Short Circuit Test

 

Short Circuit Test or Full load Cu loss of Transformer

The short circuit test of the transformer is the one of type test by which we can measure the full load cu- loss of the transformer. For testing the short circuit test all meters ( i.e ammeter, wattmeter and volt meter ) are connected to the high voltage winding and the low voltage winding is short circuited which is shown in the figure. Now the low voltage is applied at high voltage side and the applied voltage is slowly increased with the help of variac until the ammeter gives reading equal to the rated current of the HV side. When the current of HV side reach at rated current of the transformer, at that time the reading of wattmeter represents the total full-load Cu-losses of the transformer i.e both primary Cu loss and secondary Cu loss.

Now, we describe why the wattmeter represents the total full-load Cu losses of the transformer only, not represent the core losses. As the applied voltage is quite small ( usually 5 to 10 % of normal voltage ) compared to the rated primary voltage of the transformer, and the core losses of transformer is directly proportional to applied voltage, so the core losses are very small that can be taken negligible here. Let the reading of ammeter is I₁ which is rated current of the HV winding. At that time, the volt meter reading is V₁ which is quite small ( usually 5 to 10 % of normal voltage ) compared to the rated primary voltage of the transformer. And let wattmeter reading is W which is the full-load copper loss of transformer. The equivalent circuit diagram of transformer under short circuit condition is sown in Fig.-1 & 2.

W = Full-load copper loss, V₁ = Applied voltage, I₁ = Rated current, R₀₁ = Resistance as viewed from the primary, Z₀₁ = Total impedance as viewed from the primary, X₀₁ = Reactance as viewed from the primary

W=I₁²R₀₁

∴ R₀₁=W/I₁²

Z₀₁=V₁/I₁

X₀₁=√(Z₀₁²- R₀₁²)

Describe the equivalent circuit vector diagram for short circuit test & impedance triangle:-

In fig. 3 is shown the equivalent circuit vector diagram for short circuit test. From this vector diagram it has been seen that the voltage Vsc is consumed in impedance drop of the two windings. ( i.e Vsc = I₁ Z₀₁ ).And the fig. 4 is shown the impedance triangle from which we can find out the value of R₂^’ = R₀₁ – R₁.