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A 400 / 100 V, 5kVA, single-phase two winding transformer is converted into an autotransformer to supply 400 volts from a 500 V voltage source. When tested as a two winding transformer at rated load and 0.85 p.f. lagging, efficiency is 0.95, than its efficiency as auto-transformer at rated-load and 0.8 p.f. lagging will be _______ % .
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- 98.95%
- 45%
- 38.42%
- 9.995%
Correct Option: A
kVA rating of auto-transformer
= 500 × 50 = 25 kVA
As a two- winding transformer,
| η = | = 0.95 | |
| output + losses |
| ∴ | = 0.95 | |
| 5 × 0.8 + losses |
⇒ Losses = 210.52 watt
Since losses remain const ant, hence for an auto-transformer,
| η = | = 98.95% | |
| 25 × 0.8 + 0.210 |
