A single-phase transformer has no-load loss of 64 W, as

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A single-phase transformer has no-load loss of 64 W, as obtained from an open-circuit test. When a short-circuit test is performed on it with 90% of the rated currents flowing in its both LV and HV windings, the measured loss is 81 W. The transformer has maximum efficiency when operated at

1. 50.0% of the rated current.
2. 64.0% of the rated current.
3. 80.0% of the rated current.
4. 88.8% of the rated current.

Correct Option: C

P₀ = 64 watt
P_cu = 81 watt at 90% of rated load
81 = (0.9 I_fl ² )R

I_fl ² R =	81	= 100
	(0.9)²

P_cu fl = 100 w
For maximum efficiency

I = 0.81 I_fl