Sumpner or Back-to-Back Test
Sumpner or Back-to-Back Test
This test is conducted simultaneously on two identical transformers and provides data for finding the efficiency, regulation and temperature rise. The main advantage of this test is that the transformers are tested under full-load conditions without much expenditure of power. The power required to conduct this test is equal to the losses of the two transformers. It may be noted that two identical transformers are needed to carry out this test.
Fig. (7.33) shows the connections for back-to-back test on two identical transformers T1and T2. The primaries of the two transformers are connected in parallel across the rated voltage V1 while the two secondaries are connected in phase opposition. Therefore, there will be no circulating current in the loop formed by the secondaries because their induced e.m.f.s are equal and in opposition. There is an auxiliary low-voltage transformer which can be adjusted to give a variable voltage and hence current in the secondary loop circuit. A wattmeter W1, an ammeter A1 and voltmeter V1are connected to the input side. A wattmeter W2and ammeter A2are connected in the secondary circuit.
(i) The secondaries of the transformers are in phase opposition. With switch S1 closed and switch S2 open (i.e., regulating transformer not in the circuit), there will be no circulating current (I2= 0) in the secondary loop circuit. It is because the induced e.m.f.s in the secondaries are equal and in opposition. This situation is just like an open-circuit test. Therefore, the current drawn from the supply is 2 I0where I0is the no-load current of each transformer. The reading of wattmeter W1 will be equal to the core losses of the two transformers.
W1= Core losses of the two transformers.
(ii) Now switch S2is also closed and output voltage of the regulating transformer is adjusted till full-load current I2 flows in the secondary loop circuit. The full-load secondary current will cause full-load current I1 (= KI2) in the primary circuit. The primary current I1 circulates in the primary winding only and will not pass through W1. Note that full-load currents are flowing through the primary and secondary windings. Therefore, reading of wattmeter W2will be equal to the full-load copper losses of the two transformers.
W2= Full-load Cu losses of two transformers
W1 W2= Total losses of two transforms at full load
The following points may be noted:
(a) The wattmeter W1gives the core losses of the two transformers while wattmeter W2 gives the full-load copper losses (or at any other load current I2) of the two transformers. Therefor, power required to conduct this test is equal to the total losses of the two transformers. (b) Although transformers are not supplying any load, yet full iron loss and full-load copper losses are occurring in them. (c) There are two voltage sources (supply voltage and regulating transformer) and there is no Interference between them. The supply voltage gives only 2I0while regulating transformer supplies I2 and hence I1(= K I2).
(i) The power required to carry out the test is small.
(ii) The transformers are tested under full-load conditions.
(iii) The iron losses and full-load copper losses are measured simultaneously.
(iv) The secondary current I2 can be adjusted to any current value. Therefore,
we can find the copper loss at full-load or at any other load.
(v) The temperature rise of the transformers can be noted.