The efficiency of a transformer at a given load and p.f. is expressed by the relation

$$ \eta=\frac{V_{2} I_{2} \cos \phi_{2}}{V_{2} I_{2} \cos \phi_{2}+P_{1}+I_{2}^{2} R_{e}}=\frac{V_{2} \cos \phi_{2}}{V_{2} \cos \phi_{2}+P_{1} / I_{2}+I_{2} R_{e}} $$ The terminal voluge $V_{2}$ is approximately constant. Thus for a given p.f., efficiency depends upen the lnad …