Back-to-Back Test or Sumpner's Test on Transformer

Why Sumpner's Test or Back to Back Test ?

     For a transformer, the temperature rise is an important factor to be noted before making it into operation. The temperature of a transformer changes in proportion with the load. Because when the load on the transformer increases there will be a change in copper loss and temperature of oil of the transformer.

     So it is significant to perform a temperature rise test for a transformer. Generally, short-circuit and open-circuit tests are performed to calculate the efficiency of a transformer.  But to determine the temperature rise the transformer should be operated under load.

     A small transformer can be put on full-load through an artificial load, say, a water load or lamp load, but for large units regenerative test is almost imperative, because it is difficult to arrange a suitable load for large units. This regenerative test is called " Sumpner's Test or Back-to-Back Test ". 

Determination of Efficiency and Temperature rise of Transformer by Sumpner's Test :

     Two similar transformers have their primaries ( usually LV ) connected to a supply of normal voltage and frequency as shown in the below figure. The supply voltage V1 must be equal to the rated voltage of the primary windings. The two secondaries are connected together, care being taken to have their " Polarities " in opposition. This is done as follows.

     Terminals A and C are connected together, and a voltmeter V2 of range, double the voltage of either transformer is connected across BD. If no ( voltmeter V2 ) reading is obtained, the voltages in the two secondaries are in opposition. If a reading is obtained, ( double that of the secondary voltage of one transformer ) it shows that the voltages are acting in the same direction so that it will be necessary to connect A to D, under this condition voltmeter V2 reads zero reading.

Sumpner Test of Transformer

If the two secondaries are connected in opposition no secondary current flows. Hence the transformer acts as if their secondaries are open-circuited and the wattmeter reads the iron losses of the transformer.

     Now to produce secondary current it is necessary to inject an e.m.f. into the secondary circuit through a regulating transformer. This injected voltage is adjusted until ammeter A2 reads full-load secondary current is flowing. This secondary current will induce full-load currents in the primaries, but the primary currents will circulate through the bus-bars.

     The currents induced in the two primaries are in the opposite direction. The direction of currents in the primaries of two transformers is shown by arrows in the above figure. Due to the opposition of these two currents, there will be no change in the wattmeter W1, also it carries a full-load current. Since the secondaries carry full-load current the wattmeter W2 reads the full-load copper loss of both the transformers.

     From the readings of two wattmeters iron and copper losses are noted, and knowing the input and output power efficiency can be calculated.

     The tabulation of readings of the test can be done in the following manner shown below.







Readings oberved from the test

Advantage of Sumpner's Test :

     The advantage of the test is that two large transformers can be put under full-load conditions for several hours. So that the temperature rise can be measured, with an expenditure of energy equal to that required by the losses only.

     But the disadvantage is that this test necessitates two similar transformers, and these are not always available.

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