SIMPLIFIED EQUIVALENT CIRCUIT:
While drawing simplified circuit of a transformer, the exciting circuit (i.e., exciting resistance and exciting reactance) can be omitted.
The simplified equivalent circuit of a transformer is drawn by representing all the parameters of the transformer either on the secondary or on the primary side. The no-load current I0 is neglected as its value is very small as compared to full load current, therefore, I2’ = I
1. Equivalent circuit when all the quantities are referred to secondary.
The primary resistance when referred to secondary side, its value is R1‘ = K2 R1 and the total or equivalent resistance of transformer referred to secondary, Res = R2 + R1‘. Similarly, the primary reactance when referred to secondary side, its value is X1‘ = K2X1 and the total or equivalent reactance of transformer referred to secondary, Xes = X2 + X1‘. All the quantities when referred to the secondary side are shown in Fig-1.1.
The phasor (vector) diagrams of a loaded transformer when all the quantities are referred to secondary side for resistive, inductive and capacitive loads are shown in Figs. 1.2(a), 1.2(b) and1.2(c), respectively. The voltage drops in resistances (vectors) are taken parallel to the current vector and the voltage drops in reactances (vectors) are taken quadrature to the current vector.
2. Equivalent circuit when all the quantities are referred to primary.
In this case, to draw the equivalent circuit all the quantities are to be referred to primary, as shown in Fig- 2.1.
The phasor diagram to transformer when all the quantities are referred to primary side for different types of loads are shown in Fig-2.2.