Open Circuit and Short Circuit Test on Transformer

Open circuit and short circuit test are performed to determine the core and copper losses of transformer. Open circuit test is performed to find the core losses while short circuit test determines the copper losses of transformer. These tests are quite easy to perform since they find the losses without loading the transformer.

OPEN CIRCUIT TEST OR NO LOAD TEST:

Open circuit test or no-load test is performed to determine the core losses of transformer. It is performed by keeping one of the winding open and apply normal rated voltage to the other side of the winding. Generally, high voltage winding (secondary) is kept open (no load connected) and low voltage winding (primary) is connected to normal supply. An ammeter (A), wattmeter (W) and voltmeter (V) are also connected on the LV (low voltage) side of the transformer as shown in the fig.

Now, we apply a rated voltage at the low voltage side of transformer while the other (high voltage side) is open circuited. Voltmeter connected at the low voltage side indicates us when rated voltage of low voltage side is achieved. Once the rated voltage of low voltage side is achieved, we will note down the readings of voltmeter, wattmeter and ammeter. Voltmeter indicates the rated voltage, ammeter shows us the value of no load current while wattmeter measures input power during the test. Since the other side of transformer is open circuited, there is no output power. Hence input power only consists of core and copper losses during no load condition.

Since copper losses are current dependent and no-load current is very small in case of open circuit. Hence very small amount of magnetizing current is required to set up the flux in the core. In that case, copper losses are negligible and can be neglected. Therefore, wattmeter reading completely gives us the core or iron losses of the transformer. In this way, we are able to determine the core losses of transformer by open circuit test.

Why open circuit test is performed at low voltage side?

We can conduct open circuit test at any side of the transformer. However, it is better to use low voltage side in the open circuit test due to the following reasons:

For low voltage side, required rated voltage will be less as compared to the high voltage side. Hence high rating measuring instruments are not required and low range instruments (voltmeter, wattmeter, ammeter) are sufficient to conduct the test.

Secondly, copper losses are due to the copper coils (Primary and secondary windings) of transformer. As we know that low voltage side has a less number of turns which means less amount of copper coils. If open circuit test is performed on the low voltage side, the amount of copper is less due to which copper loss is also less as compared to the high voltage side. In that case, we can neglect very small copper losses and gets pure iron losses by conducting test at the low voltage side. These are some of the specific reasons for performing open circuit test at the low voltage side.

SHORT CIRCUIT TEST:

In order to determine the copper losses of transformer, short circuit test is performed at the high voltage side of transformer. An ammeter, wattmeter and voltmeter is also connected on the HV (high voltage) side of the transformer as shown in the fig.

In this test, we generally short circuited the low voltage (secondary) side of transformer and conducted test at the high voltage side (primary). We gradually raised the applied voltage at the high voltage side till the time when the ammeter indicates the rated current of high voltage side. Once the rated current of high voltage side is achieved, we will note down the readings of voltmeter, wattmeter and ammeter. Ammeter shows us the rated current of high voltage side, voltmeter shows us fractional voltage and wattmeter gives us the input power of the transformer. Since we have short circuited the other side of transformer, there is no output power. Hence input power only consists of core and copper losses of the transformer.

As rated current of high voltage side is small and it is achieved at much lower voltage ( 5-8% of the rated voltage). In that case, flux linking within the core is also very small due to which iron losses are very less. In other words, we can say that core losses are voltage dependent and voltage  produce the rated current of high voltage side is very less. Hence core losses are negligible and can be neglected. Thus, wattmeter reading completely gives us the copper losses of transformer. In this way, short circuit test is conducted to determine the copper losses of transformer.

Why short circuit test is performed on the high voltage side?

Short circuit test is conducted on the high voltage side due to the following reasons:

In short circuit test, we are determining the copper losses and try to minimize or neglect the core losses. Since core losses are proportional to the square of applied voltage. When we perform test at the high voltage side, rated current of the high voltage side is small and it is achieved at much lower voltage. In that case, core losses are very small and can be neglected. This simply means that wattmeter reading only gives us the copper losses of transformer which is required.

If test is conducted on the low voltage side, wattmeter readings will also include the core losses which are not desirable. Hence it is recommended to perform short circuit test at the high voltage side as it makes the test easier, safer and accurate.

That’s all. Hope this will helps you.