Section IV: Transformer Model

The equivalent circuit of a single-phase transformer is shown in Fig. 1.18. In this the primary voltage and currents are denoted by V₁ and V₂ respectively. The current entering the primary terminals is I₁. The core loss component is represented by R_cwhile the magnetizing reactance is denoted by X_m . The leakage inductance of the transformer is denoted by X_eqand R_eq is transformer winding resistance. It is to be noted that all the quantities are referred to the primary side. The turns ratio of the transformer is given by N₁ : N₂ .

The impedance of the shunt branch is much larger compared to that of the series branch. Therefore we neglect R_cand X_m. Again of the series parameters, R_eqis much smaller than X_eq. We can therefore neglect the series impedance. Therefore the transformer can be represented by the leakage reactance X_eq. The single-phase transformer equivalent circuit, when referred to the primary side, is as shown is Fig. 1.19 (a). The equivalent circuit, when referred to the secondary side, is shown in Fig. 1.19 (b) where a = N₁ / N₂ .

Fig. 1.18 Equivalent circuit of a single-phase transformer

Fig. 1.19 Simplified equivalent circuit of a single-phase transformer: (a) when referred to the primary side and (b) when referred to the secondary side.