VFD cable types

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rlundsrud

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chicago, il, USA
It recently came to my attention that per the 2018 editopn of NFPA 79, VFD's must be fed with RHH, RHW, RHW-2, XHH, XHHW, or XHHW-2. It specifically forbids the use of THHN or any thermoplastic insulation. While I don't typically install the power wiring for VFD's, it is almost always a thermoplastic cable jacket and I have as yet to see any issue caused by using this. Is this an overreaction by the NFPA or does anyone else know of instances where using thermoplastic insulation caused issues or does anyone believe that there is a reasonable chance this might cause any issues?
 

Cow

Senior Member
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Eastern Oregon
Occupation
Electrician
I've never heard mention of requirements for the input wiring to a vfd needing to be a certain type. I have heard of vfd output wiring needing to be a certain type.
 
VFD cable types

I had a drive and the Motor leads was in a race way with 2 more motors already and it just kept faulting out. First l called a parts house and bought VFD cable and yes they sell it. Ran a new raceway pulled the VFD cable wired it up and no more problems. VFDS don’t llike EMF which could be you problem
 

Jraef

Moderator, OTD
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
It’s a well known and documented problem, it ONLY applies to the OUTPUT conductors. The issue is that thermoplastic insulation such as PVC (as used in THHN) is manufactured as a liquid that is extruded onto the wire, then set with heat to plasticize it. In the liquid form, it has microscopic bubbles in it that become trapped in the plastic. Under normal sine wave power at 600V, these bubbles are totally irrelevant.

But in the output of a VFD, the high speed DC pulses that make up the PWM pseudo-sine wave going to the motor can /will cause the conductors to act like weak capacitors. That capacitively coupled voltage is superimposed on the output as a standing wave that runs down the conductors until it hits an impedance change, the motor terminals. Some passes through to the motor windings but some reflects back the other way, where it adds to new waves, then hits the next impedance change, the drive terminals, where some again reflects back. This continually takes place back and forth until these reflected wave become spikes of very higher than normal voltages. We already know of the potential damage that this can cause to the motor winding insulation, but now 25+ years into widespread VFD use, what’s showing up is that those little bubbles in the PVC are showing up as weak spots in trying to resist the corona discharge effects of these high voltage spikes and waves.

I don’t see a lot of it, but I have definitely seen it now numerous times: a series of burn marks along the conductors where the insulation has failed phase-to-phase, but not necessarily phase to ground so a basic megger test doesn’t show it. At first it shows up as apparently “nuisance” tripping of the VFD as it sees the high speed spikes of current, but it’s difficult to diagnose without actually pulling the conductors and looking at them very closely. In all the cases where I have seen it, that was the last thing anyone wanted to do, but it was clearly evident once we did. Mind you, this does not always happen, there are a lot of specifics and circumstances that have to take place, but it is real.

I’ve been recommending RHH/RHW or now XHHW for years because they use Cross-Linked Poly Ethylene (XLPE) insulation. XLPE is what heat-shrink tubing is made of and in wire, the wire is inserted into the tubing, then heated to shrink it onto the wire. Since it starts out as tubing, it was not liquid and there are no bubbles (or they are even smaller maybe), so no weak points in combatting corona discharges. That’s why typical XLPE insulated wire is rated 1000V or more, even though we use it at 600V or less.

Many VFD manufactures have been making this recommendation in their installation manuals for years, but nobody has been paying attention.
 

Besoeker3

Senior Member
Location
UK
Occupation
Retired Electrical Engineer
It recently came to my attention that per the 2018 editopn of NFPA 79, VFD's must be fed with RHH, RHW, RHW-2, XHH, XHHW, or XHHW-2. It specifically forbids the use of THHN or any thermoplastic insulation. While I don't typically install the power wiring for VFD's, it is almost always a thermoplastic cable jacket and I have as yet to see any issue caused by using this. Is this an overreaction by the NFPA or does anyone else know of instances where using thermoplastic insulation caused issues or does anyone believe that there is a reasonable chance this might cause any issues?
Good grief! How many types of cable do you guys have?

For us it would usually be just XPLE.
 

paulengr

Senior Member
ALL polymer insulations are porous. The capacitance is not due to these. That makes no sense at all because where is the other conductor? I think this is being confused with the idea of eddy currents which describes where circulating currents kind of like eddy currents in a stream create self inductance in a cable.

Simply put, a capacitor is created whenever we have two conductors separated by an insulator. Distributed capacitance exists between phases as well as between all phases and the nearest grounded surface.

We dont normally worry about this at low frequencies like 60 Hz. We can calculate the wavelength at 60 Hz which is about 800 kilometers. So at around 80 kilometers the electrical power line will start to act like a transmission line but for most power distribution cases we can ignore cable inductance and capacitance. However VFD pulses and more specifically rise times are fast enough that starting at around 3 meters (10 feet) it's no longer just a cable, it's a transmission line and these minor distributed capacitances and inductances matter.

Taken together with cable resistance we can calculate a characteristic impedance of the cable. Motor characteristic impedance however is very different from cable impedance so when a pulse from the drive hits it a portion of the pulse, particularly at high frequency, reflects back along the cable. It reflects again back at the drive, back and forth as the wave dissipates.

http://alignment.hep.brandeis.edu/Lab/XLine/XLine.html

PVC and XHHW do have slightly different characteristic impedances but both will have reflected waves at about the same footage. Both must pass a high pot test using 200% of rated voltage plus 1,000 V, or 2,200 V. XHHW is slightly thicker compared to THHN. Actual testing by NEMA has shown that #14 THHN from various manufacturers fails (arcs) at around 2800-2900 V based on several samples from different manufacturers. XHHW fails at a little over 3200 V. VFD cable is rated usually for 2,000 V so must pass high potting at 5,000 V. Realistically it is probably much closer to the test compared to the 600 V cables.

However unrated motors without phase papers are only good for 1,000 V. Typical standard motors today pass up to 1200 V. Inverter duty motors following NEMA MG-1 go up to 1400 V. Most manufacturers however exceed this rating and hit around 1700-1800 V. However this is less than the test voltage for 600 V cable, never mind actual test results even on much maligned THHN. Simply put cable doesn't really matter because by the time the motors start showing fluting in the bearings and the first couple turns are burned up. Sure we can use VFD cable but so what...The motors are still shredded.

One of the final arguments for VFD cable is that since it is balanced, cable capacitance is minimized, etc., that it can improve distance limits and many VFD manufacturers include this in their motor cable tables. For instance with most Allen Bradley VFDs, it goes from 50 to 80 feet. But a simple dv/dt filter increases the distance to hundreds of feet. However no provisions are made for XHHW increasing distance over THHN.


Sent from my SM-T350 using Tapatalk
 

Kinexis

Member
Location
Ohio
ALL polymer insulations are porous. The capacitance is not due to these. That makes no sense at all because where is the other conductor?

I don't think that is what he said.


what’s showing up is that those little bubbles in the PVC are showing up as weak spots
so no weak points in combatting corona discharges

I think he's just pointing out the weak spot, not saying that is causing the capacitance.



For OP and spectators, here's a simple presentation I like on the subject https://www.rockwellautomation.com/...lautomation/noa/raotm-tech-sessions/ct479.pdf
 
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