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VFD FEDDING A POWR TRANFORMER

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petronila

Electrical
Jul 28, 2005
491
Dear All ,

We are developping a motor control test center project, and some of our collegues are suggesting to use a VFD to fed a power transformer in order to change the transformer´s voltage output with out change the frecuency, this is intendded for no load test.

Any one with similar experience ? pros and conts??

Thanks for the Inputs

Petronila
 
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Hello Petronila,

Did this about nine months ago. But used the output of the transformer to start a synchronous 11 kV 6 MW motor. The inverter was a 500 V 840 A rated inverter.

That application was OK. But, if you want to have a clean sine output from your secondary, you probably need to filter the VFD output heavily.

Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
Skogsgurra
Apparently you didn't have any problems burning through the transformer winding insulation like you can do with cheap motors. Did you specify special insulation? The distance between the inverter and the transformer was probably very close.
 
Yes, quite close. I would say 15 metres/50 feet.

The transformer was an existing one and it was oil isolated. So no problems with corona. And no special insulation.

Also, it is only used during start up of the refiner motors and that takes less than two minutes. The refiners then run for typically a week or two. So even if we got some ozone production, it wouldn't accumulate.

I think that PWM and insulation can be a problem with smaller transformers. But larger ones, like those found in a typical motor repair shop, are not exactly "rush wound" and there is plenty of distance between windings and iron, too.


Gunnar Englund
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
 
Seems like a serious overkill to just change voltage alone. You could do that with a phase angle SCR controller; it's done all the time like that. The only reason to use a VFD is if you want to change the frequency.
 
Yes jraef ,
If you see the project like you are going to buy the VFD For only voltage variation you are right,is an overkill , but this VFD are intended to powered the low voltage motors up to 660 Volts.The idea is to have the initial VFD`s investment in order to fed one dry type tap transformer and have variable secundary voltage avoiding the use of CTV Transformer.

Attach you will find the design.

Thanks and regards

Petronila
 
 http://files.engineering.com/getfile.aspx?folder=a92ee501-cb7f-4882-9578-e6f2c38df3c9&file=motor_test_center_design.bmp
I've done this in the past on a couple of jobs and I definately recommend a filter to protect the motor. I think I remember something like the capacitance of the circuit goes up by the square of the transformer ratio so the voltage spikes on the motor will be high if you do nothing. A sinewave filter on the drive O/P is one option and I used the transformer and capacitors on the HV side very succesfully.
The transformer manufacturer should be informed of the PWM characteristics of the drive output.
I had a problem on one site where the step-up transformer was heating up excessively and this was solved by reducing the motor flux parameter in the drive. The application was a 6kV fan and dry type cast resin transformer so the windings were aluminium strip/sheet.
 
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