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 at length in Sec. 2.6. 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.
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.
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 are by far the first two: Pi, the constant core (iron)-loss and P, the variable copper-loss. Therefore, only these two losses will be considered in further discussions.