I have few doubts regarding short circuit test of transformers.

[nik.jay.hil]

Hi,
What does rated output in a transformer signify?
Is power same(or almost same) on both sides?
In short circuit test, we apply reduced voltage to HV winding so as to circulate rated current in HV. This will cause rated current to flow in LV. But as per definition of rated current, rated current=(rated output/rated voltage). So how come rated current flow with reduced voltage?Is power maintained constant in this case? If power is maintained constant then what about open circuit test?
Somebody pls help me. I m terribly confused. I might sound stupid but I really want to clear this..

 

[skeptic2]

Hi,
What does rated output in a transformer signify?

The maximum power the transformer can safely handle. I have heard of instances where high power and small size were needed in a transformer, so it was intentionally designed to run hot.

Is power same(or almost same) on both sides?

Yes, except for the losses in the transformer.

But as per definition of rated current, rated current=(rated output/rated voltage).

This is not true. Unlike the relationship between voltage, current and power in a circuit, rated current, rated power and rated voltage are three independent quantities.
Rated current depends mainly on the size and number of the windings.

Rated power depends on the amount of voltage times the amount of current the transformer is passing and also on how well the transformer is able to dissipate the heat.

Rated voltage depends on the electrical insulation of the windings.

It is very likely that if you put the rated voltage and rated current into a transformer, you will exceed the rated power. Transformers are designed this way in order not to be too big. So the first rating (current, voltage or power) of the transformer that you reach is the maximum that the transformer should handle.

So how come rated current flow with reduced voltage?Is power maintained constant in this case? If power is maintained constant then what about open circuit test?
Somebody pls help me. I m terribly confused. I might sound stupid but I really want to clear this..

Since the three ratings are independent of each other, each must be tested independently. If you are testing for rated current, power is not necessarily maintained constant. An open circuit test would be used to measure rated voltage and there may no current flowing and the only power dissipated would be core losses.

 

[nik.jay.hil]

Thanks a lot Sir.. Helped me a lot..
Just 1 more question- In SC and OC tests, is power maintained the same on both sides?

 

[skeptic2]

In a short circuit test there will be losses but ignoring the losses, the power will be approximately the same on both sides.

In an open circuit test with the secondary open, there can be no power dissipation in the secondary. There still will be some power dissipation in the primary because of losses.

 

[nik.jay.hil]

Thanku so much Sir..