**Subject :**Electrical Machines I (DC Machines)

**Unit :**Transformer

## 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.

**Circuit**

Fig. (7.33) shows the connections for back-to-back test on two identical transformers T_{1}and T_{2}. The primaries of the two transformers are connected in parallel across the rated voltage V_{1 }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 W_{1}, an ammeter A1 and voltmeter V_{1}are connected to the input side. A wattmeter W_{2}and ammeter A_{2}are connected in the secondary circuit.

**Operation**

(i) The secondaries of the transformers are in phase opposition. With switch S_{1 }closed and switch S_{2 }open (i.e., regulating transformer not in the circuit), there will be no circulating current (I_{2}= 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 I_{0}where I_{0}is the no-load current of each transformer. The reading of wattmeter W1 will be equal to the core losses of the two transformers.

W_{1}= Core losses of the two transformers.

(ii) Now switch S_{2}is 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 I_{1 }(= KI_{2}) in the primary circuit. The primary current I_{1 }circulates in the primary winding only and will not pass through W_{1}. Note that full-load currents are flowing through the primary and secondary windings. Therefore, reading of wattmeter W_{2}will be equal to the full-load copper losses of the two transformers.

W_{2}= Full-load Cu losses of two transformers

W_{1 } W_{2}= Total losses of two transforms at full load

The following points may be noted:

(a) The wattmeter W_{1}gives the core losses of the two transformers while wattmeter W_{2 }gives the full-load copper losses (or at any other load current I_{2}) 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 2I_{0}while regulating transformer supplies I_{2 }and hence I_{1}(= K I_{2}).

**Advantages**

(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.