EQUIVALENT CIRCUIT

Figure 1.32 shows the equivalent circuit of a single-phase transformer having load impedance Z_L. Figure 1.33 is the equivalent circuit of Figure 1.32. The secondary winding resistance (R₂), leakage reactance (X₂) and load impedance (Z_L) connected to secondary terminal is transferred to primary side.

Figure 1.32 Equivalent Circuit

Figure 1.33 Exact Equivalent Circuit

Figure 1.34 Approximate Equivalent Circuit

Since I₀ is small compared to full-load current, we can shift the excitation circuit towards the terminal voltage side shown in Figure 1.34. Figure 1.34 is the approximate equivalent circuit, whereas Figure 1.33 is the exact equivalent circuit of Figure 1.32.

From Figure 1.33, the total input impedance between input terminals becomes

where

and is the impedance of the exciting circuit.

R₀, X₀, R₀₁ and X₀₁ are the four important parameters of the transformer. From open circuit and short circuit test of a transformer, R₀, X₀ and R₀₁, X₀₁ can be determined respectively.