Jim1959
Senior Member
- Location
- Longmont, CO
Is there an advantage to using VFD cable over copper conductors in conduit for short 20-30' lengths to motors?
VFD cable
- Thread starter Jim1959
- Start date
- Status
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
In steel conduit with proper bonding to ground, not much of an advantage to using VFD cable.
In PVC conduit, it's a must.
In PVC conduit, it's a must.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
What about when you have the line and the load (VFD output) in the same conduit?
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
You would be a glutten for punishment then. That's a big no-no.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Exactly but isn't that one reason why you would a shielded VFD cable from the drive to the motor?
Not so much drive to motor. You’d just potentially leak more noise onto the line side. A worse case is two or more VFDs in the same conduit in which case cut your estimated maximum line length in half. So if AB says you can go 80 feet, plan on only 40 feet with 2 VFD loads in the same conduit.
VFD cable is largely a gimmick. It is rated 2 kV. So it gets tested to 5 kV. Sounds much better than garden variety building wire, right? Cable is tested (hi potted) to pass 200% of the rated voltage plus 1 kV. THHN which has the thinnest insulation at 600 V is tested to 2.2 kV. But according to NEMA with third party tests where they tested to failure all of it passes at least 2.8 kV with #14. So far looks like VFD cable is superior and it is. It’s similar to Type W or DLO or R in this respect.
BUT even if you get an inverter duty NEMA standard motor it’s only rated to around 1.5 kV while most handle up to 1.8 kV. So well before you shred cable, the motor turns burn out. If you run three phases plus a ground as per all the VFD installation guides, you already pretty much balance the fields. As in you can’t detect the VFD with a tic, and I have yet to measure any significant current imbalance. I run power quality tests (PdMA) for customers all the time and this is one of the test results I always look for. So far as far as this goes I think it’s again just a sales pitch to engineers, not real world.
I’m pretty much anti-VFD cable. I have seen several bad installations that use it but fail to fix the actual problem. To me it’s just a ripoff. Kind of like insisting every VFD needs a line reactor. Some do some don’t and usually if they do the line reactor is the worst option.
Finally while I’m on a roll I’m definitely not anti-metallic conduit. Under fault conditions a properly bonded steel conduit (RMC, EMT, or IMC) is vastly superior to PVC and better than aluminum. So much that shielding is pretty much redundant.
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Would you recommend XHHW-2 over THHN/W? I have seen many cases of THHN/W failing when it gets wet, esp in corrosive enviroments
Forqnc
Member
- Location
- Elizabeth City, NC
- Occupation
- Electrical Controls Engineer
In 30 years of being around controls, I have never used VFD cable and never had a problem, except 1 install.
About 6 months ago I was designing/building a control cabinet for a machine which would be in a Class 1 Div 1 environment.
So in a 36" wide enclosure, I had thermocouple barriers on the left hand side, my VFD mounted on top and my motor cables would run down the right hand side to a distribution block, for customer field wiring. During testing, my thermocouple values to my PLC would bounce all over the place. Eventually found that even though my motor cable was 12" away from the barriers, it was causing the problem. Installed a piece of VFD cable from the VFD to the DB, problem solved, even when using regular cable from the DB to the motor.
About 6 months ago I was designing/building a control cabinet for a machine which would be in a Class 1 Div 1 environment.
So in a 36" wide enclosure, I had thermocouple barriers on the left hand side, my VFD mounted on top and my motor cables would run down the right hand side to a distribution block, for customer field wiring. During testing, my thermocouple values to my PLC would bounce all over the place. Eventually found that even though my motor cable was 12" away from the barriers, it was causing the problem. Installed a piece of VFD cable from the VFD to the DB, problem solved, even when using regular cable from the DB to the motor.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
the VFD and cable manufacturers want you to use VFD cable instead of wire type conductors. Experience suggests very few people actually do so especially with smaller VFDs and short runs, which are the majority of installs. It is not like there are huge numbers of problems with these installs.
is it ideal? probably not. is it adequate? probably in most cases. Until it isn't.
is it ideal? probably not. is it adequate? probably in most cases. Until it isn't.
