# VFD connected to Transformer, just thinking here



## JimDawson (Oct 15, 2017)

I have never heard of this being done, nor have I tried it myself.  But..... What do you think would happen if you connected a VFD output to a 3 phase transformer then used the output from the transformer to power up a system?

Just looking for some opinions.


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## Karl_T (Oct 15, 2017)

about the same as using a reactor, except you get a voltage bump or drop


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## mksj (Oct 15, 2017)

I do not think it will work for multiple reasons. Reactors act like a choke or filter and are inline, they are not a transformer across the lines.
There is a high inrush current with transformers that will probably trip the VFD
A transformer is not designed at the higher frequencies that make up the PWM signal of VFDs, nor operate above or below its designed frequency, it may saturate and overheat.
A transformer will change the phase, would not work with a vector drive. Not sure if it would work at a fixed V/Hz, one would probably be better off with something like a Phase Perfect Converter.
Unclear as to how the VFD would see the transformer load and affect phase detection circuity.
I would check with the VFD manufacturer as to recommendations, and/or the machine manufacturer you want to run off of it.

If you need a higher voltage, I usually see a step-up transformer before the VFD and the VFD is run at a higher voltage. When using an input transformer, there is a significant inrush current, so the power input end needs careful consideration.


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## Karl_T (Oct 15, 2017)

Yep, I can see the VFD will think the transformer is a really big motor and kick out.


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## JimDawson (Oct 15, 2017)

Yeah, that's pretty much what I thought.  Think I'm gonna forget that idea and go with a RPC.  Thanks for the replys.


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## markba633csi (Oct 15, 2017)

Actually I don't see why it wouldn't work. Windings are windings and flux is flux. Transformers and induction motors are very similar in terms of 
electrical characteristics. 
In fact I believe I did see someone on the forum doing something just like that- VFD, step-up, and motor. And it worked. Don't remember who. 
Mark


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## whitmore (Oct 15, 2017)

markba633csi said:


> Actually I don't see why it wouldn't work.



As long as there's a motor load on the transformer, the transformer ought not to change the loading much
if at all.   The extra iron being magnetized is a small power drain, and resistive losses might go up
a tad, but the only foreseeable issue would be if the motor were OFF when the transformer is
activated (the feedback inside the VFC might register abnormal operation when there's only
a transformer, and not a back-EMF from a motor).


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## markba633csi (Oct 15, 2017)

Thank you Whit, my thoughts exactly.
Mark
of course 3 phase trannnies are pricey...


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## mksj (Oct 16, 2017)

I still think it is a bad and dangerous proposal, and I would not recommend it unless there is some validated information either by the VFD manufacturer or transformer winder to support using a transformer between VFD output and the motor. I do not proclaim to have a definitive answer, but I have run into issues with both transformers and variacs being operated at different frequencies then designed, they can quickly heat up with negative consequences. I participate in a few other machinery forums where there have been discussions of stepping up voltage output applications, the general recommendation is to do it at the VFD input. Below is a discussion links on the same topic, general comment is the transformer might last a couple of days, unless you have a specially made transformer and it would need to operate at a fixed frequency. You would be much better off running off of a RPC and a step-up transformer, or step-up transformer followed by a VFD.

http://www.practicalmachinist.com/v...and-vfd/transformer-between-vfd-motor-153443/
http://www.practicalmachinist.com/v...d-vfd/220v-vfd-output-stepped-up-480v-192439/
"To pile on a bit, it can be done but it takes a specially designed custom built transformer to be used on the output side of a VFD. Core losses (eddy currents from imperfections in the steel laminations) get extreme in PWM outputs and create not just extra impedance as mentioned above, but a rapidly increasing heat losses inside transformer where it was never designed to dissipate it from. So a standard transformer would last maybe a couple of days before smoking unless grossly oversized, in which case then your core (copper resistance) losses go up and you end up with an overloaded motor because you loose too much torque, so you then have to oversize the motor. All in all, not worth the effort."


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## markba633csi (Oct 16, 2017)

Yep I agree, above 60 hertz anything could happen with losses and heat, etc. 
Not a real practical approach and too expensive anyhow
M


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## strantor (Oct 16, 2017)

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.


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## JimDawson (Oct 16, 2017)

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.

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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## strantor (Oct 16, 2017)

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.


yeah sounds like your best option assuming the servo drives will fault out if you feed them single phase.


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## markba633csi (Oct 16, 2017)

Good thread I learned something too
Mark
ps Strantor how much loss in your undersea sine filters?


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## strantor (Oct 16, 2017)

markba633csi said:


> Good thread I learned something too
> Mark
> ps Strantor how much loss in your undersea sine filters?


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)


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## markba633csi (Oct 16, 2017)

Yes I figured it was a multipole design.  Cool stuff.  High falutin' math for sure. 
M


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## strantor (Oct 16, 2017)

markba633csi said:


> Yes I figured it was a multipole design.  Cool stuff.  High falutin' math for sure.
> M


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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