Automation Canada - Cables & Connectors Issue - 11

Here is where you look at me like you have got me.
You say, " Well the CMC can simply flow back to the inverter
through the ground wires. "
I respond, " Why do you think the CMC will flow back through
the ground wires? "
You reply, " because it's the lowest impedance return path. "
" Not at high frequency! " I state with a smile.

This is what gets most engineers and electricians in trouble with these
systems. They apply all their years of experience and knowledge about
60-Hz power systems directly to VFD systems and don't take into
account that electrical systems operating at 60-Hz and 30MHz are quite
different. If you remember this fact alone it will save you lots of trouble.
It will help you question things that you otherwise might take for
granted. Just keep reminding yourself that you are dealing with
frequencies almost 1 million times greater than the power you are used
to dealing with.
SURFACE AREA RULES ALL
In calculating resistance at high frequency, surface area becomes
king. Your little ground wires become a rela- tively high impedance
path. The current is lazy and it wants to take the path of least resistance.
The factory's infrastructure looks very appealing to it. Building steel with
its large surface area is a very attractive path. The CMC may also choose
to wander through large motor shafts (via the bearings) to factory
equipment and then through building steel on its way back to the
inverter. If it chooses this path it often leaves tracks behind. Those
tracks, also called bearing fluting, are a common cause of premature
C A N A D I A N A U TO M AT I O N

motor failure.
What we need is a nice, short, low impedance at high frequency path
for the CMC to flow to on its way back to the inverter. An overall shield,
be it a copper braid, a copper tape, or a continuously welded aluminum
ar- mor, provides such a path. Now we have yet another reason to use
shielded cable - to provide that controlled CMC path!
By the way, that nice, short, low impedance at high frequency shield
is not going to do you a lot of good unless it has a nice, conductive, low
impedance termination at both ends of it. By termination I am talking
about terminating the shield. Rockwell recommends a shield
termination that gives you 360° of electrical contact around the shield.
But please understand, not all terminations are created equal. Some
terminations are better than others just like some VFD cables are better
than others. We could do a whole other paper just
on that topic. Just remember that if you buy VFD cable but don't
terminate it correctly, you lose many of the advantages VFD cable has to
offer.
There you have it. Now you know about the issues of EMI, CIV, CMC
in VFD systems. If you allow me to use CAP for Capacitance, we have yet
another 3 letter acronym to make all this easier to remember!
Now imagine an innocent little PLC someplace far far away from the
VFD system and cable. It seems isolated from the VFD system but it
happens to be sitting next the return path for the CMC. How do you
think that PLC is going to act when its ground gets exposed to, say, 100
amps of high frequency noise? It's not going to be pretty. This is why you
want to provide a controlled path for the CMC back to the inverter. You
don't need or want lots of amps wandering around your building steel,
smoking your PLCs and other control and communica- tion equipment.
See? That mystical " We Are All Connected " philosophy has more to
do with the wonderful world of VFD engi- neering than you ever
thought. One more mystery of the universe explained!
Steve Wetzel works for Southwire and chairs the ICEA working group that is developing a standard for
VFD cables. Southwire Company, LLC, based in Carrollton, Georgia, is a participating EncompassTM
Product Partner in the Rockwell Automation Partner NetworkTM. Southwire manufactures electrical wire
and cable products.

VOLUME 3, ISSUE 2

11


https://www.southwire.ca/

Automation Canada - Cables & Connectors Issue

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