The open-circuit test in a transformer is used to measure

The open-circuit test in a transformer is used to measure

Right Answer is:

Core loss


The purpose of the open-circuit test’ is to determine the excitation admittance of the transformer- equivalent circuit, the core loss, the no-load excitation current, and the no-load power factor.

The figure shows the connections for the open-circuit test. The low voltage winding is supplied with the rated voltage which should result in the rated voltage on the high voltage side. The current drawn should be low enough so that the copper losses are very low and the power measured 1s almost all from the core losses. The current drawn during the open Circuit test will be the excitation current.

  • In the open-circuit test, the transformer load terminal is kept open. Open circuit test is also known as the no-load test.
  • The current drawn by shunt parameters is a no-load current a very small current. Therefore the current that will flow in the circuit in the open circuit test is very low so the measurement of the quantities voltage, current, and power must be on the low voltage side so that the corresponding value will be readable in the instruments. And therefore, the open circuit test must be performed on the low voltage side. This means the high voltage side must be kept open and for the measurement of power, voltage and current on the low voltage side the wattmeter, voltmeter and ammeter must be connected
  • We know that as the output terminal is open the parameters that are in the shunt can be found out by this test. Since the shunt circuit has the core parameter so we can say that the open circuit test gives the core parameter.
  • The open-circuit test on the transformer is performed to determine magnetizing reactance and equivalent resistance due to iron loss.
  • As the normal rated voltage is applied to the primary, therefore, normal iron losses will occur in the transformer core. Hence, a wattmeter will record the iron losses and small copper loss in the primary. Since the no-load current is very small (2 to 5% of rated current), copper losses in the primary under no-load conditions are negligible as compared with iron losses. Hence, wattmeter reading practically gives the iron losses in the transformer.
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