Transformer Equivalent Circuit

The approximate equivalent circuit, referred to the primary:

• series winding resistance $R_{eq} = R_1 + R_2'$ and leakage reactance $X_{eq} = X_1 + X_2'$, where $R_2' = R_2 (N_1/N_2)^2$;
• a shunt magnetising branch: $R_c$ (core loss) in parallel with $X_m$ (magnetising current).

Parameters come from two tests:

• Open-circuit test (rated voltage, secondary open) → core loss and $R_c$, $X_m$.
• Short-circuit test (reduced voltage, rated current, secondary shorted) → copper loss and $R_{eq}$, $X_{eq}$.

The per-unit impedance (typically 4–10 % for distribution transformers) also sets the fault current a transformer can deliver.