Ans: The ordinary or commercial efficiency of a transformer is defined as the ratio of the output in watts to the input in watts. Since the efficiency is based on the power output in watts and not on volt-amperes, the power factor plays an important role in determining the efficiency.
Efficiency =(Output power)/(Input power) =(E2 I2×P.F)/(E2 I2×P.F+Losses) = (E2 I2 cos∅)/(E2 I2 cos∅+W1+I1’ R1+I2’ R2 )
Where E2 =Secondary voltage.
I2 = Secondary current.
I1 =Primary current.
R1 =Primary resistance.
R2 = Secondary resistance.
W1 =Iron losses.