281. In a transformer, the resistance between the primary and secondary must be

- Zero
- 1 kΩ
- 100 kΩ
- Infinite

282. Current drawn by the primary of a transformer which steps down from 220 V to 22 V to operate a device having an impedance of 220 Ω will be

- 1 A
- 0.1 A
- 0.01 A
- 10 A

283. In a step-up transformer, turn’s ratio is 1 : 2. A Leakage cell (emf 1.5V) is connected across the primary. The voltage across the secondary is

- 3.0 V
- 0.75 V
- 0 V
- 1.5 V

284. In order to achieve all-day efficiency in a transformer

- The copper losses should be less
- The operating temperature should be low
- The shell-type construction should be used
- The iron losses should be less

285. For higher voltage ranges __________ transformers are used.

- Core type
- Shell type
- Both core type & Shell type
- None of these

286. A 200 KVA transformer has an iron loss of 1 kW and full load copper loss of 2 kW. Its load kVA corresponds to maximum efficiency is

- 100 kVA
- 70.7 kVA
- 200 kVA
- 141.4 kVA

287. A T-T transformer can not be paralleled with __________ transformer.

- V-V
- Y-Δ
- Y-Y
- Δ-Δ

288. An ideal transformer does not change

- Voltage
- Power
- Current
- None of the above

289. In a single-phase transformer the magnitude of leakage reactance is twice that of resistance of both primary and secondary. With secondary short-circuited, the input power factor is

- $\frac{1}{\sqrt{3}}$
- $\frac{1}{\sqrt{5}}$
- $\frac{2}{\sqrt{5}}$
- $\frac{1}{3}$

290. The emf per turn for a single phase 2200/220V, 50Hz transformer is approximately 12V. The number of primary and secondary turns are

- 18,183
- 183,18
- 155,26
- 172,21

291. The maximum efficiency of a 200 kVA transformer having an iron loss of 1800 W and full load copper loss of 3200 W occurs at

- 100 kVA
- 150 kVA
- 200 kVA
- 125 kVA

292. Which test is performed to determine core loss at full load of a transformer?

- Short circuit test
- Sumpner test
- Open circuit test
- None of these

293. If the transformer frequency is changed from 50 Hz to 60 Hz, the ratio of eddy current loss at 60Hz to 50Hz at constant voltage will be

- 5/6
- 6/5
- 25/36
- 36/25

294. A pulse transformer is used in a driver circuit

- To prevent dc triggering
- To shape the trigger signal
- To generate high-frequency pulses
- To provide high-frequency pulses

295. Biased differential protection for transformers are used for

- Low fault settings and high operating speeds
- High fault settings and high operating speeds
- Low fault setting and low operating speeds
- High fault setting and low operating speeds

296. Single-phase transformer voltage is maximum when the primary and secondary coils are

- Aligned along the same axis
- Have a phase shift of 120° from each other
- Normal to each other
- Have a phase shift of 90° to each other

297. When moist air passes through a bed of silica gel, the

- Dry bulb temperature of air decreases
- Dry bulb temperature of air increases
- Specific humidity of air increases
- It undergoes adiabatic saturation

298. The full-load copper loss (P_{cu}) and iron loss (P_{i}) of the transformer are 6400 Watts and 5000 Watts, respectively. The copper loss (P_{c}) and iron loss at the half-load will be, respectively

- 1600 Watts and 5000 Watts
- 3200 Watts and 5500 Watts
- 3200 Watts and 2500 Watts
- None of these

299. The iron losses in a 100 kVA transformer are 1 kW and the full load copper losses are 2 kW, then maximum efficiency occurs at a load of

- $100\sqrt{2}$
- $\frac{100}{\sqrt{2}}$
- 50 kVA
- None of these

300. What is the cause of occurrence of stray load losses in the transformer?

- under frequency operation
- overloading
- poor cooling
- leakage flux