- Location
- Illinois
- Occupation
- retired electrician
I have been specifying XHHW for VFDs and for any underground conductors. There have been reports of issues with THHN/THWN and the all of the drive cable I have seen has XHHW conductors.
petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
both are 600 V conductors. I thought VFD cables were 1000 V conductors.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
On the other hand, I do mostly VFDs and I see a LOT of cable issues (relative to non-VFD motor connections). I never recommend using THHN on the output of VFDs, I only recommend XLP insulation, such as found in XHHW, RHH/RHW conductors. Most of the XHHW I see being sold is 1000V rated, because it's almost the same price as the 600V stuff so the distributors only stock the 1000V. I have seen several installations where THHN has regularly space line-to-line burn marks between phases, basically the wave length of the standing waves created in the conductors laying next to each other and the resulting corona discharge from the high voltages in the waves and spikes. They didn't show up on basic megger tests because the leakage was not to ground, it was phase-to-phase. But the VFDs were picking up the higher amperage and nuisance tripping on short circuit. THHN uses PVC insulation that is injected onto the wire as a liquid, then heated up to set it ("the "T" in THHN is for Thermalset). that liquid PVC can have microscopic bubbles in it and while that makes no difference in standard sine-wave 600V power, it represents a weakness in the insulation when you get 1500-2000V pikes of DC pulses from the standing / reflected waves from the PWM output of the VFD. It's not a hoax, it's very real. But the severity depends on a lot of circumstances, so not everyone experiences it. Luck however is not a strategy for success.
Again, if you use STEEL conduit, I don't think VFD cable is necessary. But if you use PVC conduit (or something like fiberglass or aluminum), or cable tray and no conduit, you MUST use shielded cable (and ground the shields at BOTH ends). VFD output cables are like radio broadcast antennae, creating and emitting RF signals that will bleed into everything else in the area. I once did a project in Nevada near a military site where we used unshielded SO cord for the 5HP VFD powered motors on a machine because of needing flexibility. The USAF MPs triangulated on us from 25 miles away because of the radio signals they were picking up (mind you, they are using more sensitive detection systems than most, but 25 miles is still a LONG way away). You need to have some form of magnetic shielding to prevent that. Steel does it, PVC doesn't. THAT is the main reason for using the shielded VFD cable. There is a less common benefit in that there is a "consistent geometry" in the conductor relationships because they are twisted, which can help to reduce the creating of the standing waves, but it's not that much on moderate run lengths and can be duplicated by simple triplexing (twisting together) individual conductors in conduit.
Most VFD manuals will call for shielded cable because basically everywhere else in the world, they DON'T use steel conduit, they use what we would consider something like MC cable or tray cable that is run without conduit at all and goes into cabinets via "glands". For them, it's a requirement.
Again, if you use STEEL conduit, I don't think VFD cable is necessary. But if you use PVC conduit (or something like fiberglass or aluminum), or cable tray and no conduit, you MUST use shielded cable (and ground the shields at BOTH ends). VFD output cables are like radio broadcast antennae, creating and emitting RF signals that will bleed into everything else in the area. I once did a project in Nevada near a military site where we used unshielded SO cord for the 5HP VFD powered motors on a machine because of needing flexibility. The USAF MPs triangulated on us from 25 miles away because of the radio signals they were picking up (mind you, they are using more sensitive detection systems than most, but 25 miles is still a LONG way away). You need to have some form of magnetic shielding to prevent that. Steel does it, PVC doesn't. THAT is the main reason for using the shielded VFD cable. There is a less common benefit in that there is a "consistent geometry" in the conductor relationships because they are twisted, which can help to reduce the creating of the standing waves, but it's not that much on moderate run lengths and can be duplicated by simple triplexing (twisting together) individual conductors in conduit.
Most VFD manuals will call for shielded cable because basically everywhere else in the world, they DON'T use steel conduit, they use what we would consider something like MC cable or tray cable that is run without conduit at all and goes into cabinets via "glands". For them, it's a requirement.
