Open and short circuit tests are carried out to determine the equivalent circuit parameters of a single phase or 3 phase Transformer. An open circuit or the OC test of a transformer is used to measure “CORE losses“. Whereas the Short circuit or SC test of a transformer is used to measure the “Full load copper loss“.

Open circuit or OC test of a transformer

OC test measures core loss at RATED voltage and frequency. Which then is treated as constant despite minor variations in the voltage and frequency doing the actual operation. This is an Open circuit or OC test.

In OC (Open Circuit) test instruments are placed on the low voltage(LV) side while the high voltage (HV) side is open. This is the only practical because arranging rated voltage supply at low voltage levels is easier. Also, it is easier to work on the low-voltage side.

Ignore primary copper loss because the no-load current is very low also the primary impedance voltage drop is very low at this low value of no-load current.

The no-load power factor is very low, therefore it is best to use a low-power factor wattmeter.

Since the no-load current is less than 5% of the rated current, neglect the primary copper loss and primary impedance drop, without much loss of accuracy. Therefore in an exact equivalent circuit, we remove the primary impedance.

These are the parameters determined by this test.

Poc = no load input power

Voc = rated voltage

Io = no-load current

Accordingly here are the minimum formulas to determine any of the parameters, which are from the phasor diagram of the OC or test.

Trick

R_c = \frac{V_{oc}^2}{P_{oc}}

\phi_o = cos^{-1}(\frac{P_{oc}}{V_{oc}I_{o}})

X_\phi = \frac{R_{c}}{tan\phi}

Short circuit or SC test of transformer

A short circuit or SC test for a transformer determines the full load copper loss. Accordingly, this test is set at rated current and frequency although the requirement of rated frequency is not necessary. This is a Short circuit or SC test.

Instruments connect to the high voltage side with low voltage short-circuited by a very thick wire of less resistance. To circulate full load current at short circuit, an input voltage of 8-10% is usually sufficient and therefore ignores the core loss during this test. Moreover, the no-load current at such low voltage also becomes negligible and therefore should be ignored. This means that the exciting part of the equivalent circuit may be ignored during a short circuit test.

In the SC (Short circuit) test, instruments are placed on the high voltage side because this rated current on the high voltage side is lower than the low voltage side. Hence, we should connect low-current rating ammeters, wattmeters, and instrument transformers. This reduces the cost of the experiment.

Earlier Parameters of the short circuit test are –

Psc = full load input power

Vsc = short circuit voltage

Isc = full load current

To find :

Req = equivalent resistance of windings

Xeq = equivalent reactance of the windings

Trick:

Z_{eq} = \frac{V_{sc}}{I_{sc}}

angle~of~Z_{eq} = cos^{-1}(\frac{P_{sc}}{V_{sc}I_{sc}})

Z_{eq} = R_{eq}+jX_{eq}