Transformer Open Circuit and Short Circuit Test

Open Circuit Or No Load Test and Short Circuit Or Impedance Test On Transformer can carry out economically without loading a Transformer.

Open Circuit Test And Short Circuit Test on Transformer are performed to find the parameters of the equivalent circuit of transformer and losses of the transformer. These two tests are conducted to determine the core loss, copper loss, and equivalent circuit parameters of a transformer.

The performance of a transformer can be calculated on the basis of a Transformer  Equivalent Circuit which contains four main parameters:

  1. the equivalent resistance R01 as referred to primary (or secondary R02); 
  2. the equivalent leakage reactance X01 as referred to primary (or secondary X02);
  3. the core-loss conductance G0 (or resistance R0); 
  4. the magnetising susceptance B0 (or reactance X0). 

These constants or parameters can be easily determined by two tests:
  1. open-circuit test and 
  2. short-circuit test. 

These tests are very economical and convenient 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-circuit and short-circuit tests of a transformer.

Transformer Open Circuit and Short Circuit Test

Open Circuit Or No Load Test On Transformer

Open circuit test is performed in a transformer to find the resistance of core and shell France we apply rated voltage in open circuit test because to find maximum admittance or resistance of core and excitation branch.

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 an equivalent circuit of the transformer (X0 and R0) can be calculated as


X0 = V1/Iμ  and  R0 = V1/Iw.


(These values are referring to the LV side of the transformer.)
Hence, it is seen that the open-circuit test gives core losses of transformer and shunt parameters of the equivalent circuit.


Short Circuit Or Impedance Test On Transformer

we apply a short circuit test to find the series impedance of a transformer by adjusting primary voltage until in secondary winding current equal to rated current.

Sense input voltage is low current flowing through excitation branch is negligible and all voltage will drop in a transformer due to series element in a circuit.

The ammeter reading gives the primary equivalent of full load current (Isc).

The voltage applied for full load current is very small as compared to rated voltage. Hence, core loss due to small applied voltage can be neglected. 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, the equivalent reactance of the 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 the transformer and approximate equivalent resistance and reactance of the transformer.

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