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.

 

[jghrist]

Applying higher than rated voltage to either side will cause saturation of the core. 260V is 18% above the 220V rating.

 

[AliBakBak]

Thank you for your reply - I do realise that it is slightly higher than the rated nameplate value of the transformer.

But what i meant was - Can i connect it the other way around, if I apply 220V and use the resulting voltage on the 380V side? (i.e. in reverse).

Additionally, what is the limit on the transformer in terms of percentage voltage above rating?

 

[magoo2]

Normally the transformer standards allow you to go 5% high at rated load, but 18% is too much.

 

[jghrist]

But what i meant was - Can i connect it the other way around, if I apply 220V and use the resulting voltage on the 380V side? (i.e. in reverse).

Yes

Additionally, what is the limit on the transformer in terms of percentage voltage above rating?

This is probably governed by an IEC standard. I'm not familiar with the IEC transformer standards.

 

[jpankask]

I'm confused. First you say you want to connect 260V to the 220V side, then you say you want to connect 220V to the 220V side. 260V is not slightly higher than the rated voltage, it's almost 20% higher.

Most transformers will work in reverse, though some won't handle it so well, but you need to watch the rated voltage you apply. But even if you do this and get the 380V out of the transformer, you still need 440V, or close to it, for your test so I don't see where you are gaining anything. It sounds like you want to apply 260V and get about 445V out the other side. This may not be possible based on the transformer limitations.

 

[AliBakBak]

Thank you guys for your help.

I will only ever be applying higher than rated value to the transformer (higher than 220V) when I perform a no-load (light-running) test on the machine under test. Thus, there will be minimal current (so nowhere near rated current of the transformer).

 

[edison123]

If you plan to run the 440 V motor only on no-load (for checking mechanical run & vibration ?), then the motor will most likely run at 260 V itself, especailly if you start the generator along with the motor.



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

 

[AliBakBak]

Unfortunately I need to run the motor up to rated voltage (it is fo characterisation of the motor accrding to an IEC standard).

I dont think I will be able to do this now that I think of it. Applying 260V to the 220V side of the transformer is probably not a good idea. I dont think? Like already said - it is 18% more than rated voltage.

I may need to make another plan!

 

[edison123]

If this is wound rotor motor with a rotor voltage of 260 V, you can run with 260 V supply to the rotor and map the characteristic.

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

 

[AliBakBak]

It is a squirrel cage induction motor. So that option goes out of the window.

I think what I will do is leave th transformer out of the equation and rather use a completely different setup.

I will drive a DC generator to power a DC machine. This machine is coupled to a synchronous generator (at 400V) and then I will have my required voltage for the motor.

 

[edison123]

You mention synchronous machine in the first post. What is the voltage of that machine ?

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

 

[AliBakBak]

That machine is hard-wired onto the 380V 3-phase busbars of the lab. And it is the machine that drives the AC generator.

 

[edison123]

So, you do have a 380 V supply. May be a booster transformer to get to 440 V ?

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

 

[AliBakBak]

Unfortunately i dont have one of those?

And to be honest I am not sure what you mean by "booster" transformer.

Also - I need to have the motor start on soft-starter.

 

[waross]

Tell us a little more about what you have. Several possible solutions come to mind.
Is the motor single phase or three phase?
If three phase, is it a 3 lead motor, a 6 lead motor, a 9 lead motor or something else?
Is your transformer three phase or single phase?
Is your transformer star delta, delta star, etc?
Is the transformer re-connectable?

Are you testing the soft starter as well as the motor?

A booster transformer is usually an autotransformer.
It may be possible to re-connect your transformer as an auto-transformer.
As an auto-transformer your 220:380 volt transformer will output 600 volts.
If you trim your generator's output voltage down to 161 volts you will have 440 volts out of the autotransformer.


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

 

[AliBakBak]

The motor is a three phase squirrel cage induction motor as is the transformer.

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

By "soft starter" i mean the voltage (260V) that i will be supplying the transformer 220V side with can be trimmed from 0-260V. This is how I will start the motor.

Surely, the transformer cant output 600V (that is 58% higher than rated!!!!). Th insulation of the windings surely aren't designed to handle this.

All I need is to apply 0-260V on the 220V side of the transformer to get 0-450V on the output 380V side of the transformer to run the motor at rated voltage 440V.

 

[edison123]

If you can open up transformer and winding connections, then you can reconnect the 380 delta to star. Then you apply 147 V on the secondary (220 V winding) to get 440 V on the other side.

A 380 V winding will definitely withstand 600 V.

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

 

[waross]

How many wire are there comming out of the motor?
Is the motor rated for more than one voltage?
What is the KVA rating of the transformer?
Are you a student?

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

 

[SteveWag]

First, operation of the transformer "in reverse" is not a problem. The transformer will work just as well in either direction.
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.
Starting with a primary voltage of 100 volts and ramping up to provide a secondary voltage of 440 should work very well as a "soft-start".
I agree with edison123 and would say that any of the windings will withstand operation at 600 volts, insulation wise.
Just see that the transformer is generously oversized, current or watts wise.
Steve Wagner