We just used SWAPVC.
- Location
- Illinois
- Occupation
- retired electrician
In the US XHHW is most often marked as 600 volt, however the exact same wire, other than the markings is marked at 1000 volt in other countries. I expect with the code language changes in the past couple of cycles from "600 volts or less" to "1000 volts or less" we will start seeing the 1000 volt marking here.
XHHW-2 is just thicker. I think you mean THWN-2. Once you get to this point both are rated for wet locations and both are rated 90 C. The big difference in the past (THHN, XHHW) was the dry vs wet location and 90 vs 65 C rating.
XHHW-2 according to the NEMA report increases the hi pot rating from around 2850 for THHN to 3200 for XHHW no matter the brand or variety. Either way again it’s vastly better than magnet wire in the motor. Motor wiring is typically rated a few Volts turn to turn and we run slot insulation and phase papers between the coils where the insulation has to be higher rated. So theoretically it shouldn’t handle even a hundred Volt transient but it all tests to around 1200-1800 V depending on the motor which is still below the test voltages for building wire and almost half of the point where building wire fails. So again...where is my weakest link? If all building wire is rated and performs far better than the motor itself, it doesn’t matter which one you use.
As to manufacturing ALL polymer insulation’s have air voids. XLPE in particular has a bad history with this at higher voltages yet you’re kind of pushing XLPE which is XHHW. The difference is the THHN family uses a PVC insulation layer with a clear nylon jacket where XHHW just uses a single thicker layer of polyethylene. PVC is better insulation but sensitive to liquids including sucking up oils so they added the outer protective nylon jacket. Either way I’ve seen failures of both types and VFD cable too due to improper installs. Newer insulation adds anti-treeing/water blocking to the insulation that eliminates the problems and the differences between the insulation types.
The key isn’t the type of cable. The first thing is making sure the VFD chassis is bonded to the motor frame. Don’t care how it gets done but this must happen. VFD manufacturers insist on a supplemental ground but steel conduit is better. Second there is a section in the VFD manual talking about maximum cable length. Past a certain point dv/dt or even sinus filters on the drive output are necessary. Read it, follow it, and don’t guess. Dv/dt filters are cheap, cheaper than VFD cable, and bump line limits to hundreds of feet. The VFD manual may refer to using inverter duty motors and VFD cable. But the gains are modest, cost more than the filter, and they set up a maintenance department for future failures when somebody forgets to use the special material. So either keep cables short or install the filter. A third issue is isolation between VFDs. Don’t run them in the same conduit or if you do, cut everything in half.
I run into situations all the time where somebody “upgraded” from a starter to a VFD or maybe had an old soft start or years ago an old forced commutation VFD or DC drive. They keep the same wiring then have problems because it’s hundreds of feet from the VFD especially conveyors. Motors fail within months and eventually the burned spots in the cabling cause shorts or ground faults, Insert filter and the problems go away.
The key isn’t the type of cable. The first thing is making sure the VFD chassis is bonded to the motor frame. Don’t care how it gets done but this must happen. VFD manufacturers insist on a supplemental ground but steel conduit is better. Second there is a section in the VFD manual talking about maximum cable length. Past a certain point dv/dt or even sinus filters on the drive output are necessary. Read it, follow it, and don’t guess. Dv/dt filters are cheap, cheaper than VFD cable, and bump line limits to hundreds of feet. The VFD manual may refer to using inverter duty motors and VFD cable. But the gains are modest, cost more than the filter, and they set up a maintenance department for future failures when somebody forgets to use the special material. So either keep cables short or install the filter. A third issue is isolation between VFDs. Don’t run them in the same conduit or if you do, cut everything in half.
I run into situations all the time where somebody “upgraded” from a starter to a VFD or maybe had an old soft start or years ago an old forced commutation VFD or DC drive. They keep the same wiring then have problems because it’s hundreds of feet from the VFD especially conveyors. Motors fail within months and eventually the burned spots in the cabling cause shorts or ground faults, Insert filter and the problems go away.
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