EFFICIENCY OF A TRANSFORMER

Since the equivalent circuit contains two winding resistances and a core-loss resistance then power is lost as heating energy inside the transformer. Hence the conversion of power through the transformer cannot be 100%, a small loss of efficiency occurs. This is usually less than about 2% for power transformers. Assume all resistances and reactances are referred to the secondary winding. The efficiency can be expressed as,



Where cosØ is the power factor of the load

Pc is the core-loss
Is is the secondary current
Vs is the secondary voltage
Es is the secondary emf.

This formula applies to single-phase transformers, or to one phase of a three-phase transformer.