VFDs and impedance matching for cables 1

[jantoffman]

Currently I'm reading a bunch of documents on power cable selection for VFDs. I kinda understand the potential problem of impedance mismatch, but I'm not sure how to DESIGN around it.
Do people actually measure the impedance of their motors before hand and try to match it? There are a lot of suggestions to use manufacturer recomendations, but at design time we haven't selected a manufacturer, and are writing the spec. Do we spec load reactors, chokes, or terminators? If you had a motor in place (480V 90HP Inverter Duty rated, for Variable Speed application), how would you go about insuring you don't end up with a consdierable impedance mismatch? We're locating the vfds as close as possible.

A related question:
My boss read in a Beleden document that impedance matching "is a 'green' thing". In hopes to improve effeciency she wants to make sure we incorporate this into the design.
On a simplistic level, I don't think this is correct though. Sure, if you match impedance you'll maximize power output, but that doesn't mean you'll maximize efficiency. The added impedance on the cable will just create more heat! Is Belden wrong?

 

[Skogsgurra]

You do not match cables or motor. You apply motor reactors, du/dt filters or somtimes even sine filters.

Belden and impedance matching sounds weird. Very. A sales thing.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[Skogsgurra]

I should have added that the impedance is the wave impedance. It is equal to sqrt(L/C) and has very little effect on efficiency. It is responsible for wave reflections, though. Google "transmission line theory", "reflected wave" and such terms.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[hooverdale]

Matching source and load impedance to achieve maximum power transfer does not result in maximum efficiency. If we define efficiency as the ratio of power dissipated by the load to power developed by the source:

Efficiency = ZLoad/(ZLoad+ZSource)

If ZLoad = ZSource ; then efficiency is 0.5
If ZLoad = Infinity ; or ZSource = 0 ; then efficient is 1
If Zload = 0 ; then efficiency is 0

The efficiency is only 50% when maximum power transfer is achieved, but approaches 100% as the load resistance approaches infinity, though the total power level tends towards zero.

Efficiency also approaches 100% if the source resistance can be made close to zero.

When the load resistance is zero, all the power is consumed inside the source (the power dissipated in a short circuit is zero) so the efficiency is zero.

 

[controlsdude]

If you have wet versus dry location you would have to pay attention to what type of cable you use.

If dry location, you can get away with just THHN cable, no special VFD cable.

If wet location, that is cable built for that enviroment, but VFD rated.

VFD to motor distances usually dictate any special filtering as Gunnar suggested. Usually manufacturers have a table on vfd to motor distances and recommended filtering.

 

[jantoffman]

Thanks everyone!