Equivalent Circuit of a Transformer

EQUIVALENT CIRCUIT OF A TRANSFORMER

Equivalent circuit of transformer is shown in Figure 1.29. It is resolved into an equivalent circuit in which the resistance and leakage reactance of the transformer are imagined to be external to the winding which transform the voltage.

At no load condition, the primary of a transformer draws no load current I₀. It is mainly used to supply the iron loss and to produce the flux in the core. Iron loss is represented by R_m and the magnetizing current is X_m. R_m and X_m are connected in parallel to primary winding. It is called as no load branch (or) exciting branch. (R_m and X_m).

R₁ - Primary winding resistance in Ω

X₁ - Primary winding reactance in Ω

R_m - No load resistance in Ω

X_m - No load reactance in Ω

I₁ - Full load primary current in Amps

I₀ - No load primary current in Amps

I₂'-Load component of primary current in Amps

I_w - Working component

I_m - Magnetizing component

E₁ - Induced emf in primary winding in Volts

E₂ - Induced emf in secondary winding in Volts

R₂, X₂ - Secondary winding resistance in Q, secondary winding reactance in Ω

I₂ - Full load secondary current in A

k - Transformation Ratio.

Circuit Referred to Primary

Secondary parameters like Resistance, Reactance, impedance were transferred to the primary side, shown in Figure 1.30. Resistance and reactance values should be divided by k² if secondary parameters were transferred to primary. Voltages are divided by k and currents are multiplied by k.

Approximate equivalent circuit:

The no-load current I is only 1-3% of rated primary current. So I₂' is practically equal to I₁. Due to this, the equivalent circuit can be simplified by transferring the exciting branch (R_m and X_m) to the left position of the circuit shown in Figure 1.31.