VFD's....

[xformer]

Do variable frequency drives cause a resonant circuit effect in the motor circuit?

 

[GoldDigger]

Hopefully not, but the fast rising and falling waveform edges can allow reflections on long VFD-to-motor cables to form standing waves with high voltage points.
That acts a lot like a resonant circuit for some purposes and is not a good thing.


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[xformer]

Hopefully not, but the fast rising and falling waveform edges can allow reflections on long VFD-to-motor cables to form standing waves with high voltage points.
That acts a lot like a resonant circuit for some purposes and is not a good thing.


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Why hopefully not? Is it not desirable to cause a unity power factor for continuous loads for efficiency?

 

[GoldDigger]

Why hopefully not? Is it not desirable to cause a unity power factor for continuous loads for efficiency?

Please believe me that a resonant circuit is not the way to do power factor correction.
A match of inductive and capacitive reactance in the load, yes, but with such a high damping factor (low Q) that you hardly notice that it is a resonant circuit.
Also, FWIW, the VFD itself may incorporate power factor correction in its input circuit, but will in any case hide the actual motor power factor from the AC input wiring.
Since the VFD will be operating the motor over a wide frequency range (starting and/or reduced speed running) you again do not want to create a resonant circuit on the VFD output side.

 

[Besoeker]

Do variable frequency drives cause a resonant circuit effect in the motor circuit?

To add to what the GoldMinerExcavatorDigger posted about long cables, it does happen in real life.

This is a waveform I recorded on a 660kW VSD.



It shows damped resonance but note that the horizontal time scale is in microseconds. The time frame for the fundamental motor frequency is milliseconds - three orders of magnitude different.
The waveform was measured at the motor terminal box. It's not something I like to do - exposed live terminals with the attendant risks you see.
But, as part of the contract for supplying the VSDs, we were required to demonstrate that the waveform presented to the motor was within prescribed limits for peak voltage and dv/dt.

There's a little bit of history to this and not entirely off topic in the context of the thread title, VSDs. And, hopefully, informative.

The customer, our customer, in this case had had a bad experience with another installation with three 800kW VSDs. Our customer purchased the inverters from us and the motors separately from an established motor manufacturer. The motors had a high attrition rate. About one a month. As one engineer put it at the time, an 800kW motor can't be considered a consumable item.

And, of course, the blame game started. Was the inverter or the motor to blame? Huge bucks at stake for all parties. And I lost sleep over it.
In the event, it was a baptism by fire for the motor manufacturer. They had been advised of the application but provided their standard fare which, as we later determined, just wan't up to the duty. Mesh wound and form wound will be forever etched in my brain.

It was a steep learning curve for all involved.