- Open Circuit Test Definition: The open circuit test of a transformer checks the core losses and parameters of the shunt branch by connecting instruments to the LV side and keeping the HV side open.
- Short Circuit Test Definition: The short circuit test of a transformer determines copper losses and equivalent circuit parameters by applying a low voltage to the HV side and short-circuiting the LV side.
- Core Losses Measurement: During the open circuit test, the wattmeter reading is used to measure the core losses, as there is no output power.
- Copper Losses Measurement: In the short circuit test, the wattmeter reading measures copper losses since the applied voltage is low and core losses are negligible.
- Equivalent Circuit Parameters: Both tests are essential for calculating the equivalent circuit parameters, which are crucial for understanding transformer performance.
Open and short circuit tests are performed on a transformer to determine the:
- Equivalent circuit of transformer
- Voltage regulation of transformer
- Efficiency of transformer
The power required for open circuit tests and short circuit tests on a transformer is equal to the power loss occurring in the transformer.
Open Circuit Test on Transformer
The connection diagram for open circuit test on transformer is shown in the figure. A voltmeter, wattmeter, and an ammeter are connected in LV side of the transformer as shown. The voltage at rated frequency is applied to that LV side with the help of a variac of variable ratio auto transformer.
The HV side is kept open. Using the variac, increase the voltage slowly until the voltmeter reads the rated LV voltage. Then, record the readings of the voltmeter, ammeter, and wattmeter.
The ammeter reading gives the no load current Ie. As no load current Ie is quite small compared to rated current of the transformer, the voltage drops due to this current that can be taken as negligible.
The voltmeter reading V1 is taken as the secondary induced voltage. The wattmeter shows the input power, which, since there is no output, consists of core losses in transformer and copper losses. However, because the no-load current is small, we can ignore the copper loss, making the wattmeter reading equal to the core losses.
Let us consider wattmeter reading is Po.
Where, Rm is shunt branch resistance of transformer.
If, Zm is shunt branch impedance of transformer.
Therefore, if shunt branch reactance of transformer is Xm,
These values are referred to the LV side of the transformer due to the tests being conducted on the LV side of transformer. These values could easily be referred to HV side by multiplying these values with square of transformation ratio.
Therefore it is seen that the open circuit test on transformer is used to determine core losses in transformer and parameters of the shunt branch of the equivalent circuit of the transformer.
Short Circuit Test on Transformer
The connection diagram for the short circuit test on the transformer is shown in the figure below. A voltmeter, wattmeter, and an ammeter are connected in HV side of the transformer as shown. A low voltage of around 5-10% is applied to that HV side with the help of a variac (i.e. a variable ratio auto transformer). We short-circuit the LV side of the transformer. Now with the help of variac applied voltage is slowly increased until the wattmeter, and an ammeter gives reading equal to the rated current of the HV side.
After reaching the rated current of the HV side, we record all the three instrument readings (Voltmeter, Ammeter and Watt-meter readings). The ammeter reading gives the primary equivalent of full load current IL. As the voltage applied for full load current in a short circuit test on the transformer is quite small compared to the rated primary voltage of the transformer, the core losses in the transformer can be taken as negligible here.
Let’s say, voltmeter reading is Vsc. The watt-meter reading indicates the input power during the test. As we have short-circuited the transformer, there is no output; hence the input power here consists of copper losses in the transformer. Since the applied voltage Vsc is short circuit voltage in the transformer and hence it is quite small compared to the rated voltage, so, we can neglect the core loss due to the small applied voltage. Hence the wattmeter reading can be taken as equal to copper losses in the transformer. Let us consider wattmeter reading is Psc.
Where, Re is equivalent resistance of transformer.
If, Ze is equivalent impedance of transformer.
Therefore, if equivalent reactance of transformer is Xe.
These values are referred to the HV side of the transformer as the test is conducted on the HV side of the transformer. These values could easily be converted to the LV side by dividing these values with the square of transformation ratio.
Hence the short-circuit test of a transformer is used to determine copper losses in the transformer at full load. It is also used to obtain the parameters to approximate the equivalent circuit of a transformer.
