Open-circuit test

The open-circuit test, or no-load test, is one of the methods used in electrical engineering to determine the no-load impedance in the excitation branch of a transformer.

The no load is represented by the open circuit, which is represented on the right side of the figure as the "hole" or incomplete part of the circuit.

An ammeter is connected in series with the primary winding.

Hence the iron loss is maximum at rated voltage.

This maximum iron loss is measured using the wattmeter.

Since the impedance of the series winding of the transformer is very small compared to that of the excitation branch, all of the input voltage is dropped across the excitation branch.

The two losses can be separated by driving the transformer from a variable frequency source since the hysteresis loss varies linearly with supply frequency and the eddy current loss varies with the frequency squared.

Since the secondary of the transformer is open, the primary draws only no-load current, which will have some copper loss.

Therefore, power is no longer transferred from primary to secondary in this approximation, and negligible current goes through the secondary windings.

Since no current passes through the secondary windings, no magnetic field is created, which means zero current is induced on the primary side.

The parallel shunt component on the equivalent circuit diagram is used to represent the core losses.

These core losses come from the change in the direction of the flux and eddy currents.

In contrast to the parallel shunt component, the series component in the circuit diagram represents the winding losses due to the resistance of the coil windings of the transformer.

Current, voltage and power are measured at the primary winding to ascertain the admittance and power-factor angle.

Another method of determining the series impedance of a real transformer is the short-circuit test.

is the core loss component of no-load current