The correct answer is C. core loss.

An open-circuit test on a transformer, also known as a no-load test, is conducted to measure the core loss of the transformer. During this test, the primary winding is connected to the power supply and the secondary winding is open. By measuring the no-load current and voltage, the no-load or core loss can be calculated.

Core loss is caused by the excitation of the core material’s magnetic domains, which results in energy dissipation in the form of heat. Measuring the core loss helps in determining the efficiency of the transformer and also to check the health of the transformer.

[A] insulation resistance – Insulation resistance is the resistance offered by the insulation between the various windings and the core of the transformer. An insulation resistance test is performed on a transformer by applying a high voltage between the primary and secondary windings, and measuring the current flowing through the transformer.

The ratio of voltage to current is used to calculate the insulation resistance. This test is used to detect any insulation defects in the transformer, which can cause the transformer to fail if not addressed. However, the open-circuit test is not primarily used to measure insulation resistance.

[B] copper loss – Copper loss is the energy dissipation in the form of heat caused by the resistance of the transformer’s windings. Copper loss is a function of the current flowing through the transformer windings and is directly proportional to the square of the current.

The copper loss can be measured by performing a load test on the transformer, where the transformer is connected to a known load and the input and output power is measured. However, the open-circuit test is not primarily used to measure copper loss.

[C] core loss – Core loss is caused by the excitation of the core material’s magnetic domains, which results in energy dissipation in the form of heat.

Measuring the core loss helps in determining the efficiency of the transformer and also to check the health of the transformer. The open-circuit test is primarily used to measure core loss.

[D] total loss – Total loss is the sum of core loss and copper loss. It is the total energy dissipation in the form of heat caused by the transformer. However, the open-circuit test is not primarily used to measure total loss.

The correct answer is C. core loss.