The transformer is a Static device, i.e., It has no moving parts so by common sense it must have the highest efficiency among all the other electrical machines and also higher than that of rotating machines.
The different losses in a transformer are as follows:
1.Core loss
These are hysteresis and eddy-current losses resulting from alternations of magnetic flux in the core. It may be emphasised here that the core-loss is constant for a transformer operated at constant voltage and frequency as are all power frequency equipment.
2.Copper loss (I^2R losses)
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.
3.Load loss (Stray Losses)
It largely results from leakage fields inducing eddy-currents in the tank wall, and conductors.
Dielectric loss, this loss occurs is in the insulating materials, particularly in oil and solid insulations.
However there are only Two major losses by far - Pi, the constant core (iron-loss) and Pc, the variable copper-loss.
Therefore, only these two losses will be considered in further discussions.