The losses occurring in the transformer core due to hysteresis and eddy currents are called core or no load losses. These losses are represented by the shunt branch parameters in the equivalent circuit.

The open circuit test is done to determine the core/no load losses, turns ratio and parameters of the shunt or no load branch like resistance (Ro) and reactance (Xo) of a transformer's equivalent circuit.

During the open circuit test, the high voltage side of the transformer is kept open while the low voltage side is connected to the variable AC supply.

The I2R losses occurring in the transformer windings due to the flow of current are called copper or load losses. These losses are represented by the series branch parameters in the equivalent circuit.

The short circuit test helps determine the full load copper losses, equivalent resistance (Re) and reactance (Xe) of a transformer which are the series parameters of the equivalent circuit used for modeling transformer performance.