VFD connected to Transformer, just thinking here

Transformers connected to VFD output is possible but not advisable if I understand your application correctly.

Where I work we make subsea tools that operate at the end of a 10,000ft cable. To reduce voltage drop along that cable we use 3,000V motors. We use 480V/100A VFDs into a sine filter, into a 480>3,200V transformer. We have done it successfully with many Omron and ABB VFDs.

Off-the-shelf 60Hz Transformers have a specified reluctance (measure of magnetic "resistance") appropriate for 60Hz. When you operate below 60Hz they saturate too easily. In order to maintain variable speed with this setup, we must use special-order 30Hz transformers which are huge, heavy, and expensive. If you were to use this setup to get a fixed 60Hz, this wouldn't be a consideration.

The output of VFDs is very nasty, electrically speaking. Many spikes and much noise. The sine filter helps, but it's still gross coming out of the sine filter. The transformer acts as a second filter, but the spikes that do make it through, are amplified. If you are using the setup to power an entire 480V system, expect your electronics to see some nasty power. Sine filter is a must.

If you're doing this just for one motor, fixed 60Hz, it's feasible. But it sounds like you want to power a whole machine with it, which may include electronics, therefore I don't recommend it.
 
Thank you all for your replies. This has been an education. :)

My thinking was to feed a 3 phase, 1:1, star/delta isolation transformer with a 240V VFD. Vin = Vout, operating at 60 Hz. It would be driving three AC servo drives, three small 3 phase motors, and the other electronics. Given the nasty waveform and noise out of the VFD, just the filtering required would probably make the whole idea infeasible due to cost. I could split the power and run the 3 phase motors on single phase input VFDs, then run the computer system on a completely separate feed. That would leave only the servo drives on the VFD/transformer. But given the cost of the servo drives, it would be best not to let the magic smoke out.:cautious:

I am going to build a RPC and go that route. I just happen to have a 15HP, 3 phase motor in stock. I should be able to build one for < $200.
 
JimDawson said:
I am going to build a RPC and go that route. I just happen to have a 15HP, 3 phase motor in stock. I should be able to build one for < $200.
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yeah sounds like your best option assuming the servo drives will fault out if you feed them single phase.
 
Good thread I learned something too
Mark
ps Strantor how much loss in your undersea sine filters?
 
markba633csi said:
Good thread I learned something too
Mark
ps Strantor how much loss in your undersea sine filters?
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A lot. 480 goes in, 420-460 comes out. They arent simple reactors. There is a cap bank too, and the voltage loss has to do with phase angle. So a 60V loss @ 100A would correspond to >10kW of loss, but there isn't actually that much loss or else the filter would burn up (but it doesn't)
 
Yes I figured it was a multipole design. Cool stuff. High falutin' math for sure.
M
 
markba633csi said:
Yes I figured it was a multipole design. Cool stuff. High falutin' math for sure.
M
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Yeah then on the other side of the transformer you have transmission line impedance math. Can't say that I enjoy the math.
 
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