Back-to-Back Test on Transformer

Sumpner’s test provides data for finding the regulation, efficiency and heating under load conditions and is employed only when two similar transformers are available.The O.C. and S.C. tests give us the equivalent circuit parameters but can not give heating information under various load conditions.The Sumpner’s test requires two identical transformers.One transformer is loaded on the other and both are connected to supply. The power taken from the supply is that necessary for supplying the losses of both transformers and the negligibly small loss in the control circuit. Sumpner’s test is also called as back to back test on transformers because of connections are in parallel at input side and connections are in series at output side.

Procedure Sumpner’s Test on Transformers :

As shown in Fig, primaries of the two transformers are connected in parallel across the same a.c. supply. With switch S open, the wattmeter W1 reads the core loss for the two transformers.

Finding Losses With Sumpner’s Test On Transformer

The secondaries are so connected that their potentials are in opposition to each other. This would so if VAB = VCD and A is joined to C whilst B is joined to D. In that case, there would be no secondary current flowing around the loop formed by the two secondaries. T is an auxiliary low-voltage transformer[Regulation Transformer].which can be adjusted to give a variable voltage and hence current in the secondary loop circuit. By proper adjustment of T, full-load secondary current I2 can be made to flow as shown. It is seen, that I2 flows from D to C and then from A to B. Flow of I1 is confined to the loop FEJLGHMF and it does not pass through W1. Hence, W1continues to read the core loss and W2 measures full-load Cu loss (or at any other load current value I2). Obviously, the power taken in is twice the losses of a single transformer.

i.e. copper loss per transformer PCu = W2/2.
i.e. iron loss per transformer Pi = W1/2.

From results of sumpner’s test, the full load efficiency of each transformer can be given as

Advantages of Sumpner’s Test