Why does the open-circuit test gives core losses while short-circuit test gives copper losses?

Open circuit tests and short circuit tests are two fundamental methods used in transformer testing to separately determine core losses and copper losses.

Open Circuit Test (No-Load Test)

In an open circuit test, a rated voltage is typically applied to one winding while the other winding is left open. This setup is primarily used to measure core losses for the following reasons:

Core losses mainly consist of hysteresis losses and eddy current losses, which occur in the transformer's core. When an AC voltage is applied to the primary winding, it magnetizes the core, generating an alternating magnetic field. The hysteresis and eddy current losses produced during this process can be quantified by measuring the input power.

In the open circuit test, because the secondary winding is open, there is virtually no current flowing through the windings, so copper losses can be neglected. This means that the measured input power almost entirely reflects the core losses.

Short Circuit Test

In a short circuit test, a sufficiently low voltage is applied to one winding to avoid saturation, while the other winding is short-circuited. This test is primarily used to measure copper losses for the following reasons:

Copper losses are mainly due to I²R losses caused by the resistance of the windings. During the short circuit test, since the secondary winding is short-circuited, a significant current (close to the rated current) flows through the primary winding, resulting in substantial copper losses.

Because the applied voltage is low, the core does not reach saturation, so core losses are relatively small and can be neglected. Therefore, under these conditions, the measured input power primarily reflects the copper losses.

By using these two testing methods, core losses and copper losses can be effectively separated and evaluated independently. This is crucial for design optimization, fault diagnosis, and ensuring efficient transformer operation.