In the circuit diagram for the modelled practical transformer the resistor that is supposed to represent core losses is in parallel with the voltage supply and the primary winding. Why is this so? I would also just like to know in general what determines whether the loss impedances are in series or parallel with the voltage supply/primary winding.

Cheers

 

Iron (magnetising) losses are related to the transformer primary (or excitation) voltage and are therefore modelled by a resistance across the source which is in parallel with the "ideal" primary winding. The primary winding magnetising inductance is also modelled as an ideal inductor in parallel with both the source voltage and the magnetising loss equivalent resistance.

Series loss impedance includes the primary and secondary winding copper losses and leakage inductances. The effect of these is not generally obvious unless current is flowing both primary and secondary windings - with the transformer operating under load.