A time-varying magnetic flux superimposed on a magnetic material causes losses by hysteresis and losses originating from the circulation resulting from parasites. These losses are usually called losses in the iron, losses in the core, or even losses in a vacuum, since they are present even when the transformer is not connected to any load.
The losses in the core are proportional to the square of the voltage taken to the winding. In this way, it is possible to approximate the losses in the iron by a Gm conductance or, obviously, a resistance Rm in parallel with the transformer terminals.
Having enumerated each of the losses occurring in the transformer under load and also those to which the equipment is subjected even in a vacuum, it is possible to produce the equivalent circuit of the power transformer.
The model focuses on an ideal transformer core, in series with resistors R1 and R2, representing losses in copper, and with X1 and X2, representing primary and secondary dispersion reactances, respectively. It is possible to identify in parallel with the transformer windings the conductance Gm and the magnetization reactance Xm (by introducing the effect from the magnetization current necessary to establish the magnetic flux in the nucleus).