It is essential to conduct a sumpner’s test on a full-load transformer to determine the maximum temperature rise. Hence, this test is also known as the temperature rise test.
In the case of a small transformer, a full load test is possible, but in a large transformer, a full load test is very challenging.
In a large transformer, an appropriate load to absorb full load power may be readily available. During this test, a large amount of energy is wasted.
Maximum temperature rise can find out for large transformers using sumpner’s test. The sumpner’s test is also known as the back-to-back test or regenerative test.
Connection Diagram of Sumpner’s Test
The connection diagram of the sumpner’s test on two identical single-phase transformers Tr1 and Tr2, is given in the figure below.
An ammeter, a voltmeter, and a wattmeter are connected to the supply, as shown in the figure.
The secondary windings of both transformers are connected in series with their polarities in phase opposition. Polarities can be checked by voltmeter V2. The range of voltmeter V2 should be double the rated voltage of either transformer secondary.
To check that secondary windings are connected in series opposition, any two terminals (i.e., B and C) are joined together, and voltage is measured between remaining terminals (i.e., A and D).
AC supply is given to the circuit. Now, if the voltmeter V2 reads twice of rated secondary voltage, the polarities of both secondary windings are similar.
If voltmeter V2 reads zero, the polarity of both secondary windings is different. Then terminals A and C are short, and terminals B and D are used for the test.
The total voltage across the two secondary windings in series will be zero if the primary circuit is closed. So, no current flows in secondary windings.
The transformer will behave as their secondary windings are open circuits. So, the reading of wattmeter W1 gives the iron losses of the transformers.
A small voltage is injected into the secondary circuit by a regulating transformer TR excited by the main supply in the circuit diagram.
The magnitude of injected voltage is varied till ammeter A2 reads full-load secondary current.
The secondary current produces a full-load current to flow through the primary windings. This current will flow in the circulating path through the main bus bar, as shown dotted in a circuit diagram.
The reading of wattmeter W2 will not affect by this current. Thus, wattmeter W2 gives the full load copper losses of the two transformers.
The ammeter A1 gives a total no-load current of the two transformers. So, in this method, we have loaded the two transformers to full load, but the power taken from the supply is necessary to supply the losses of the transformers.
The time duration of this test is very long (48 hours), and take a reading of temperature oil periodically, say every one hour.
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