Open circuit and short circuit tests on single phase transformer:-
Why we need to conduct OC & SC test on transformers?-To obtain the equivalent circuit parameters from Open circuit and short circuit tests , and to estimate efficiency & regulation at various loads.
-To understand the basic working principle of a transformer.
This method also called as indirect loading method of transformers.
Open Circuit Test (O.C. Test):-
This test mainly to find out 'no load loss (core loss)' and 'no load current I0'.it is also called no load test because we kept high voltage (HV) winding is kept open and the low voltage (LV) winding is connected to its power supply.to measure voltage,current,power wattmeter (W), ammeter (A) and voltmeter (V) are connected to the LV winding as shown in the above figure.
1.Apply voltage to the LV side in steps up to the rated voltage and record primary current and power drawn from the source when voltage reached to rated value of the LV winding.
2.The ammeter reading gives the no load current I0. As I0 itself is very small, the voltage drops due to this current can be neglected.
3.The total power sending to the transformer is calculated by wattmeter (W),this reading is also sum of core losses and copper losses,beacuse no load is appiled on transformer so output is zero.
4.Hence, this input power only consists of core losses and copper losses.
The LV side of transformer is short circuited and wattmeter (W), voltmere (V) and ammeter (A) are connected on the HV side of the transformer.we short circuit only on LV side because to get high current,if we short circuit on HV side we get low current.
1. As secondary is shorted, its resistance is very very small and on rated voltage it may draw very large current. Such large current can cause overheating and burning of the transformer. To limit this short circuit current, primary is supplied with low voltage which is just enough to cause rated current to flow through primary which can be observed on an ammeter. The low voltage can be adjusted with the help of variac. Hence this test is also called low voltage test or reduced voltage test.
2.Voltage is applied to the HV side and increased from the zero until the ammeter reading equals the rated current.
1.Apply voltage to the LV side in steps up to the rated voltage and record primary current and power drawn from the source when voltage reached to rated value of the LV winding.
2.The ammeter reading gives the no load current I0. As I0 itself is very small, the voltage drops due to this current can be neglected.
3.The total power sending to the transformer is calculated by wattmeter (W),this reading is also sum of core losses and copper losses,beacuse no load is appiled on transformer so output is zero.
4.Hence, this input power only consists of core losses and copper losses.
Sometimes, a high resistance voltmeter is connected across the HV winding. Though, a voltmeter is connected, HV winding can be treated as open circuit as the current through the voltmeter is negligibly small. This helps in to find voltage transformation ratio (K).
The two components of no load current can be given as,
Iμ = I0sinΦ0 and Iw = I0cosΦ0.
cosΦ0 (no load power factor) = W / (V1I0). ... (W = wattmeter reading)
From this, shunt parameters of equivalent circuit parameters of equivalent circuit of transformer (X0 and R0) can be calculated as
X0 = V1/Iμ and R0 = V1/Iw.
Short Circuit(SC)Or Impedance Test :-
1. As secondary is shorted, its resistance is very very small and on rated voltage it may draw very large current. Such large current can cause overheating and burning of the transformer. To limit this short circuit current, primary is supplied with low voltage which is just enough to cause rated current to flow through primary which can be observed on an ammeter. The low voltage can be adjusted with the help of variac. Hence this test is also called low voltage test or reduced voltage test.
2.Voltage is applied to the HV side and increased from the zero until the ammeter reading equals the rated current.
Thus, the wattmeter reading can be taken as copper loss in the transformer.
Therefore, W = Isc2Req....... (where Req is the equivalent resistance of transformer)
Zeq = Vsc/Isc.
Therefore, equivalent reactance of transformer can be calculated from the formula Zeq2 = Req2 + Xeq2.
These, values are referred to the HV side of the transformer.
Hence, it is seen that the short circuit test gives copper losses of transformer and approximate equivalent resistance and reactance of the transformer.
