Open and Short Circuit test of Transformer

Open and short circuit test are carried out to determine the equivalent circuit parameters of a single phase or 3 phase Transformer. Open circuit or the OC test of a transformer is used to measure “CORE losses“. Whereas 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 actual operation. This is Open circuit or OC test.

This test take the instruments placing on the low voltage(LV) side while the high voltage (HV) side is open. This is the only practice in practical because arranging rated voltage supply at low voltage level is easier. Also, it is easier to work on low voltage side.

Ignore primary copper loss because 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 vary low, therefore it is best to use 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 exact equivalent circuit we remove the primary impedance.

These are the parameters determine by this test.

Poc = no load input power

Voc = rated voltage

Io = no load current

Accordingle here are the minimum formulas to determine any of the parameter, which are from 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

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

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

The instruments places on high voltage side because this rated current on high voltage side is lower than low voltage side. Hence, we should connect low current rating ammeters, wattmeters and instruments 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}

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