- 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-circuit and short-circuit tests on a transformer give:
- Equivalent circuit of transformer
- Voltage regulation of transformer
- Efficiency of transformer
The power used for open circuit tests and short circuit tests on a transformer equals the loss in the transformer during the test.
Open Circuit Test on Transformer
The connection diagram for the open circuit test on transformer is in the figure. A voltmeter, wattmeter and an ammeter sit on the LV side as shown. Rated-frequency voltage is applied to that LV side through a variable-ratio auto transformer (variac).
The HV side is left open. Raise the variac slowly until the voltmeter shows rated LV voltage. Then record the voltmeter, ammeter and wattmeter readings.
The ammeter reading is the no-load current Ie. That Ie is small next to rated current of the transformer, so voltage drops from this current can be ignored.
Voltmeter reading V1 is taken as the secondary induced voltage. The wattmeter shows input power. With no output, that power is core losses in transformer plus copper loss. No-load current is small, so copper loss is ignored and the wattmeter reading is taken as core loss.
Let the wattmeter reading be Po.
Where Rm is shunt-branch resistance of the transformer.
If Zm is shunt-branch impedance of transformer.
If shunt-branch reactance of the transformer is Xm,
These values are referred to the LV side because the test is run on the LV side. Refer them to the HV side by multiplying by the square of the transformation ratio.
So the open circuit test on transformer gives core losses and the shunt-branch parameters of the equivalent circuit.
Short Circuit Test on Transformer
The connection diagram for the short-circuit test on the transformer is in the figure below. A voltmeter, wattmeter and an ammeter sit on the HV side as shown. About 5-10% of rated voltage is applied to that HV side through a variac (a variable-ratio auto transformer). The LV side is short-circuited. Raise the variac slowly until the wattmeter and an ammeter show HV-side rated current.
Once HV-side rated current is reached, record all three instrument readings (Voltmeter, Ammeter and Watt-meter readings). The ammeter reading is the primary equivalent of full-load current IL. The voltage needed for that current in a short-circuit test is small next to rated primary voltage, so core losses in the transformer can be ignored here.
Let the voltmeter reading be Vsc. The wattmeter shows input power during the test. The secondary is shorted so there is no output, and that input is copper loss. Applied voltage Vsc is the short-circuit voltage, small next to rated voltage, so core loss from that small voltage is ignored. The wattmeter reading is therefore taken as copper loss. Let that reading be Psc.
Where Re is equivalent resistance of the transformer.
If Ze is equivalent impedance of the transformer.
If equivalent reactance of the transformer is Xe.
These values are referred to the HV side because the test is run on the HV side. Convert them to the LV side by dividing by the square of the transformation ratio.
So the short-circuit test of a transformer gives copper losses at full load. It also supplies the parameters for the approximate equivalent circuit of a transformer.
