Automation Canada - Cables & Connectors Issue - 8

WHAT IS A VFD CABLE
BY STEVE WETZEL, SR. PRODUCT ENGINEER

A variable-frequency drive (VFD) cable is a special cable construction
for the inverter-to-motor cable that has some or all of the following
attributes:
* An overall shield that keeps bad stuff such as electrical magnetic
interference (EMI) from escaping.
* A robust insulation system that keeps good stuff such as voltage and
current from escaping.
* A symmetrical design that reduces the amount of bad stuff in the
cable, such as common mode current and EMI.
Not all VFD cables offer each attribute mentioned; but each attribute
helps the cable operate in such a way as to reduce problems that occur
in installations that use VFDs. These problems aren't restricted to
premature cable failure. They include interference with radios, controls
and communication systems; shock hazards; premature motor failure;
bearing fluting; drive trips; drive failures; and even having that precious
magic smoke leak out of programmable logic con- trollers (PLCs),
causing them to fail!
All these problems can make it harder to keep a facility up and
running, and the wrong invert- er-to-motor cable can contribute to any
or all of these issues. How can a power cable at one end of a plant affect
a PLC at the other end of the plant when it's not even connected to it?
No, it's not the mystical " We Are All Connected " philosophy, although,
in a way, that's not too far off. Before I discuss this mystery of the
universe, here's the simple stuff.
A VFD CABLE HAS AN OVERALL SHIELD
VFD cables have an overall shield. Unlike traditional slow 60-Hz
power systems, the newer light- ning-fast VFD waveforms have
frequency measured in tens of MHz. Electrical Engineering 101 taught
C A N A D I A N A U TO M AT I O N

us that higher frequency means more energy. We need a way to stop
that high energy from escaping the inverter-to-drive system, and a
shield is a great way to limit electromagnetic radiation, interference and
cross-talk.
Consider, for instance, two inverter-to-motor cables traveling
together for a good distance in cable tray. One is energized, and the
other is locked out. The locked-out circuit may receive enough
electromagnetic coupling from the energized cable to produce a
combination of voltage and current that can be dangerous or even fatal.
It may shock you to learn that not using shield- ed (VFD) cable can result
in safety issues. I'm pretty sure you don't need additional safety issues.
Of course, with a VFD cable, this safety issue goes away!
A VFD CABLE HAS ROBUST INSULATION
While not all of VFD cables have this attribute, most of them do.
Robust insulation means:
* Adequate thickness to handle the higher than traditional voltage
peaks the cable sees.
* A high corona inception voltage (CIV) so the cable insulation is not
damaged by corona discharge.
* A low dielectric constant to minimize cable capacitance.
This may seem like a lot of cable manufacturer mumbo-jumbo, but it's
important. The first two items, insulation thickness and CIV, are
important because inverter-to-motor cables see much higher voltages
than you would expect because of reflected (also called standing)
waves. These waves are caused by an impedance mismatch between
the cable and the motor. I won't bore you with all the math, but in a
480V drive system, the cable can see voltage peaks of 1,300V, and that
is a conservative number.
VOLUME 3, ISSUE 2

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Automation Canada - Cables & Connectors Issue

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