# When is a VFD cable actually "required" ?



## Vintage Sounds

When I was a first year I helped a journeyman pipe a mechanical room in a commercial building. We connected probably twenty 600v motors and drives and I distinctly remember using EMT, metallic liquid tight flex and 600v rated T90 conductors. 

From what I learned in school and on this forum since that time, that installation would be less than ideal because the voltage spikes associated with rectifying AC would create corona discharge that pinhole the insulation. By my calculations, on a 600v system the rectified DC would be 846v.

I see that most cable manufacturers sell at least one type of VFD cable. Since VFDs are everywhere now, when is one of these special cables actually a necessity? I ask because I've never actually seen any in person. Whenever I've seen a VFD in an industrial application the motor has generally been connected using 1000v Teck 90. Is this acceptable or are you theoretically supposed to use VFD cable "every time"?


----------



## don_resqcapt19

We have always used THWN in rigid conduit for our industrial jobs and have had no issues.


----------



## Fibes

We do the same as Don. We would use FMC or LFMC for the connection to the motor.

VFD cable would only be for open wiring designs.


----------



## Cow

That's a good question that'd I'd like to know the answer to also.

I realize VFD manufacturers most likely recommend it for every install, but myself, I've really only installed it when I've also used EMI/RFI filters because of potential issues with RFID readers for cow ear tags.

Another thing I've read is to keep VFD motor leads in their own metallic conduit and away from other motor leads. I've been on one job where the VFD leads have been XHHW or THHN in their own pvc conduit, but mixed in with a bank of other conduits, some 400+ feet long, which include other VFD conduits as well.

What's surprising is everything I've read says this type of install should cause VFD issues and the THHN/XHHW insulation should break down, but this particular install is 4+ years old and hasn't had any wire or drives replaced to my knowledge for this issue. 

This makes it difficult to justify the cost for me of VFD cable plus oversized conduits unless I'm missing something in the bigger picture that I don't know about yet?


----------



## micromind

I've yet to see this 'special' cable as well. I use basic pipe, flex and THHN. Never had an issue, even on 500' runs. 

I've also run 120AC controls in the same pipe as the VFD output. 

Seems to me that the 'requirement' for special cable is a lot like the requirement for AFCIs.


----------



## JW Splicer

Never seen an issue, I've always piped them with emt and lfmc and wired with thhn/thwn...


----------



## dawgs

99% of the time we use metallic conduit and thwn wire. We are starting a job next week and the engineer Speced VFD cable. Not sure why exactly.


----------



## John Valdes

I always used pipe and THHN myself. However, special cable was just starting to appear on the market right before I left the business.
I also never had any issues with this wiring method though I can promise you, there will be several who will disagree.

I can't really take a side on this as I only know the conduit and THHN side.


----------



## don_resqcapt19

I have started specifying XHHW for VFDs now, just because that is what is used in VFD cables, but it is not based on any type of failure. It is also a tougher cable and I specify it for all underground runs too.

We have also connected may older non-inverter rated motors to VFDs without any issues and we typically don't use load side reactors unless the wiring between the VFD and the motor is over 500'


----------



## ponyboy

I think Jreaf taught me vfd cable in a steel raceway is just redundant and unnecessary. Like don said the xhhw and a properly bonded steel raceway is the way to go


Sent from my iPhone using Tapatalk


----------



## bill39

This is something that you should not rely on as "Rule of Thumb" type of thing. If you're wrong and have only pulled single conductors and need to instead pull cable then your conduit may be too small. Plus you'll end up looking bad.

I try to call the VFD's manufacturer and let them tell me. Running the cable drives up the installation cost but at least you've done your due diligence.


----------



## EC2253

I was looking at a installation manual from a major manufacturer, one of their requirements says to run motor cables from multiple vfds in separate conduits.
Do most manufactures have this requirement?


----------



## telsa

IIRC, the universal standard test for 600 V conductors is TWICE the labeled voltage PLUS + 1000 VAC. ( It's a NEMA thing, IIRC. )

Hence the run-of-the-mill THWN-2/ MTW/ XHHW-2 is routinely tested at 2,200 Volts before it's kicked out the door. (So they say. I've seen the occasional conductor with factory flaws -- which should've busted out into a corona at 2,200 volts.)

So, it's no surprise that such conductors can tolerate ordinary VFD harmonics.


----------



## AK_sparky

Vintage Sounds said:


> From what I learned in school and on this forum since that time, that installation would be less than ideal because the voltage spikes associated with rectifying AC would create corona discharge that pinhole the insulation. By my calculations, on a 600v system the rectified DC would be 846v.


A regular AC sine wave from your favorite local utility at 600 Vrms will also peak at 846V, so I don't see why that would be any different on a VFD from a purely "peak voltage" standpoint.


----------



## Jlarson

Anytime you aren't running a metallic raceway all the way to the motor you should be using drive cable. Or if you absolutely have to run multiple drive outputs in one conduit. Those are the biggies.


----------



## JRaef

I feel compelled to correct some slight misinterpretations here (and confirm other statements). I had a doozy of a reply all typed up on this, and my internet went down before I was able to post it... I hate when that happens. 

This is part of what I do for a living, so I'm pretty sure I have posted all that someplace at one time or another and rather than re-do it now, I'm going to try to find it and link to it.


----------



## AK_sparky

And upsize your conduit one or two sizes so the watts can move around quicker!


----------



## ponyboy

bill39 said:


> This is something that you should not rely on as "Rule of Thumb" type of thing. If you're wrong and have only pulled single conductors and need to instead pull cable then your conduit may be too small. Plus you'll end up looking bad.
> 
> I try to call the VFD's manufacturer and let them tell me. Running the cable drives up the installation cost but at least you've done your due diligence.



If I called the manufacturer every time I hooked up a vfd I'd get less done than I do now. Of course drive manufacturers will tell you it's best to use their brand drive cable but why wouldn't they. I couldn't even begin to guess the amount of vfds I've hooked up in the last 5 years alone that weren't simple pipe and wire. 


Sent from my iPad using Tapatalk


----------



## bill39

ponyboy said:


> If I called the manufacturer every time I hooked up a vfd I'd get less done than I do now. Of course drive manufacturers will tell you it's best to use their brand drive cable but why wouldn't they. I couldn't even begin to guess the amount of vfds I've hooked up in the last 5 years alone that weren't simple pipe and wire.
> 
> 
> Sent from my iPad using Tapatalk


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.


----------



## micromind

bill39 said:


> 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.


This is the reason why a lot of tech support is useless. 

Always blame the installer.........


----------



## ponyboy

bill39 said:


> 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


----------



## cabletie

I believe recommendations and requirements are the same thing. 

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


----------



## AK_sparky

cabletie said:


> I believe recommendations and requirements are the same thing.


Not the same.


----------



## Fibes

cabletie said:


> 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.


----------



## bill39

ponyboy said:


> ..................... I don't need anybody telling me how to do my job. ......
> 
> Sent from my iPhone using Tapatalk


Some would say that only a very confident person would make a statement like that.

Others might use the term over-confident.


----------



## ponyboy

bill39 said:


> 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


----------



## JRaef

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.


----------



## don_resqcapt19

JRaef said:


> ...
> 
> 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.


----------



## JRaef

don_resqcapt19 said:


> 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.


----------



## garfield

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.


----------



## micromind

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.


----------



## cabletie

Fibes said:


> cabletie said:
> 
> 
> 
> 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.
Click to expand...


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.


----------



## JRaef

garfield said:


> 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.



micromind said:


> 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.


----------



## Vintage Sounds

Thanks JRaef. That's some serious information.


----------



## Jlarson

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.


----------



## KennyW

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.


----------



## CaptKarnage

JRaef said:


> 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.


----------



## bill39

micromind said:


> 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.


----------



## Lyn Wilson

micromind said:


> This is the reason why a lot of tech support is useless.
> 
> Always blame the installer.........


Especially when they don’t follow the installation instructions!


----------



## OKmarts

Here's a case of our client,

The client called and said that the incoming cable of the dynamometer equipment (an equipment similar to VFD, but it can also generate electricity and feed back to the power grid) should be shielded, and the capacity of the dynamometer is about 12x500 = 6000kVA. The initial operation consumes electricity and then generates electricity. Its power supply is required to be shielded. 

After receiving the call, we asked the client to confirm with the supplier that if neutral line is needed for the power supply of the dynamometer equipment. The result is unnecessary. 

So when designing its products, AVL (the world's largest dynamometer) has considered the possibility of using 3 + 3 structure cable.

Check our website for all brands of VFDs and other parts.








Okmarts.com | Industrial supply refrigeration compressor, Servo motor, Transmitter, Light Curtain, hmi, Encoder and many more


Okmarts industrial online store provides various quality industrial products, servo amplifier, servo motor, servo driver, frequency converter, solenoid valve, overhaul kits, Thyristor, soft starter, HMI, refrigeration compressor, Injector, transmitter and many more.




okmarts.com


----------



## paulengr

OKmarts said:


