Open Circuit Test:

This test is performed to determine the core losses and shunt branch parameters G_i and B_m  of the equivalent circuit of the transformer. This test is performed on the low voltage winding side whereas high voltage winding is kept open circuited. The connection diagram is illustrated in the Figure. Rated voltage is applied to the low voltage winding and meter readings are recorded.

Voltmeter reading = V₁ Volts

 Ammeter reading = I₀

 Amps Power input = P₀ Watts

The no load current I₀ drawn by the transformer in very low usually 2-6% of the rated current. The winding resistances and reactances are of extremely low ohmic value. Thus, ohmic losses may be neglected.

The equivalent (approximate) circuit is illustrated in Figure

Figure: Equivalent Circuit for Open Circuit

The watt meter reads the core losses (P₀), watts :

(V₁ )² G_i  = P₀

G_i  =     P₀ /(V₁ )²

Y  = I₀ /V₁

We must know the G_i and B_m are referred to the low voltage winding side. We may easily transform them to the HV winding side.