Transformer... in reverse? 1

[AliBakBak]

Goodday

I am going to be testing a new 55kW induction machine rated at 440V and 93A.

The supply I have in the laboratory will be via an AC generator (driven through a synchronous machine). The AC generator gives me 260V out.

As you may have noticed, I need 440V to the induction motor under test.

To do this I thought of placing the 260V from the AC generator through a transformer (380:220). Unfortunately, the original primary side of the transformer is the 380V HV side.

Can I connect the 260V from the AC generator onto the 220V (original secondary) side of the transformer and use it in 'reverse' so to speak? Or will this saturate the core of the transformer?

I dont see how it should? The same voltage will be applied to both sides (or at least quite close to it as I am placing a slightly higher voltage on it to get the required 440V). The current will thus also be the same if it was operating under normal conditions except that the current will be flowing in the opposite direction.

Your help is greatly appreciated.

 

[waross]

If the operating current of the motor is small compared to the winding rating of the transformer, then 18% over voltage should be no problem. If over 50% of the rated winding current I might rethink.

NOT SO.
Saturation of the transformer core has nothing to do with the load. 18% overvoltage is likely to cause saturation and overheating. Without specific knowledge of the design of the transformer in question, it should not be done.
Some transformers may survive, many will not survive.
This has nothing to do with insulation levels or load.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[edison123]

The problem with saturation (due to over-voltage) is not only high no-load current but also the fact that the output voltage will not increase proportionately with input. So, you may not get 440 V out of 220/380 V trafo with a 260 V input.

* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *

 

[AliBakBak]

I agree with edison123 and also did that calculation, I am not sure whether I can rewire it as described.

But, if I can and then only need o apply 147V to the 220V side of the transformer and get the required 40V to the motor, then will the transformer not saturate??

 

[edison123]

No, it would not saturate if you apply 147 V to the 220 V side. If you reconnect the 380 V delta to star, you will get a higher line to line voltage and that does not mean saturation.

One problem of both sides being star/star is unstable neutral, which for your motor testing purpose, isn't a problem at all.

The real question is whether you have the wherewithal to do the winding modification. Otherwise, take the motor to nearby motor repair shop to get this simple test done.

* Women are like the police. They can have all the evidence in the world and yet they still want a confession - Chris Rock *

 

[waross]

Will you recheck your transformer configuration?
You said that:

the transformer is connected a 380:220V transformer connected in delta-star respectively.

To us that means delta 380 volts and star 220 volts.
The common delta voltage is 220 volts. 380 volts is often derived by connecting 220 volt windings in star.
Please verify your transformer configuration.
Also, what is the KVA rating of the transformer?
It is important even if you think that you will be running at a light load.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

If you have access to all winding ends you could reconnect the transformer as an autotransformer capable, assuming two equal windings of 220V each as Bill suggests, of doubling the input voltage.


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