Sumpner's test



Sumpner's test (also known as Sumpner test, back to back test or load test ) - a type of test of transformers, which allows simulation of full load conditions and hence also thermal testing.

Sumpner's test is used for large transformers, for which ordinary full-load tests are too expensive from the viewpoint of electrical energy consumption and difficulties in finding a suitable load capable of absorbing nominal power of the transformer. This is especially difficult, because the thermal tests require operation under full load for many hours.

The test can also be used for determining total efficiency of the transformer.

The test was invented by William Sumpner, who also had several patents on various electromagnetic devices.

Principle of the test
The test requires two identical transformers (see T1 and T2 in the drawing). The primary winding of each of the transformer is connected to the main supply under nominal operating conditions. They are therefore connected in parallel.

With the secondary windings open-circuited (switch S open), the wattmeter W1 measures the sum of no-load loss of both transformers. Copper loss is very low so the wattmeter shows essentially iron loss.

The secondary windings are connected in series, but with opposing polarity. The secondary voltages cancel each other so there is no current flowing after the switch S is closed. For this reason the transformers must be identical. Otherwise complete voltage cancellation cannot be achieved, and this creates problems with injecting the current into secondary windings for simulation of copper loss.

An auxiliary variable supply (aux supply) is connected in series with the secondary windings. This can be achieved either by using a variable-amplitude voltage source with an auxiliary transformer (T aux), or a variable transformer connected directly with the secondary windings.

The variable supply is then used to drive current into the secondary windings. Because the polarities are in opposition relatively low voltage is required to achieve nominal value of currents. The secondary current is transformed to the primary side, but due to the parallel connection the current circulates in the primary windings, without flowing to the main supply and affecting the indication of the wattmeter W1. The wattmeter W2 shows therefore the sum of copper loss for both transformers.

During the operation the transformers do not deliver any significant power, so the energy use is relatively low. However, full-load losses for both copper and iron are achieved, which allows performing full-load-like thermal tests.