Two identical transformers are connected back to back ( two secondary windings are

connected in phase opposition) for determining all the information obtained from o.c.test

and s.c.test together along with temperature rise of transformer can also be determined

(Heat Run Test). Here, rated load conditions are being simulated without having

corresponding expenditure of energy.


Back-to-Back Test (Sumpner’s Test) on Transformer

The two identical transformers are used for the back to back test. Consider two transformerTr1 and Tr2

 ; those primary windings are connected in parallel to each other. The ratedvoltage and frequency is 

supplied to their primary winding (LV sides). The voltmeter andammeter are connected on their 

primary side for the measurement of the input voltage andcurrent (2I0).


The secondary winding of the transformer is connected in series opposition with each other.

A voltmeter V2 is connected across the secondary winding terminals for the measurement of

the voltage for ascertaining proper connection (keeping switch S open).


Secondary windings are connected in series opposition. For confirmation a voltmeter is

connected across their remaining terminals. If it is connected in series opposition, the

voltmeter will give zero reading (switch S open).



Determination of Temperature Rise

The temperature rise of the transformer is determined by measuring the temperature of the
oil after an regular interval of time.

Determination of Iron Loss

The wattmeter W1 measures the power loss which is equal to the iron loss of the
transformer. For determining the iron loss, the primary circuit of the transformer is kept
closed.
i.e. iron loss per transformer Pi = W1/2.


Determination of Copper Loss


The copper loss of the transformer is determined when the full load current flows through
their primary and secondary windings. The additional regulating transformer is used for
exciting the secondary windings. The full load current flows from the secondary to the
primary winding. The wattmeter W2 measures the full load copper loss of the two
transformers.
i.e. copper loss per transformer PCu = W2/2.


Advantages of Sumpner’s Test

1.Low power is required to conduct this test.
2.Full load copper losses and iron losses can be determined.
3.Information about Temperature rise on full load can be found ( Heat Run Test).