A method for determining the equivalent circuit of a transformer consists of two tests: the open-circuit test and the short-circuit test. The open-circuit test, shown in Figure P7.35 (a), is usually done by applying rated voltage to the primary side of the transformer while leaving the secondary side open. The current into the primary side is measured, as is the power dissipated.

The short-circuit test, shown in Figure P7.35(b), is performed by increasing the primary voltage until rated current is going into the transformer while the secondary side is short-circuited. The current into the transformer, the applied voltage, and the power dissipated are measured. The equivalent circuit of a transformer is shown in Figure P7.35(c), where r_w and L_w represent the winding resistance and inductance, respectively, and r_c and L_c represent the losses in the core of the transformer and the inductance of the core. The ideal transformer is also included in the model. With the open-circuit test, we may assume that _P = _S = 0. Then all of the current that is measured is directed through the parallel combination of r_c and L_c. We also assume that |r_c||j_ωL_c| is much greater than r_w + j_ωL_w. Using these assumptions and the open-circuit test data, we can find the resistance r_c and the inductance L_c. In the short-circuit test, we assume that  secondary is zero, so that the voltage on the primary side of the ideal transformer is also zero, causing no current flow through the r_c − L_c parallel combination. Using thisassumption with the short-circuit test data, we are able to find the resistance r_w and inductance L_w. Using the following test data, find the equivalent circuit of the transformer: