Electrician Talk banner

21 - 39 of 39 Posts

·
Registered
Joined
·
8,051 Posts
My experience has been if there happens to be something flakey going on and I need to call tech support they will point to any part of the installation that doesn't follow their manual or recommendations. Then my customer gets cranky and starts complaining that is wasn't installed correctly.



Better safe than sorry.

Manufacturer requirements and manufacturer recommendations are not the same thing. You do whatever makes you feel good but I don't need anybody telling me how to do my job. That's something I've noticed on this site- lots of folks would rather be told exactly what and how to do instead of providing a competent turnkey installation.


Sent from my iPhone using Tapatalk
 

·
Registered
Joined
·
1,698 Posts
I believe recommendations and requirements are the same thing.

If a judge recommends drug rehab, that person is going to rehab.
 

·
Registered
Joined
·
575 Posts
I believe recommendations and requirements are the same thing.
No, they are not the same thing. Some MC manufacturers furnish anti short bushings with every roll and actually recommend using them but, they are not required.
 

·
Registered
Joined
·
8,051 Posts
Some would say that only a very confident person would make a statement like that.

Others might use the term over-confident.

Call it anything you like, fact is you said you call tech support for every vfd you put in to see how they want you to do the simplest part of a drive/motor installation. Frankly I'm questioning your qualifications right now.


Sent from my iPhone using Tapatalk
 

·
Registered
Joined
·
5,451 Posts
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:

  1. PVC conduit or cable tray installations (aluminum conduit is problematic too); in other words you are not using steel conduit.
  2. Flexible cables, not in conduit at all; do NOT use SO or other portable cord for VFD outputs!
  3. Installations where you are exceeding the maximum recommended cable distance of the VFD in question.
  4. 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.
 

·
Registered
Joined
·
3,838 Posts
...

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.
XHHW is also XLPE and is used in some brands of VFD cables.
 

·
Registered
Joined
·
5,451 Posts
XHHW is also XLPE and is used in some brands of VFD cables.
Yes, you're right, I screwed it up on multiple levels. The operative issue is the XLPE insulation. RHH/RHW, (not RHHW) has slightly thicker insulation than XHHW, which you're right is what they usually use in the VFD cables. Thanks for correcting that.
 
  • Like
Reactions: CaptKarnage

·
Registered
Joined
·
888 Posts
So if I understand correctly I can feed a drive with Soow cord, pvc conduit and THHN wire but on the output side we should have LFMC, emt, imc with xlpe insulation and or drive cable.
 

·
Bilge Rat
motors and controls.........
Joined
·
7,503 Posts
I've run VFD outputs through buried PVC using THHN many times. Longest run was about 230' or so.

On longer runs I'll use some sort of a load reactor though.
 

·
Registered
Joined
·
1,698 Posts
I believe recommendations and requirements are the same thing.
No, they are not the same thing. Some MC manufacturers furnish anti short bushings with every roll and actually recommend using them but, they are not required.

I have not seen the manufacture recommend using anti-shorts. I have heard they supply them as a courtesy.

I do know that when the manufacture recommends a insulated ground to a spa, you can't use Romex. That would go against the manufactures recommendations. And would not pass an inspection around here.

I don't do much service work anymore, and have never had to do service work on VFD's. All of the motors on VFD's that I have done were always conduit/flex with THWN and as far as I know we're never a problem. This thread will make me question the next install I do.
 

·
Registered
Joined
·
5,451 Posts
So if I understand correctly I can feed a drive with Soow cord, pvc conduit and THHN wire but on the output side we should have LFMC, emt, imc with xlpe insulation and or drive cable.
That's what I recommend, although the SO cord on the input would be subject to the normal restrictions, and that too can allow some EMI/RFI bleeding as well. That's why most VFD mfrs sell what they call "EMC Filter" options for the line side of their VFDs. VFDs sold here in the US have to meet basic FCC regulations with regard to EMI emissions so most people don't have issues with it, but the regulations in the rest of the world are tighter. Still, using portable cord is "poking the bear" in that regard.

I've run VFD outputs through buried PVC using THHN many times. Longest run was about 230' or so.

On longer runs I'll use some sort of a load reactor though.
Like I said, it's not a simplistic issue, there are multiple factors that go into it so it doesn't always happen, but when it does, you may not realize it.

Have you ever pulled out any of the old runs of cable and looked at them yet? That's one of the issues. I worked on a project last year that had been installed for about 9 years; parallel 2/0 THHN cables per phase in steel conduit, about 130-150' linear run to the motors from the 200HP VFDs. Multiple transistor failures on the VFDs was causing suspicion, so we pulled the cables and stretched them out on the floor. You could see the burn marks at regular distances all along the cables where the standing waves peaked and broke down the insulation, burning through to the other cables. Meggers didn't pick it up because the burn through was only when energized and creating corona discharge, and even then it was phase to phase, not phase to ground. So nobody knew until we could see it with our own eyes. But you could extrapolate the wavelength because the burn marks were fairly evenly spaced at about every 16 feet or so. Granted, this is the only one I have actually seen with my own eyes, but it's also the only time I have pulled the cables and looked.
 

·
Registered
Joined
·
14,368 Posts
We've probably pulled out about a dozen failed THHN runs on drive outputs.

A while back I pulled a bunch of small stuff out from part of a plant remodel that looked like it was all part of the same batch of wire, maybe even off the same 2500 foot spools that pin holed. I pulled XHHW back in.
 

·
Registered
Joined
·
438 Posts
Here in Canada in industrial environments it is pretty much always aluminum interlocked armored cable (ie TECK90) in tray.

The "VFD" variant of this style of cable (basically it adds symmetrical grounds + shield) is not significantly more expensive than standard TECK. So, we generally always use it for 50hp and greater, or 20hp and greater when the run is more than 150 feet or so.

So for us it's not so much a question of when is it needed, but given the small price difference, why not?

If the standard install is wires in conduit though I guess it's a drastic change to the install.
 

·
Registered
Joined
·
1 Posts
OK, I can't find anything I have already done that covers all of the issues raised, so once again, into the breech...
I had to register on this site just so I could tell you thank you for such a great post (even though it's almost 6 years later now). A lot of people miss the nuances of VFDs and all the reasons to use the cable. You summed up a complicated issue well. Thank you for that.
 

·
Registered
Joined
·
1,089 Posts
This is the reason why a lot of tech support is useless.

Always blame the installer.........
While I don’t disagree with your statement , if it’s not installed per the manufacturer’s recommendation then it is tougher to get their tech support more involved. Also, your customer will tend to think less of your abilities and may damage your reputation.
 
21 - 39 of 39 Posts
Top