\[\eta=\frac{\text { output power }}{\text { input power }}\]
\[\eta=\frac{\text { output power }}{\text { output power }+\text { losses }}\]
\[\eta=\frac{\text{E}_{2} \text{I}_{2} \cos \phi_{2}}{\text{E}_{2} \text{I}_{2} \cos \phi_{2}+\text{F} . \text{L} \text { cu losses }+\text { Iron losses }}\]