201. The polarity of a transformer is determined by
A) Open-circuit test
B) Polarity test
C) Short-circuit test
D) Load test
Answer: B
202. The all-day efficiency of a transformer depends on
A) Load factor and power factor
B) Iron and copper losses
C) Copper loss only
D) Iron loss only
Answer: A
203. Which loss in transformer varies with square of the load current?
A) Iron loss
B) Copper loss
C) Eddy current loss
D) Hysteresis loss
Answer: B
204. Transformer efficiency can be maximum when
A) Copper loss = Iron loss
B) Iron loss > Copper loss
C) Copper loss = 0
D) Iron loss = 0
Answer: A
205. If load on a transformer is increased, its efficiency
A) Increases
B) Decreases
C) Constant
D) First increases then decreases
Answer: D
206. The purpose of breather in transformer is
A) To filter air entering conservator
B) To cool oil
C) To prevent moisture
D) To measure temperature
Answer: C
207. Which of the following connections is used for small three-phase transformers?
A) Star–Star
B) Delta–Delta
C) Open Delta
D) Star–Delta
Answer: C
208. The iron loss in a transformer depends upon
A) Voltage and frequency
B) Load current
C) Temperature
D) Supply power factor
Answer: A
209. A 230/115 V transformer has full-load copper loss of 200 W and iron loss of 150 W. Efficiency at full-load, unity pf = ?
A) 97.8%
B) 98.1%
C) 96.4%
D) 95.5%
Answer: B
210. In transformer, the magnetizing current is
A) Lagging by 90°
B) Leading by 90°
C) In phase with voltage
D) Opposite to voltage
Answer: A
211. The kVA rating of transformer depends on
A) Voltage and current
B) Power factor
C) Frequency
D) Efficiency
Answer: A
212. The leakage flux in a transformer produces
A) Leakage reactance
B) Copper loss
C) Eddy current loss
D) None of these
Answer: A
213. For distribution transformers, the design aims at
A) Maximum efficiency at full-load
B) Maximum efficiency at ½ load
C) Minimum losses
D) Minimum regulation
Answer: B
214. The purpose of Buchholz relay is to
A) Detect incipient faults
B) Measure oil temperature
C) Detect overcurrent
D) Indicate oil level
Answer: A
215. The open-circuit test is performed to measure
A) Iron loss
B) Copper loss
C) Total loss
D) Leakage reactance
Answer: A
216. In short-circuit test, the applied voltage is
A) Full rated voltage
B) Reduced voltage
C) Increased voltage
D) Zero
Answer: B
217. The open-circuit test on transformer is performed at
A) Rated frequency and voltage
B) Reduced voltage
C) High voltage
D) DC voltage
Answer: A
218. The no-load current in transformer is about
A) 1–3% of full-load current
B) 10% of full-load current
C) 25% of full-load current
D) 50% of full-load current
Answer: A
219. The impedance of a transformer is represented as
A) Vector sum of resistance and reactance
B) Sum of resistance and reactance
C) Ratio of resistance to reactance
D) None
Answer: A
220. When transformers operate in parallel, circulating current flows due to
A) Unequal voltage ratio
B) Unequal impedance
C) Phase difference
D) All of these
Answer: D
221. The transformer’s regulation at leading power factor is
A) Negative
B) Positive
C) Zero
D) Infinite
Answer: A
222. A step-up transformer increases
A) Voltage
B) Current
C) Power
D) Both A and C
Answer: A
223. The dielectric strength of good transformer oil is about
A) 10 kV/mm
B) 30 kV/mm
C) 50 kV/mm
D) 100 kV/mm
Answer: B
224. In an auto-transformer, secondary is
A) Electrically connected to primary
B) Isolated
C) Magnetic only
D) None
Answer: A
225. In star–delta transformer, primary current is
A) √3 times secondary current
B) Equal to secondary current
C) 1/√3 of secondary current
D) Zero
Answer: C
226. The main advantage of auto-transformer over two-winding transformer is
A) Higher efficiency and lower cost
B) Better isolation
C) Higher leakage
D) Lower efficiency
Answer: A
227. The emf equation of transformer is . The factor 4.44 arises from
A) RMS value of sinusoidal wave
B) Peak value of flux
C) Frequency conversion
D) Unit change
Answer: A
228. In a 3-phase transformer, the vector group indicates
A) Phase displacement between windings
B) Frequency
C) Voltage ratio
D) Efficiency
Answer: A
229. The percentage impedance of a transformer determines
A) Short-circuit current
B) Efficiency
C) Regulation
D) All of the above
Answer: A
230. When load power factor is lagging, voltage regulation is
A) Positive
B) Negative
C) Zero
D) None
Answer: A
231. The main cause of noise in transformer is
A) Magnetostriction in core
B) Vibration of windings
C) Cooling fan
D) Both A and B
Answer: D
232. The function of conservator in transformer is
A) To provide space for expansion of oil
B) To cool oil
C) To reduce vibration
D) To store moisture
Answer: A
233. In a transformer, the magnetic flux is confined to
A) Core
B) Air gap
C) Windings
D) Insulation
Answer: A
234. For a transformer working at constant load, efficiency can be improved by
A) Improving power factor
B) Increasing voltage
C) Reducing iron loss
D) Increasing frequency
Answer: A
235. The transformer tank is grounded to
A) Avoid electric shock
B) Provide insulation
C) Maintain flux
D) None
Answer: A
236. In dry-type transformer, cooling is done by
A) Air natural
B) Oil natural
C) Oil forced
D) Water
Answer: A
237. Transformer iron loss occurs mainly due to
A) Alternating flux
B) Alternating current
C) Temperature
D) Vibration
Answer: A
238. Which type of transformer connection eliminates third harmonic component of flux?
A) Delta connection
B) Star connection
C) Zig-zag
D) Open delta
Answer: A
239. The neutral of a transformer should be earthed to
A) Protect against lightning
B) Reduce leakage
C) Avoid overvoltage
D) All of the above
Answer: D
240. The instrument used to test oil breakdown voltage is
A) Oil BDV tester
B) Megger
C) Voltmeter
D) Ohmmeter
Answer: A
241. The transformer vector group Dyn11 means
A) 30° lead of secondary
B) 30° lag of secondary
C) Delta–star 11 shift
D) None
Answer: A
242. Which cooling method is used for large power transformers?
A) ONAN
B) ONAF
C) OFAF
D) All of these
Answer: D
243. The transformer tap changer is used to
A) Regulate output voltage
B) Reduce losses
C) Increase frequency
D) Change polarity
Answer: A
244. Tap changing under load is preferred for
A) Power transformers
B) Distribution transformers
C) Auto-transformers
D) Instrument transformers
Answer: A
245. The percentage regulation of transformer at leading PF is
A) Less than lagging PF
B) Greater than lagging PF
C) Equal
D) Infinite
Answer: A
246. The load sharing between transformers in parallel is proportional to
A) kVA rating / impedance
B) kVA rating × impedance
C) Impedance ratio only
D) Turns ratio
Answer: A
247. In current transformers, the secondary should
A) Never be open-circuited
B) Always be open
C) Be grounded
D) Be shorted through a fuse
Answer: A
248. In potential transformers, secondary should
A) Never be short-circuited
B) Always be shorted
C) Have low impedance
D) Be open
Answer: A
249. The rated secondary current of CT is generally
A) 1 A or 5 A
B) 10 A
C) 0.1 A
D) 100 A
Answer: A
250. The rated secondary voltage of PT is generally
A) 110 V
B) 230 V
C) 440 V
D) 11 kV
Answer: A
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