VFD Cable With dV/dt Filter

[bwat]

I've seen a few topics on here about shielded VFD cable vs something like XHHW in metallic raceway vs etc... but I don't think I've see this specific topic. Does anybody here have experience or knowledge on the necessity of shielded cable or metallic raceway when a dv/dt filter is used on output of VFD and/or how this filter impacts this need? I know there are a number of VFD gurus on this forum that could probably weigh in on this with some insightful info.

I have a scenario where it's a decently long run from VFD to motor.. maybe >600 feet.. and it will be underground so I instantly want to go PVC, so then that would push me back toward having to use shielded cable. But because of the length, I'm using the dv/dt filter. So that makes me wonder if this dv/dt filter negates the need for shielding via cable or metallic raceway.

I have no skin in the game cost wise on the cable, so I'll do whatever is right, but I just don't want to tell the owner and contractor that we have to plan to use a bunch of VFD cable if it's unnecessary because of the filter.

Thanks in advance.

 

[__dan]

It's probably a question of required vs recommended, and that will vary with the drive model and manufacturer. If you phrase the question that way to the customer, it is their money and they can give informed consent to spending it.

So the question would go back to the drive manufacturer instructions and engineering support. Some drives are rated for the distance with no filter and some not. That's where the drive manufacturer may say, recommended (but not required). XLPE, XHHW is a good adder for the better insulation, if it's a single load in conduit the shield may not be doing much, but some customers will just not spend half a nickle more than they want to.

If the manufacturer recommends but does not require it, imo it is a decision the customer makes about their money. The specific drive model will have a rating for load side distance and cable type. Models vary a lot in what they are rated for.

 

[Jraef]

The dv/dt filter will mitigate SOME of the issues that shielded VFD cable addresses, but it does NOT mitigate RF issues and cable problems such as inconsistent geometry and resulting capacitance problems. You can further mitigate the geometery / capacitance issue by "triplexing" or twisting the conductors together before pulling them, but then you are just trading your labor time in exchange for the difference in conductor cost, while still running the risk of creating problems with RF interference. It's just better to stick with using shielded VFD cable if using PVC conduit.

 

[bwat]

Thanks for the solid replies.

Interestingly enough, digging through one of the manuals for the drive (Siemens) that might be used I see that they actually permit longer lengths if you go unshielded and don't care about EMC. And I don't happen to see anywhere in this particular manual that specifically states that you must use shielded or metallic conduit, but it hints at it several times that you should be using shielded cable and then offers this table which almost seems contradictory.

 

[Besoeker3]

I've seen a few topics on here about shielded VFD cable vs something like XHHW in metallic raceway vs etc... but I don't think I've see this specific topic. Does anybody here have experience or knowledge on the necessity of shielded cable or metallic raceway when a dv/dt filter is used on output of VFD and/or how this filter impacts this need? I know there are a number of VFD gurus on this forum that could probably weigh in on this with some insightful info.

I have a scenario where it's a decently long run from VFD to motor.. maybe >600 feet.. and it will be underground so I instantly want to go PVC, so then that would push me back toward having to use shielded cable. But because of the length, I'm using the dv/dt filter. So that makes me wonder if this dv/dt filter negates the need for shielding via cable or metallic raceway.

I have no skin in the game cost wise on the cable, so I'll do whatever is right, but I just don't want to tell the owner and contractor that we have to plan to use a bunch of VFD cable if it's unnecessary because of the filter.

Thanks in advance.

We always used XPLE armoured with no problems. And no raceways.
That is cross-linked polyethylene.

 

[Jraef]

The key there is "No EMC category"... If you are not in North America, that is basically not going to happen.
If you are in North America, then although we don't have the same EMC regulations, the issue still comes down to RFI emissions from the cables. Don't make the mistake of thinking that RF is not a problem just because the conduits are underground. My standard story on this was when some military police triangulated on a job site I was working on from over 20 miles away based on the RF signals coming from 5HP VFDs run without shielding from the VFDs to the motors. Our entire machine was underground at the time. FM radio signals pass through the earth; FM = Frequency Modulation, which is basically what a VFD is doing on the output. Those output cables are radio broadcast antennae.

 

[bwat]

We always used XPLE armoured with no problems. And no raceways.
That is cross-linked polyethylene.

Good info. Isn't MC XPLE almost the same thing as XHHW in metal conduit? Same insulation and shielding concept at least, right? And I think VFD cable is usually XPLE as well with the metallic shield of course.

It might be a much better price point to go with MC XPLE though if that's what you're getting at. And I may be able to forget the PVC.

Am I crazy for being cautious about direct burying cable, even if it's MC? It's hard for me to distinguish sometimes between my actual good practices and me being stuck in the mode of "that's just what we do".

 

[bwat]

The key there is "No EMC category"... If you are not in North America, that is basically not going to happen.
If you are in North America, then although we don't have the same EMC regulations, the issue still comes down to RFI emissions from the cables. Don't make the mistake of thinking that RF is not a problem just because the conduits are underground. My standard story on this was when some military police triangulated on a job site I was working on from over 20 miles away based on the RF signals coming from 5HP VFDs run without shielding from the VFDs to the motors. Our entire machine was underground at the time. FM radio signals pass through the earth; FM = Frequency Modulation, which is basically what a VFD is doing on the output. Those output cables are radio broadcast antennae.

