VFD connected to Transformer, just thinking here

JimDawson

Global Moderator
Staff member
H-M Lifetime Diamond Member
Joined
Feb 8, 2014
Messages
11,138
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.
 
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.
 
Yep, I can see the VFD will think the transformer is a really big motor and kick out.
 
Yeah, that's pretty much what I thought. Think I'm gonna forget that idea and go with a RPC. Thanks for the replys. :)
 
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
 
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).
 
Thank you Whit, my thoughts exactly.
Mark
of course 3 phase trannnies are pricey...
 
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."
 
Yep I agree, above 60 hertz anything could happen with losses and heat, etc.
Not a real practical approach and too expensive anyhow
M
 
Back
Top