ELECTRICAL AND ELECTRONICS ENGINEERING : The life of transformer insulation mainly depends on

231. The main source of noise in a transformer is due to

A) Magnetostriction in the core
B) Cooling fans
C) Oil flow
D) Loose bolts

232. The function of the conservator tank in a transformer is to
A) Provide space for oil expansion
B) Reduce vibration
C) Act as an oil filter
D) Store oil permanently

233. Transformer oil mainly serves to
A) Insulate and cool the windings
B) Lubricate bearings
C) Increase power factor
D) Improve voltage regulation

234. The breather in a transformer contains
A) Silica gel
B) Charcoal
C) Sodium chloride
D) Calcium oxide

235. The color change of silica gel in breather from blue to pink indicates
A) Moisture absorption
B) Temperature rise
C) Oil contamination
D) Fault in windings

236. The purpose of the Buchholz relay is to
A) Detect internal faults
B) Regulate voltage
C) Measure load current
D) Indicate temperature

237. A current transformer (CT) should never be
A) Open-circuited on secondary
B) Short-circuited on primary
C) Overloaded
D) Used at low voltage

238. A potential transformer (PT) should never be
A) Short-circuited on secondary
B) Open-circuited on secondary
C) Overloaded
D) Operated below frequency

239. The secondary of a CT is designed for
A) 5 A or 1 A current
B) 110 V
C) 230 V
D) 440 V

240. The secondary of a PT is designed for
A) 110 V
B) 230 V
C) 11 kV
D) 400 V

241. The vector group Dyn11 represents
A) Star–Delta, 30° lead
B) Delta–Star, 30° lead
C) Star–Delta, 30° lag
D) Delta–Star, 30° lag

242. The connection commonly used for distribution transformers is
A) Delta–Star
B) Star–Delta
C) Delta–Delta
D) Star–Star

243. The tertiary winding in a three-winding transformer is generally used for
A) Stabilizing unbalanced load
B) Voltage regulation
C) Power factor improvement
D) Harmonic elimination

244. A transformer rated for 50 Hz can be operated safely at
A) 40 Hz
B) 25 Hz
C) 60 Hz
D) 15 Hz

245. The efficiency of a transformer under light load is low because
A) Iron loss is constant
B) Copper loss increases
C) Leakage flux increases
D) Core gets saturated

246. The no-load current in a transformer is
A) Highly lagging
B) Highly leading
C) In phase
D) Unity

247. The magnetizing reactance of a transformer is usually
A) Very large
B) Very small
C) Equal to leakage reactance
D) Infinite

248. Transformer cores are laminated to reduce
A) Eddy current loss
B) Hysteresis loss
C) Copper loss
D) Leakage flux

249. The hysteresis loss in a transformer is proportional to
A) Frequency
B) Frequency × flux density
C) $f \times B_m^{1.6}$
D) $f \times B_m^2$

250. Eddy current loss is proportional to
A) $f^2 B_m^2$
B) $f B_m$
C) $B_m^2$
D) $f B_m^3$

251. The temperature rise in transformer winding is limited to
A) 55°C for oil immersed
B) 75°C for dry type
C) 100°C for air cooled
D) Both A and B

252. The main reason for transformer failure in service is
A) Insulation breakdown
B) Copper loss
C) Core loss
D) Tap changer defect

253. In an ideal transformer, efficiency is
A) 100%
B) 90%
C) 80%
D) 99%

254. The output voltage of a transformer changes slightly under load due to
A) Voltage regulation
B) Iron loss
C) Efficiency
D) Hysteresis

255. The type of transformer used in electrical measuring instruments is
A) Instrument transformer
B) Auto-transformer
C) Distribution transformer
D) Power transformer

256. The ratio error in CT depends on
A) Magnetizing current
B) Frequency
C) Core loss
D) Temperature

257. The phase angle error in CT is due to
A) Magnetizing and core loss components
B) Copper loss
C) Load current
D) Frequency

258. The knee point voltage in CT core is defined as
A) The point where magnetizing current increases sharply
B) Rated voltage
C) Voltage regulation limit
D) None of these

259. The burden of an instrument transformer is
A) The load connected to its secondary
B) Primary current
C) Magnetizing current
D) Core loss

260. For protection purposes, CTs are designed for
A) Accuracy up to saturation
B) Operation beyond knee point
C) High current with minimum error
D) All of these

261. The cooling method ONAN means
A) Oil Natural Air Natural
B) Oil Natural Air Forced
C) Oil Forced Air Natural
D) Oil Forced Air Forced

262. In large power transformers, forced cooling is required to
A) Increase kVA rating
B) Reduce iron loss
C) Improve voltage regulation
D) Reduce inrush current

263. The transformer tap changer regulates
A) Output voltage
B) Frequency
C) Current
D) Efficiency

264. Tap changers can be
A) Off-load or on-load
B) Primary side only
C) Secondary side only
D) None

265. The on-load tap changer (OLTC) operates
A) Without interrupting supply
B) By disconnecting supply
C) At no load only
D) With delay