> Here's a case of our client,
> 
> The client called and said that the incoming cable of the dynamometer equipment (an equipment similar to VFD, but it can also generate electricity and feed back to the power grid) should be shielded, and the capacity of the dynamometer is about 12x500 = 6000kVA. The initial operation consumes electricity and then generates electricity. Its power supply is required to be shielded.
> 
> After receiving the call, we asked the client to confirm with the supplier that if neutral line is needed for the power supply of the dynamometer equipment. The result is unnecessary.
> 
> So when designing its products, AVL (the world's largest dynamometer) has considered the possibility of using 3 + 3 structure cable.
> 
> Check our website for all brands of VFDs and other parts.
> 
> 
> 
> 
> 
> 
> 
> 
> Okmarts.com | Industrial supply refrigeration compressor, Servo motor, Transmitter, Light Curtain, hmi, Encoder and many more
> 
> 
> Okmarts industrial online store provides various quality industrial products, servo amplifier, servo motor, servo driver, frequency converter, solenoid valve, overhaul kits, Thyristor, soft starter, HMI, refrigeration compressor, Injector, transmitter and many more.
> 
> 
> 
> 
> okmarts.com


What are you talking about? So if the supply side must be shielded where does the shielding end? Which end it ends need the drain connected? What is the purpose? Do you require IEEE 386 elbow connectors to the transformer? Is this medium voltage? If so it makes sense. Otherwise not. As for structured cabling you can’t be serious? You are really going to push 6000 kVA across 6 CAT 6 cables?

So what cable do you actually specify/recommend? Please use standard IEC or UL terminology and specifications, not some made up AWM.

As to regeneration not being a VFD may I suggest you do your homework? Have you ever heard of an active front end (AFE). Sure the common pump/fan drive with a diode front end can’t regenerate but regeneration in general has been around for decades. It just requires slightly more expensive VFDs that are often used for centrifuges, conveyors, cranes, and various machine tools. In medium voltage drives due to the already complicated multi pulse architecture AFEs are rare but they certainly exist. It’s just that most MV drives are for process equipment that doesn’t need regenerative capabilities.

Some type of power conversion is always at the heart of dynos. They are for torque testing and are effectively large brakes that can perform sustained braking. Even the most primitive just convert it to heat and exhaust it while the most fancy regen and back feed the recovered power back into the device under test.


----------



## SWDweller

Unless your working above 6000 feet above sea level and your greater than 2500v. Corona is not much of a problem. I had a job located at 7500 asl and working voltages were 12.47kv. We had to wrap the exposed bus. The breakers had arc shoots and that contained the arc when opening or closing. 
Also had to plastic bag all of the end of the conductors when high poting.


----------



## paulengr

SWDweller said:


> Unless your working above 6000 feet above sea level and your greater than 2500v. Corona is not much of a problem. I had a job located at 7500 asl and working voltages were 12.47kv. We had to wrap the exposed bus. The breakers had arc shoots and that contained the arc when opening or closing.
> Also had to plastic bag all of the end of the conductors when high poting.


Repeat and simply not true. I can show you lots of photos of cables that have corona damage roughly at sea level. I haven't seen much of any of it at 2500 V (phase to phase) or even 4160 (phase to phase) but by the time you get to around 6 kV or higher, it is definitely there and gets worse as the voltage goes up. It is highly noticeable after a couple years on unshielded substation cabling especially when it goes through panels or through CT windows.

Mild cases of it but this is what it looks like:



https://www.ee.co.za/wp-content/uploads/2016/08/Installations-Malaysian-Switchgear-fig2.jpg



Typically where we see it is when they jam cables into too small of a CT window for a zero sequence CT, or they drape cables through a knife-edge piece of steel instead of cutting and mounting a piece of glastic to keep it away from the grounded edge, or failing to use some kind of spacers to route cable bundles (using C-channel cut to make small spacers with cable ties works well), or just letting the cables lay against the sides/floor of grounded metal substation enclosures without a sheet of glastic or tieing it up neatly to avoid getting too close, or letting cables lay up against each other at the terminations when they are all different lengths or one is twisted up to the point where an unshielded cable lays up against a stress cone or two stress cones lay against each other. How good/bad of a job you do is the difference between having problems in a year or two or a decade or two, and voltages increase the problem exponentially. Starting at around 15 kV for instance is the point where you need to start using a rasp to clean every last rough edge off of the lugs and start wrapping them in semicon where at 4160 it's usually good enough to do a little careful routing and liberal use of glastic.


----------



## sledge

hmmm

I am reading all this information posted above and I am thinking wow are we ever doing it wrong. 
We have literally thousands of vfds that have been operating for around 30 years with absolutely none of these issues considered. Never heard of any of the concerns listed. If we add a vfd we use whatever wire is kicking around, especially in a breakdown. We have run vfds in the past using cab tire as a temporary fix that due to shifting priorities remains in service for years. We mix low voltage and high voltage all the time. We have 3 and 4 door panels with a dozen vfds jammed inside all over the plant. no issues with noise.
The first time I have even heard of it being an issue is with a shiny new seimens controller that has issues with 4-20mA signals due to vfd noise. From what I can see, Seimens stuff is flimsy junk anyway.


----------



## paulengr

sledge said:


> hmmm
> 
> I am reading all this information posted above and I am thinking wow are we ever doing it wrong.
> We have literally thousands of vfds that have been operating for around 30 years with absolutely none of these issues considered. Never heard of any of the concerns listed. If we add a vfd we use whatever wire is kicking around, especially in a breakdown. We have run vfds in the past using cab tire as a temporary fix that due to shifting priorities remains in service for years. We mix low voltage and high voltage all the time. We have 3 and 4 door panels with a dozen vfds jammed inside all over the plant. no issues with noise.
> The first time I have even heard of it being an issue is with a shiny new seimens controller that has issues with 4-20mA signals due to vfd noise. From what I can see, Seimens stuff is flimsy junk anyway.


Number of VFDs close together doesn’t matter. The one thing to avoid there is the VFD SCCR can be an issue. There are models with very good SCCR and others that are quite poor. But you won’t know anything is wrong except they seem to die frequently when a motor takes one out. If the total VFD load approaches the transformer size then the nonlinear current on the line side does affect the transformer (overheating issue) and other electronics. And cap banks are affected faster than transformers.

On the load side it’s a cable length and motor size issue but when it becomes a problem “depends”. At 50 feet of cable or less you can do almost anything without ever seeing any issues whatsoever because the cable is electrically “short”. That is the case with a lot of equipment. Worst case you may have trouble with plant radios nearby. As cable length increases the problems start to show up but it depends on a combination of VFD make/model, motor size, cable type, raceway type, installation, and cable length. So hard rules like “50 feet” don’t work well.

I’d say 95%+ of installations go without a hitch. 5% have issues. I’ve seen drives with 30 feet of rigid conduit, VFD cable, supplemental ground, line and load reactors, EMC filters, and semiconductor fuses on an inverter duty motor that always runs around 45-55 Hz on a centrifugal pump. Was this preventing a problem or ripping someone off? This is compounded by the fact that while many of the multiple vendors involved are selling legitimate solutions to specific issues that do not apply to every situation they also spread a bunch of lies around and attempt to sell their products in places where they do little or nothing. Most of it is mixing some truth and a lot of lies. For example a line reactor reduces triplen harmonics, something a VFD does not generate. It does nothing or very little for other harmonics. It increases line impedance (and losses) if you have an SCCR issue. It has NO influence on the load side. Yet it is rare that I see line reactors installed for the one thing they do well.

The case for VFD cable is far worse. For instance JRaef states that THHN insulation thickness is a minimum of 15 mils. True only for #14. Minimum thickness no surprise increases with size. He then states it can be up to 50% thinner on one side. This is a half a fib because spec is 25%. Then he tries to apply both specs which is a big fat 5 pinnochio lie. It’s 15 mils minimum, period. The.n he throws our very high voltage specs based on line to line numbers. I’ve worked in plenty of plants making Listed product as an employee. UL shows up one day randomly and takes a sample and drives the lab guys nuts. The spec is 15 mils. If your extruder sucks you might have to make it 19 mils and try to keep it under the +/-20%. If you have good control you can cut costs by going to 16

At that point we either take the insulation thickness at 200% (wire to wire) or phase to ground so we divide by 1.732. In any case he continues to ignore the fact that THHN (not to mention THHN-2) equals or exceeds motor insulation with sham evidence. I have seen my share of motors destroyed by VFD reflected waves. Quite often where there is smoke there is fire...pull out the cable and you may find damage. I have yet to see cable damage without a history of motor damage. Simple physics (transmission line theory) predicts how high voltages can get and they are (somewhat) easily measured with the right tools. It cannot be higher in the line than the load without a surge impedance increase exceeding that of the motor. Hence again another lie.

VFD cable can solve grounding issues where it was not properly installed. It can fix phase imbalance issues like very uneven cable lengths where improper installation was done. Shielded VFD cable can fix problems with VFDs sharing conduits, another installation error, and RFI problems from using nonmetallic conduit. In some cases it may prevent damage to an improper install due to line length but not to the motor: What do all of these situations have in common? VFD cable used to fix installation errors.

As an example I have a customer that has six 20 HP screw conveyors mounted almost exactly100 feet cable length from each of 6 VFDs in IMC conduit with grounds. This is almost text book that nothing extra should be required. They used AB power flex drives originally with cheap rolled steel motors. The first motors died and got replaced with 1750 V rates motors which died a while longer. Inspection showed the first couple turns melted. The issue was the installer looked at the print and the layout and saved install costs by running multiple motors in a single conduit. Induced pulses were causing reflections normally seen on cables 3 times longer. Possible solutions were dv/dt filters, run new conduits and wire, or run shielded VFD cable. Customer chose the cheapest option (filters). VFD cable was even more expensive than running more conduits and wire.


----------



## sledge

Wow Great information. After 40 years of doing this, still lots to learn.


----------

