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