OK, I can't find anything I have already done that covers all of the issues raised, so once again, into the breech...
There are SEVERAL issues to contend with regarding the VFD to motor lead wiring; capacitive coupling, voltage spikes from reflected / standing waves, motor winding insulation damage, motor bearing damage, EMI/RFI interference and Common Mode Noise problems. They are mostly all interrelated, starting with the capacitive coupling issue and similar effects. But to avoid a long boring story, let's just say that VFD cable solves SOME problems, not all of them.
The main one it addresses is the EMI/RFI issue. The output cables of a VFD are, to over simplify it, like a powerful local FM radio transmitter. FM is "Frequency Modulation", which is exactly what a
VFD is doing
(radio purists, please excuse my over generalization here, it's for effect...) This is where the idea that putting VFD cable INSIDE of steel conduit is redundant. The steel conduit has the same effect as far as keeping the RF inside. But as was mentioned, if you have MULTIPLE VFD cables in a single conduit, then you MUST use shielded VFD cable, otherwise the different frequencies of the multiple outputs will cause induction between each other, as WELL as the RF bleeding from one to another.
The VFD cable also uses a more symmetric geometry of the cables, and combined with better insulation, can help reduce the cable capacitance issues you may encounter using separate conductors in conduit, which is what leads to refelcted / standing wave spikes. But if your distances are shorter, that might not be an issue anyway, so that alone is not a good enough reason to always use it.
The fact that the VFD cable always has a good over sized ground conductor, or multiples, and a shield that is grounded on both ends, also helps cut down on common mode noise creating in the cables and helps avoid transmitting CM noise created in the VFD to other nearby equipment. But again, that alone is not usually a good enough reason to use it all the time.
Lastly, there is evidence that by using a good VFD cable with XLPE insulation on the conductors, can help to reduce the surge capacitance of the cable itself. That can, again based on circumstances, be cause for concern related to capacitive charging current required from the VFD, which can "rob" your motor of available current and decrease the shaft torque, and is also a contributor to reflected wave creation.
So bottom line, the only time I tell people the MUST use VFD cable is under the following conditions:
- PVC conduit or cable tray installations (aluminum conduit is problematic too); in other words you are not using steel conduit.
- Flexible cables, not in conduit at all; do NOT use SO or other portable cord for VFD outputs!
- Installations where you are exceeding the maximum recommended cable distance of the VFD in question.
- Places where you ALREADY know you have a problem.
Some of the same issues mentioned above can be ALSO mitigated by using filters on the output of the VFD, but the only thing that CANNOT be mitigated is the EMI/RFI issue.
Re: Insulation type.
The voltage spikes that can damage the motor insulation is based on the Corona Inception Voltage (the point at which a corona discharge occurs) level of the insulation in the magnet wire. So on 600V insulation, the peak rating of older motors was 1200V. But on a 480V line, the reflected wave spikes can reach almost 1600V, so far above what the old motor could tolerate. "Inverter Spike Resistant" (ISR) magnet wire raised the peak level to 1600V or more, with a CIV of over 2400V. That is what you get when you buy an "inverter rated" motor., so that solved THAT problem. Still, if you DON'T have an Inverter Duty motor, you need to worry about this.
But the CIV issue takes place in the motor leads too, regardless of whether the motor insulation can take it or not. PVC insulation, as found in THHN / THWN cable, is typically 15mil thick and has a CIV of at least 2400V, but that can go down by as much as 50% if the wire is wet, and even further if it is nicked in pulling. In addition, over time now we have seen that because the PVC is injected in a liquid form around the wire, it can have microscopic bubbles in it, which allow the CIV to be even lower yet. So even if the wire passes muster for standard testing based on sine wave power, it might not be suitable for VFD outputs without compromising the longevity of the installation. Because cable is EXPECTED to last 25-50 years, and high speed transistor VFDs have only been around for 20 or so, the foreshortened life of THHN is only now coming to light. I have seen 3 different installations now in the last 2 years in which older THHN cables were pulled out, and you can see the burn marks that are tell tale signs of corona discharge happening.
XLPE (Cross Linked PolyEthylene) insulation, as is used in
RHHW cable, is 30mil thick, heat shrink applied to the wire so there are no bubbles, and is rated for at least 1000VAC RMS with a CIV of over 4,000V. It is also less susceptible to water infiltration and has been shown to lose less than 30% of it's CIV capacity when wet, which is still FAR above the levels that can be seen on VFD outputs. Many of the VFD cables on the market will be made with XLPE insulation instead of PVC (but not all, so check). So if you ARE going to use steel conduit, I am recommending that people start using RHHW conductors now, not THHN. This only applies to the OUTPUT side of the VFD, nothing special goes on on the input side.