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A 100 kVA, single-phase transformer has a full-load copper loss of 600 W and iron loss of 500 W. The maximum efficiency occurs at a load of nearly
1. 82.1 kVA
2. 83.3 kVA
3. 91.3 kVA
4. 98.1 kVA

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Correct Answer - Option 3 : 91.3 kVA

Concept:

The maximum efficiency in a transformer that occurs at copper losses is equal to iron losses.

Efficiency is maximum at some fraction 'k' of full load.

\(k = \sqrt {\frac{{{W_i}}}{{{W_{cu}}}}}\)

Where Wi = iron losses

Wcu = copper losses

kVA at maximum efficiency is given by:

\(kVA\;at\;{\eta _{max}} = full\;load\;kVA × \sqrt {\frac{{{W_i}}}{{{W_{cu}}}}}\)

Calculation:

For the given copper and iron loss:

\(k = \sqrt {\frac{{{500}}}{{{600}}}}\)

k = 0.913

Now, the load at which maximum efficiency occurs is given by:

= k × 100 kVA

= 91.3 kVA

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