Transformer Back-to-Back or Sumpner Test

Sumner's Test or Back-to-Back Test of a Transformer

A full-load test of the large transformer is needed to determine the maximum temperature rise of a transformer. This kind of Transformer test is called a back-to-back test. The back-to-back test is also known as Sumpner's test or regenerative test of transformer.


Sumner's Test Circuit Diagram of Transformer

The circuit diagram or the connection diagram to carry out the transformer Sumpner's test also known as the transformer back-to-back test is shown below. Find herewith the following connection diagram where two transformers are connected in a parallel way to supply the AC voltage V1 and the low voltage supply V2
Watt meter  W1 and W2, Volt meter V and Amp meter A are connected to monitor the result as below:

Transformer Back-to-back or Sumpner Test


How to perform the Back-to-back or Sumpner Test of a Transformer?

Transformer Back-to-back or Sumpner Test provides data for finding the transformer regulation, transformer efficiency, and transformer heating under load conditions.

 The condition of Sumpner or Back to Back test of a transformer is only employed when two similar transformers are available and connected in parallel.

One transformer is loaded on the other and both are connected to the supply. The power taken from the supply is necessary for supplying the losses of both transformers and the negligibly small loss in the control circuit. 

As shown in the figure, the 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.

T1 and T2 are identical transformers and secondaries of them are connected in voltage opposition.

EMF opposite each other,  EEF and EGH 

 So, both the emf's cancel each other, as transformers are identical.

The current drawn from V1 is 2I0

where I0 is equal to the no-load current of each transformer. 

Thus input power measured by wattmeter W1 is equal to iron losses of both transformers.
i.e. iron loss per transformer Pi = W1/2.

 copper loss per transformer PCu = W2/2.


Sumpner Test Calculation procedure

  • Ammeter A1 = No-load current = 2I0
  • Voltmeter V1 = Applied rated input voltage (Primary voltage)
  • Wattmeter W1 = Core losses (iron losses) of both transformers = 2Pi
  • Ammeter A2 = Full load secondary current of both transformers = 2I2
  • Voltmeter V2 = Total voltage across series connection of both secondary windings
  • Wattmeter W2 = Full load copper loss of both transformers = 2Pcu

Calculation of Sumpner or Back-to-back-Test

Why is Sumpner's test used?

Sumpner's test is used for large transformers, A full-load test on the large transformer is to be conducted to determine the maximum temperature rise of the transformer which is called a back-to-back test.


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