No load Test on Transformers; Open Circuit test on Transformers

Open Circuit Or No Load Test on Transformer

Open circuit test is also known as no load test. This test is performed on Transformer to calculate core losses at no load.

As indicated by name Open circuit means transformer one winding is kept. So usually High voltage side is kept open as at load voltage and current readings are very low which could not be feasible to measure on high voltage side. So in this test High voltage winding is kept open and Voltmeter, Ammeter, Wattmeter are connected on Low voltage side and source of supply is also connected on Low voltage side for measurement of parameter.

For short circuit test on Transformers visit:-
http://electrialstandards.blogspot.in/2015/09/short-circuit-test-on-transformerswhy.html

Connection diagram for the same is as shown below:-

In open circuit test as discussed earlier that this test is performed on low voltage side so all measuring instruments are connected on low voltage side i.e. A voltmeter, wattmeter, and an ammeter are connected in LV side of the transformer as shown.

Low voltage is applied to that LV side through a variac. The High voltage side of the transformer is kept open.  This applied voltage is increased till the rated voltage of low voltage winding. Readings of the measuring instruments are recorded.

Ammeter gives the no load current which is usually very small in comparison to rated current of transformer so losses due to that current will be very small and can be ignored i.e. copper losses can be ignored. Now as voltmeter gives the rated voltage and wattmeter connected in low voltage winding gives Input power without load at secondary side. As secondary winding is open circuited so wattmeter gives core losses of the transformer.

So Open circuit test gives core losses of the transformer. Let’s consider that power indicated by wattmeter was P(o) and voltmeter reading as V(o).

Then P(o)= V(o)²

                    Rm

Rm is the shunt resistance of the transformer.

Thus we have seen that Open circuit test will give Core losses of the transformer.