Open Circuit and Short Circuit Test on Transformer
We conduct open circuit and short circuit test on single phase transformer to determine the efficiency and regulation of a transformer on any load condition and at any power factor.Open circuit & short circuit test are also called as OC test & SC test on the transformer.This method of finding the parameters of a transformer is called as an indirect loading method.
Open Circuit(OC) and Short Circuit(SC) Test on Transformer
What is the need for OC & SC test on transformers?
Open circuit test and short circuit test on the transformer are very economical and convenient because they are performed without actually loading of the transformer.because they furnish the required information without actually loading the transformer. In fact, the testing of very large a.c. the machinery consists of running two tests similar to the open and short-circuit tests of a transformer.
Open-circuit or No-load Test On Single Phase Transformers
The circuit diagram for open circuit test is shown in the figure. A voltmeter(V), wattmeter(W), and an ammeter(A) are connected in LV side of the transformer.Usually, high voltage (HV) winding is kept open and the low voltage (LV) winding is connected to its normal supply.Because we are going to find the max output voltage of transformer which is present at HV side.This is the reason why HV winding is open circuited.
Procedure of Open Circuit(OC) Test
Switch on single phase AC supply, with the help of variac, increase the voltage until the voltmeter raeds the rated voltage of the LV side.
At the instant of rated LV voltage, Note down the readings of all three instruments.(Voltmeter, Ammeter and Wattmeter readings) are recorded.
The ammeter reading gives the no-load current I0.As no load current (I0) very small, the voltage drop due to this current also neglected.The input power supplied to the transformer is indicated by the watt meter (W).Output power of transformer in open circuit test is zero because another side of transformer is open circuited.That means input supply is to just compensate the core losses and copper losses.By neglecting some voltage drop due to small no-load current, Watt meter (W) reading gives the core losses of the transformer.
Here,Wo = Pi = Iron losses
Calculations : We know that,
Wo = VoIo cos Φ(watt meter reading)
cos Φo = Wo /(VoIo) = no load power factor
Once cos Φo is known we can obtain,
Ic = Io cos Φo
and Im = Io sin Φo
Once Ic and Im are known we can determine exciting circuit parameters as,
Ro = Vo /Ic Ω
Xo = Vo /Im Ω
where, X0, R0 are equivalent exciting reactance, resistance of transformer.
Key Point: We should use LPF (low power factor)watt meter to get error-free results.Because cos Φo is very low in the above case.The above values are calculated by taking LV side of transformer as reference.If the meters are connected to secondary and primary is kept open then from O.C. test we get Ro’ and Xo’ with which we can obtain Ro and Xo knowing the transformation ratio K.
The equivalent circuit derived by the OC test is shown below.
Hence, we can conclude that open circuit test on transformer gives core losses of transformer and shunt parameters of the equivalent circuit.
Short Circuit Or Impedance Test On Transformer
The connection diagram for short circuit test on transformer is shown in the figure. A voltmeter(V), wattmeter(W), and an ammeter(A) are connected in HV side of the transformer as shown.The secondary winding is short-circuited with the help of thick copper wire or solid link. As high voltage side is always low current side, it is convenient to connect high voltage side to supply and shorting the low voltage side.
Procedure of Short Circuit (OC) Test
Apply voltage to HV side.
Increase the voltage from the zero until the ammeter reading equals the rated current.
At the instant of rated HV current, Note down the readings of all three instruments.(Voltmeter, Ammeter and wattmeter readings) are recorded.