoidal
B) Non-sinusoidal due to saturation
C) Zero
D) Constant DC

162. In an ideal transformer, the input and output power are
A) Equal
B) Different
C) Dependent on voltage
D) Dependent on current

163. In a transformer, the percentage resistance is
A) $\frac{I_{sc} R_{eq}}{V_{rated}} \times 100$
B) $\frac{V_{sc} R_{eq}}{I_{rated}} \times 100$
C) $\frac{R_{eq}}{Z_{eq}} \times 100$
D) $\frac{V_{rated}}{I_{sc}} \times 100$

164. Which test gives the equivalent circuit parameters of transformer?
A) OC and SC tests
B) Load test
C) Regulation test
D) Temperature rise test

165. The leakage flux in transformer produces
A) Voltage drop
B) Efficiency increase
C) Core loss reduction
D) Temperature decrease

166. The short-circuit impedance of transformer is expressed as
A) % impedance voltage
B) Ohmic resistance
C) Turns ratio
D) Load factor

167. The emf induced per turn in primary and secondary of transformer is
A) Equal
B) Unequal
C) Proportional to number of turns
D) Zero

168. The dielectric strength of transformer oil decreases due to
A) Moisture content
B) Aging
C) Dust
D) All of the above

169. For parallel operation of transformers, the per-unit impedance should be
A) Equal
B) Unequal
C) Maximum
D) Minimum

170. The leakage reactance of a transformer depends mainly on
A) Leakage flux
B) Core area
C) Frequency
D) Turns ratio

171. The magnetostriction effect in transformer causes
A) Noise and vibration
B) Overheating
C) Loss of efficiency
D) Voltage drop

172. The zero-sequence impedance of a transformer is
A) Same as positive-sequence
B) Depends on winding connection
C) Always zero
D) Always maximum

173. The purpose of using high permeability core is to
A) Reduce hysteresis loss
B) Reduce magnetizing current
C) Increase mutual flux linkage
D) All of the above

174. The polarity test of transformer is used to determine
A) Voltage ratio
B) Correct phase relation
C) Efficiency
D) Impedance

175. The efficiency of a transformer is highest when
A) Copper loss = Iron loss
B) Copper loss > Iron loss
C) Copper loss < Iron loss
D) Load is full

176. In an auto-transformer, the transformation ratio $k = \frac{N_2}{N_1}$ affects
A) Saving of copper
B) Efficiency
C) Cost
D) All of the above

177. The harmonics in no-load current of transformer are predominantly
A) 2nd and 4th
B) 3rd and 5th
C) 3rd and 7th
D) Odd harmonics only

178. In a distribution transformer, which loss is considered constant throughout the day?
A) Copper loss
B) Iron loss
C) Stray loss
D) Dielectric loss

179. In transformer equivalent circuit, magnetizing branch represents
A) Core losses and magnetizing current
B) Leakage reactance
C) Load
D) None

180. For a 230/115 V transformer, the turns ratio is
A) 2:1
B) 1:2
C) 1:1
D) 4:1

181. A transformer designed for 400 Hz operation cannot be used for 50 Hz because
A) Core will saturate
B) Copper loss increases
C) Efficiency reduces
D) Regulation improves

182. The efficiency of a transformer decreases with
A) Decrease in load
B) Increase in load
C) Constant load
D) Higher power factor

183. For a transformer to share load proportionally, both should have same
A) Voltage ratio and impedance ratio
B) Voltage ratio only
C) Impedance only
D) Current ratio

184. If the applied voltage to a transformer is increased by 10%, core loss increases by approximately
A) 10%
B) 20%
C) 21%
D) 44%

185. The main reason for transformer failure is
A) Insulation breakdown
B) Copper loss
C) Overvoltage
D) Overload

186. In an auto-transformer, saving in copper is maximum when
A) Ratio of transformation is small
B) Ratio is large
C) Ratio = 1
D) None

187. The leakage reactance in transformer affects
A) Voltage regulation
B) Efficiency
C) Core loss
D) Cooling

188. A 1000 kVA transformer has 1% resistance and 5% reactance. The percentage impedance is
A) 4%
B) 5%
C) 5.1%
D) 5.02%

189. The tertiary winding of a transformer is usually connected in
A) Star
B) Delta
C) Open-delta
D) Zigzag

190. The copper loss in transformer varies as
A) $I$
B) $I^2$
C) $V^2$
D) $f^2$

191. Transformer breathers are filled with
A) Silica gel
B) Activated carbon
C) Calcium chloride
D) Sodium chloride

192. Silica gel in breather becomes pink due to
A) Absorption of moisture
B) High temperature
C) Oil vapor
D) Dust

193. In a three-phase transformer, open-delta connection is used when
A) One transformer fails
B) Full load needed
C) Voltage increased
D) Power factor correction required

194. In star–delta transformer, the secondary line voltage leads the primary line voltage by
A) 0°
B) 30°
C) 150°
D) 180°

195. The magnetizing reactance of transformer is
A) Large
B) Small
C) Zero
D) Equal to leakage reactance

196. The temperature of transformer oil is measured by
A) Thermometer pocket
B) Thermistor
C) RTD sensor
D) Buchholz relay

197. Transformer losses are minimum when operated at
A) Rated load
B) Maximum efficiency load
C) No-load
D) Full-load

198. The primary induced emf in transformer lags flux by
A) 0°
B) 90°
C) 180°
D) 45°

199. If a transformer is connected to DC supply
A) It will burn due to high current
B) Work normally
C) Give constant output
D) Reduce efficiency only

200. The inrush current in transformer is due to
A) Residual flux and saturation
B) High leakage reactance
C) Overvoltage
D) Low copper loss

ELECTRICAL AND ELECTRONICS ENGINEERING

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Tuesday, October 21, 2025

If a transformer is connected to DC supply

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