# The efficiency of two identical transformers under load conditions can be determined by

The efficiency of two identical transformers under load conditions can be determined by

### Right Answer is: Back to Back test (Sumpner Test)

#### SOLUTION

To determine the maximum temperature rise of a transformer Sumpner’s test is performed. This test can also be performed to find out the efficiency of a transformer. With the help of Sumpner’s test both the open-circuit and short-circuit tests can be performed simultaneously.

• Sumpner’s test is essentially a load test. It requires two identical transformers whose primaries are connected in parallel.
• The two secondaries are connected in series with their polarities in phase opposition. The primary windings are supplied at rated voltage and frequency. A voltmeter, ammeter, and wattmeter are connected to the input.
• As the two secondaries are connected in phase opposition, the two secondary EMFs oppose each other and no current can flow in the secondary circuit.
• A regulating transformer excited by an ac mains supply is used to inject voltage into the secondary winding. The injected voltage is adjusted till the ammeter A, reads full load secondary current. The secondary current causes full load current to flow through the primary windings.
• The wattmeter W1, indicates total core losses, W2 indicates total copper losses, and ammeter A1 indicates the total no-load current of the two transformers.
• Thus by this method, we can load the transformer to full load but the supplying energy is only equal to that required for the losses only. This test can be continued for a long time to determine the maximum temperature rise of a transformer.
• The transformers are kept in this condition for 48 hours and the temperature is noted on an hourly basis and a curve is a plot that should become constant after some time also it should be within the limit.