Losses in Transformer:

The transformer has no moving parts so that its efficiency is much higher than that of rotating machines. The various Losses in Transformer are enumerated below:


These are hysteresis and eddy-current losses resulting from alternations of magnetic flux in the core. Their nature and the remedies to reduce these have already been discussed. It may be emphasized here that the core-loss is constant for a transformer operated at constant voltage and frequency as are all power frequency transformers.

Copper-loss (I2R-loss):

This loss occurs in winding resistances when the transformer carries the load current; varies as the square of the loading expressed as a ratio of the full-load.

Load (stray)-loss:

It largely results from leakage fields inducing eddy-currents in the tank wall, and conductors.


The seat of this loss is in the insulating materials, particularly in oil and solid insulations.

The major Losses in Transformer are by far the first two: Pi, the constant core (iron)-loss and Pc, the variable copper-loss. Therefore, only these two losses will be considered in further discussions.

It will be seen already that transformer losses and the parameters of its equivalent circuit can be easily determined by two simple tests without actually loading it.