sumpner's test on transformer

sumpner's test or back to back test on transformer is another method for determining transformer efficiency, voltage regulation and heating under loaded conditions. short and open circuit tests on transformer can give us parameters of equivalent circuit of the transformer, but they can not help us in finding the heating information. Unlike O.C. and S.C. tests, actual loading is simulated in Sumpner's test. Thus the Sumpner's test gives more accurate results of regulation and efficiency than O.C. and S.C. tests.

sumpner's test:







sumpner's test

Sumpner's test or back to back test can be employed only when two identical transformers are available. Both transformers are connected to supply such that one transformer is loaded on another. Primaries of the two identical transformers are connected in parallel across a supply. Secon

daries are connected in series such that emf's of them are opposite to each other. Another low voltage supply is connected in series with secondaries to get the readings, as shown in the circuit diagram shown below.In above diagram, T1 and T2 are identical transformers. Secondaries of them are connected in voltage opposition, i.e. EEF and EGH. Both the emf's cancel each other, as transformers are identical. In this case, as per superposition theorem, no current flows through secondary. And thus the no load test is simulated. The current drawn from V1 is 2I0, where I0 is equal to no load current of each transformer. Thus input power measured by wattmeter W1 is equal to iron losses of both transformers.


i.e. iron loss per transformer Pi = W1/2.
Now, a small voltage V2 is injected into secondary with the help of a low voltage transformer. The voltage V2 is adjusted so that, the rated current I2 flows through the secondary. In this case, both primaries and secondaries carry rated current. Thus short circuit test is simulated and wattmeter W2 shows total full load copper losses of both transformers.i.e. copper loss per transformer PCu = W2/2.


From above test results, the full load efficiency of each transformer can be given as -

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Main Difference between contactor and Starter

The magnetic starter is very similar to the magnetic contactor in design and operation. Both have the feature of operating contacts when the coil is energized. The important difference between contactorsand starters is the use of overload heater element in the starter.

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Why is transformer efficiency higher than motors or generators?

Why is transformer efficiency higher than motors or generators?

Since the transformer is a static machine it has no moving parts, means no frictional and windage losses, it only has the copper loss and core loss. In the case of generators and motors, in addition to copper losses and core losses, they have frictional and windage losses due to rotating parts. That is the reason why transformers have higher efficiency.






transformer

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why the efficiency of a generator is more than a motor??

In general, the efficiency of a generator will be greater than that of motor.


Some of the reasons are:

The generator windings are made of more thicker windings hence have lesser resistance and hence lower copper losses.
In a motor, the armature produces back emf which tries to oppose the motion of the motor (there are two opposing voltages )
but in the generator, there are no opposing voltages.
Also, the leakage of magnetic flux produces power losses in a motor but the leakage of the flux produced by the field winding in the generator is minimum and also not so critical and so it does not pose a problem to lose some flux.

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a:

3-point starter

The shunt connection in 4- point starter is provided separately from the line where as in 3-point state it is connected with line which is the drawback in 3- point starter.

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