June 3, 2017

# Open circuit test of transformer

Open circuit test of transformer … this test very important, it considered backbone of transformer testing .. through this test we will calculate no load losses of transformer at no load condition .. open circuit test of transformer classified under type test and routine test .. you can find full types of transformer tests here >>> transformer testing

Purpose of this test:

1. Determine the no load iron losses (P0)
2. Determine the no load current (I0) of the transformer at rated voltage and frequency.
3. Verify guaranteed values to prove the quality of the core steel and the workmanship of core staking.
4. Determine core resistance and reactance.

Precautions before do this test:

1. Disconnect the electrical current from the transformer.
2. Clean the terminals of the transformer.

## Steps of open circuit test of transformer :

• Connect secondary winding with AC source 380 Volt as shown in fig.
• Keep primary winding without any load (open circuit).
• Measure Voltage, Current and power at secondary side with voltmeter, ammeter and watt-meter as shown in fig.
• Current Drawn at this case (I0) should not exceed 1% of rated current of transformer as transformer opened at primary.
• So, we can neglect power loss at primary side.
• We can also neglect voltage drop that’s mean the measured power at this case represent iron loss (P0) only.
• For more accuracy we can repeat this test 5 times at 90% , 95% , 100%, 105% , 110% of rated voltage and at every time we can measure Voltage, Current and power at secondary side.
• If the measured values differ significantly from the nameplate values, it means that we have an iron core problem and there may be short in lamination or there are gaps between the iron core layers or protrusions in the iron core sections.
• This test is also useful to see if there is an unsymmetrical structure of the iron core or not.
• If it is observed that the current drawn in one of the phases is very different from the other two phases, there were an unsymmetrical structure of the iron core.
• After obtaining values of voltage, current and power at open circuit test of transformer we can now calculate some parameters of transformer as iron core resistance and reactance as follows :