266. The dielectric strength of transformer oil should be at least
A) 30 kV
B) 50 kV
C) 70 kV
D) 10 kV

267. The impedance voltage of transformer is used for
A) Fault current calculation
B) Core loss determination
C) Efficiency test
D) Voltage regulation only

268. Percentage impedance of transformer =
A) $\frac{V_{SC}}{V_{Rated}} \times 100$
B) $\frac{I_{SC}}{I_{Rated}} \times 100$
C) $\frac{P_{Loss}}{V_{Rated}} \times 100$
D) $\frac{R_{eq}}{X_{eq}} \times 100$

269. The per unit impedance of a transformer is independent of
A) Voltage rating
B) kVA rating
C) Frequency
D) Design

270. The transformer’s leakage flux depends on
A) Leakage reactance
B) Winding separation
C) Core design
D) All of the above

271. When frequency increases, eddy current loss
A) Increases
B) Decreases
C) Remains same
D) First increases then decreases

272. The efficiency of distribution transformer is high at
A) 60–70% load
B) 90% load
C) Full load
D) 25% load

273. Transformer regulation depends on
A) Load current and PF
B) Iron loss
C) Frequency
D) None

274. Open-circuit test gives
A) Core loss
B) Copper loss
C) Leakage reactance
D) Efficiency directly

275. Short-circuit test gives
A) Copper loss
B) Iron loss
C) Efficiency
D) Power factor

276. In a transformer, if secondary is open, then
A) No-load current flows
B) Full-load current flows
C) High current flows
D) None

277. The transformer equivalent circuit is referred to
A) Either primary or secondary
B) Primary only
C) Secondary only
D) Both separately

278. The all-day efficiency of transformer is
A) (Output energy / Input energy) × 100
B) (Output power / Input power) × 100
C) Based on instantaneous losses
D) Constant

279. The efficiency of transformer at unity PF is always
A) Higher than lagging PF
B) Lower than lagging PF
C) Equal
D) None

280. In a transformer, eddy current losses are reduced by
A) Laminating the core
B) Increasing flux
C) Increasing frequency
D) Using thick core sheets

281. The phase angle between applied voltage and magnetizing current in transformer is
A) 90° lagging
B) 60° lagging
C) 0°
D) 30° lagging

282. Hysteresis loss can be minimized by using
A) Cold-rolled silicon steel
B) Cast iron
C) Aluminium core
D) Thick laminations

283. The efficiency of transformer depends on
A) Load and power factor
B) Voltage ratio
C) Frequency
D) Cooling

284. The kVA rating of transformer is independent of
A) Power factor
B) Voltage
C) Current
D) Frequency

285. The leakage reactance of transformer is proportional to
A) Square of leakage flux
B) Leakage flux × current
C) Leakage flux × number of turns
D) Leakage flux only

286. The rating of a transformer is expressed in
A) kVA
B) kW
C) kVAR
D) HP

287. The polarity of transformer is important during
A) Parallel operation
B) Short-circuit test
C) Efficiency test
D) OC test

288. The function of tap changer is to
A) Control output voltage
B) Improve efficiency
C) Control frequency
D) Reduce current

289. The resistance of transformer windings increases with
A) Temperature
B) Voltage
C) Current
D) Frequency

290. When a transformer is switched on at zero voltage, inrush current is
A) Minimum
B) Maximum
C) Zero
D) Constant

291. When a transformer is switched on at voltage peak, inrush current is
A) Maximum
B) Minimum
C) Zero
D) Constant

292. The inrush current in transformer may reach
A) 5–10 times rated current
B) 2 times rated current
C) 15 times rated current
D) Equal to rated current

293. The magnetizing current in transformer causes
A) Core losses
B) Leakage flux
C) Load losses
D) Voltage drop

294. In an auto-transformer, copper saving is
A) $1 - k$ times of two-winding transformer
B) $k$ times
C) $k^2$ times
D) Constant

295. An ideal transformer has
A) No losses and 100% efficiency
B) No copper loss
C) No core loss
D) All of the above

296. The transformer primary current under no-load condition is
A) Very small
B) Large
C) Equal to secondary current
D) Zero

297. The leakage reactance of transformer is responsible for
A) Voltage drop and poor regulation
B) Core loss
C) Hysteresis
D) None

298. If supply frequency decreases, core losses
A) Decrease
B) Increase
C) Remain constant
D) Depend on voltage

299. The most efficient transformer operates at
A) Copper loss = Iron loss
B) Maximum flux
C) Minimum voltage
D) Zero current

300. The life of transformer insulation mainly depends on
A) Temperature
B) Frequency
C) Voltage
D) Humidity

ELECTRICAL AND ELECTRONICS ENGINEERING

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Tuesday, October 21, 2025

The life of transformer insulation mainly depends on

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