Current Transformer in VFD application
Current Transformer in VFD application
(OP)
Can a traditional Current Transformer be used in a VFD application? Is it capable to read correct current? For example , 10 HZ, 20HZ .
A traditional Current Transformer I 've used normally works in a 50HZ application.
A traditional Current Transformer I 've used normally works in a 50HZ application.





RE: Current Transformer in VFD application
There is also the problem with HF interference from the PWM voltage. It is not so much of a problem if all you want to do is sending the secondary current to an analogue instrument, but using it in a recording device or a controller usually needs quite a lot of filtering, shielding and grounding.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Current Transformer in VFD application
RE: Current Transformer in VFD application
And, with no extra hardware! I think that's called a "bargain", even in the king's English!
RE: Current Transformer in VFD application
RE: Current Transformer in VFD application
Such cases exist and sometimes an ordinary CT works quite well if the operating range doesn't go down to frequencies where the CT core saturates.
As to the complexity of the waveform, I would say that it is the least problem. VFDs deliver an almost pure sinusoidal current to the motor windings with very low residual PWM traces.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Current Transformer in VFD application
As mentioned above, I have found them barely acceptable where purely comparative measurements are to be taken but that is all. On multiple motor applications like you mention, comparative might be good enough depending on the situation but I don't see any mention of multiple motors in the original post.
RE: Current Transformer in VFD application
I may expand a little on this VFD/CT theme. I was asked to go to a city named V---s and help a company named A-B measure output current from a VFD. I was surprised - after all, that company is well-known for excellent engineering and support.
They had done everything right. But they didn't get any consistent measurements from their CTs. They had even tried shunts with isolation amplifiers, they had tried LEM type transducers. And now, they were back to standard CTs.
Burden was low enough and no saturation. But still problems. It was only when we wrapped the cables with brass foil and soldered a short braid between foil and cabinet ground rail that we got any meaningful measurements.
So, most problems with using CTs on PWM outputs are, as I have experienced it, caused by EMI that hits the secondaries and (this was an application with a rectifier between CT and burden resistor) is rectified. The current waveform as such was not a problem in this case. But EMI was. And I think that is why there are so many problems with CTs on VFDs. And, of course, saturation when frequency gets low.
It is interesting to hear that you tested CTs and got poor results. I do not know what the final accuracy was in the case I cited. Are we talking a couple percent or tens of percent in your case?
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Current Transformer in VFD application
Also are there some VFD that do not use sinusoidal PWM. If so then the results might seem a little crazy.
RE: Current Transformer in VFD application
RE: Current Transformer in VFD application
But I would keep shield connection separate and probably ground it as close to CT as possible. Long wires are no good HF conductors.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Current Transformer in VFD application
I guess it depends on your accuracy requirements but yes, the measured value in the VFD is typically good enough for itself so would be good enough to provide as an output.
It depends on your VFD type but most these days can 'connect' the measured current to an analogue output or use the current as a monitoring function and define internally thresholds to activate a reaction (via a relay or other switching device).
RE: Current Transformer in VFD application
Some, if not all, the LEM transducers suffer from E-field coupling when they are installed on an unscreened cable at a non-earthy potential. The easy solution is much as you described.
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RE: Current Transformer in VFD application
Next time I run into one of these CT's, I'm going to try your shielding technique and see what happens.
The testing we did was on an ABB ACS600 so the PWM algorithm was not necessarily sinusoidal. That may have been part of the problem too.
RE: Current Transformer in VFD application
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...