Two test are performed on the transformer which are open circuit test and short circuit test. These tests are performed to determine the parameter or constants of transformer, efficiency and regulation.
1. Open Circuit Test
This is also called no load test. It determined the iron losses and the no load current. One winding of the transformer, usually the low voltage side is connected to its normal supply with an ammeter to measure the voltage applied to the winding and a wattmeter to measure taken by transformer at no load. the high voltage winding is kept open. Under these conditions normal flux will be set up in the core, therefore, normal iron losses will occur. The current taken will be wattmeter will indicate the iron losses.
2. Short Circuit Test
This test is used to determine the full load copper losses and the equivalent resitances and reactances referred to the metering side. In this test on the high voltage winding a reduced value of the voltage is increased until full load current is flowing in this winding. The applied voltage is a small fraction of the normal working voltage, the mutual flux produced is very small and hence the core losses at this voltage can be neglected. The wattmeter during this test gives the total coppery losses.