As always, very insightful. Thank you. I think I remember seeing your military triangulation story a time or two on here. That's a good one to remember.

 

[Besoeker3]

Good info. Isn't MC XPLE almost the same thing as XHHW in metal conduit? Same insulation and shielding concept at least, right? And I think VFD cable is usually XPLE as well with the metallic shield of course.

It might be a much better price point to go with MC XPLE though if that's what you're getting at. And I may be able to forget the PVC.

Am I crazy for being cautious about direct burying cable, even if it's MC? It's hard for me to distinguish sometimes between my actual good practices and me being stuck in the mode of "that's just what we do".

You probably know that I'm from UK.
I'm not familiar with XHHW cable. We don't as a rule bury cables. If you have to run from one building to another, is is normally run in a cable trench.

 

[bwat]

You probably know that I'm from UK.
I'm not familiar with XHHW cable. We don't as a rule bury cables. If you have to run from one building to another, is is normally run in a cable trench.

Ahhh the cable trench. Yes I was aware of you being from UK and some of the differences, but didn't know about the cable burying and cable trench. Thanks again.

 

[Besoeker3]

Ahhh the cable trench. Yes I was aware of you being from UK and some of the differences, but didn't know about the cable burying and cable trench. Thanks again.

Welcome sir!
A little further background that you may already know. Pretty much all of what we as a company did was industrial power electronics. We did the design, build, test, and commission. Mostly, installation and cabling was done by others.

 

[Jraef]

You probably know that I'm from UK.
I'm not familiar with XHHW cable. We don't as a rule bury cables. If you have to run from one building to another, is is normally run in a cable trench.

XHHW (XLPE High Heat Wet environment) is a designation for cable using insulation made from Cross-Linked Poly Ethylene (abbreviated XLPE, hence the X).

XLPE is considered superior in VFD output applications to the more common THHN (Thermoplastic High Heat Nylon jacketed) building wire because XLPE is made as "shrink tube" and shrunk onto the wire, as opposed to the Thermoplastic (PVC) that is injected around the wire as a liquid and will contain microscopic bubbles. Those bubbles can become weak points when subjected to corona discharge due to standing waves in PWM outputs.

 

[Besoeker3]

XHHW (XLPE High Heat Wet environment) is a designation for cable using insulation made from Cross-Linked Poly Ethylene (abbreviated XLPE, hence the X).

XLPE is considered superior in VFD output applications to the more common THHN (Thermoplastic High Heat Nylon jacketed) building wire because XLPE is made as "shrink tube" and shrunk onto the wire, as opposed to the Thermoplastic (PVC) that is injected around the wire as a liquid and will contain microscopic bubbles. Those bubbles can become weak points when subjected to corona discharge due to standing waves in PWM outputs.

Thank you.

 

[paulengr]

XHHW (XLPE High Heat Wet environment) is a designation for cable using insulation made from Cross-Linked Poly Ethylene (abbreviated XLPE, hence the X).

XLPE is considered superior in VFD output applications to the more common THHN (Thermoplastic High Heat Nylon jacketed) building wire because XLPE is made as "shrink tube" and shrunk onto the wire, as opposed to the Thermoplastic (PVC) that is injected around the wire as a liquid and will contain microscopic bubbles. Those bubbles can become weak points when subjected to corona discharge due to standing waves in PWM outputs.

XLPE has bubbles too aka voids. ALL plastics do. Not sure where you git this information. At one time XLPE had a problem with water trees because it would soak up water and start partial discharging. This has been solved by TRXLPE (tree resistant...). There is a ton of information out there about the issues of voids in XLPE. It applies to ALL thermoplastics and thermoses polymers though as long as they are formed via cross linking.

The big difference is that the smaller wire sizes (14 and 12) on THHN/THWN are down to 15 mils plus or minus a couple mils. If you gouge up the nylon jacket while pulling and scuff the PVC you can reach a point where there is simply not enough insulation to prevent partial discharge, something not normally see in 609 V cable. Pure (not triple rated) MTW is even worse because it’s THHN mints the nylon jacket. In contrast XHHW (1 or 2) starts at 20 mils and goes up from there. So at the two smallest sizes either be careful when pulling or switch to XHHW.

Plenty of data out there shows that the breakdown voltage on THHN is around 2809 V and goes up to 3200 V on XHHW but since the highest rated motors are around 1700 V and really we need to design for replacements so 1200 V, these stories about VFD cable magic properties are hogwash.

There is something to be said about voltage unbalance and it is better in a magnetic raceway (MC, RMC) but the big thing is keep your cables together and run a supplemental ground cable from drive to motor. On small motors bearing currents are not generally an issue. As you get up over 250 HP or so this starts to matter no matter the shielding. We automatically install insulated bearings in motors starting at 1000 HP because even without a VFD the bearing currents even from small winding imperfections and end rings tend to cause bearing fluting. Fluting problems increase with motor size naturally. And that’s where a grounded magnetic raceway really makes a difference.