Open and Short Circuit test of transformer – EEE | electrical4u

Question :

  • Discuss Open Circuit (O.C.) and Short Circuit (S.C.) test on a single phase transformer.
  • In transformer open circuit test gives iron losses and short circuit test gives copper losses’. Justify.

Open Circuit and Short Circuit Test

Experimental circuit diagram of a open circuit and short circuit test to be carried out on a transformer.

These tests are helpful to determine parameters such as efficiency and regulation Of transformer at full load or at any other load. These tests are simple, low power required, accurate and no need Of actual loading.

Open Circuit Test

In this Open Circuit Test, secondary is kept Open and volt meter, ammeter and wattmeter are connected in primary side shown in fig. Ammeter reads no load primary current I0. Rated voltage is applied to primary winding. Small copper losses of primary winding is neglected. Thus, wattmeter reads constant loss or Iron loss.

Let W0 = (wattmeter reading) = Iron  loss.

I0 = no load current = Ammeter Reading

V0 = Rated supply voltage = Voltmeter reading.

∴ W0 = V0 I0cos ϕ0 watt

Thus no load power factor = (W0/V0I0) = cos ϕ0

Magnetizing current Iu = I0 sinϕ0 amp

Active current component = Iw= I0 cos ϕ0 Amp

Magnetising reactance = X0 =(V/Iu) Ω

Resistance equivalent of core loss = R0 = (V/Iw) Ω

Hence parameter X0 and R0 can be determined by open circuit test .

Short Circuit Test

In the short circuit test one of the windings is short circuited by thick wire. Wattmeter, ammeter and voltmeter are connected as shown in figure. Reduced voltage is applied through auto figure. Reduced voltage is applied through auto transformer , such that ammeter reads rated value. As very less voltage is applied , iron losses are very small and hence is applied , iron losses are very small and hence neglected. Thus wattmeter gives full load copper loss.

Wsc = Wattmeter Reading = (I2sc)R01

Isc = Ammeter Reading = Read Short circuit current

Vsc = Voltmeter reading = short circuit voltage applied

Equivalent resistance from primary side

R01 = Wsc / (I2sc) Ω

Z01 = Vsc / Isc Ω

Z01 = (Z201 – R201 ) Ω

Knowing equivalent resistance and reactance referred to particular winding, regulation and efficiency at any power factor can be determined.

Question : While performing open circuit test , why the high voltage winding is generally kept open circuited ?

During open circuit test, we apply rated voltage to primary side. Power taken by the transformer is very less and load current I0 is less. If we connect meters on high voltage. Side then for the same input power and the rated voltage of high voltage side, the input current will be very small and may not be measurable. Also very high range voltmeters are not available to read input voltage. Hence high voltage winding is Kept open circuited.