Open-circuit test
Open circuit test, sometimes called no-load test, is one of the methods used in electrical engineering to determine the value of the impedance in the excitation branch of a real transformer.
Methods
The secondary winding of the transformer is left open-circuited. A full-rated line voltage is applied to the primary winding of the transformer. Since the value of the impedance of the series winding of the transformer is too small compared to the value of the impedance of the excitation branch, therefore all of the input voltage is dropped across the excitation branch.
All the values of current, voltage and power are measured from the primary winding in order to get the magnitude value of admittance and power factor angle.
Another method in determining the value of the series impedance of a real transformer is called short circuit test.