What is the issue with VFDs and oversizing cables, shields, etc? 2

[bdn2004]

We have installed some 1000V, 3/C shielded, #8AWG cables between a VFD and a 2hp, 460V 3-phase induction motor.

Our client is having us remove the cable, saying it is not a VFD cable. It is grossly oversized. It is shielded. I've seen motors run from VFDs using standard wiring methods, without oversizing or shielding and there hasn't been any issues that I'm aware of.

Can anyone supply some info on WHY the oversizing and shielding is required?

 

[Skogsgurra]

Lots and lots of information available. Look at Belden's site. Look at ABB Technical Guide No 5 and all other VFD supplier's literature.

It is mostly about EMI, but also other issues, too numerous to be even start talking about here. There's an FAQ about VFDs available here. It could be a good start.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[bdn2004]

Attached is a cut sheet of the cable that was pulled. I'll read the literature, but I wish someone would tell me why this stuff won't work.

And how it is you test it to make sure there is really all the havoc they say is happening?

Again, I used to work at a place that changed out a number of FVNR motor starters, replaced with VFDs left all the power wiring in place and it has worked ever since.

Is there a code requirement for VFD cables for VFD fed motors?

 

[dpc]

There is no special NEC requirement regarding motor feeders from VFDs.

 

[Skogsgurra]

bdn

I am looking at this from a European perspective. Here, we have standards that we must comply with. And EU Directives that turn those standards into laws.

If you are in the US, you may very well ignore the EMI part of the problem complex and then have issues with corrupted measurement signals or problem with communication. There's also FCC 15, which may come into play. I'd better let them Americans do the talking here.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[jraef]

That cable would have been fine by me in this application, except for being grossly over sized for a 2HP motor(???). To say it is not "VFD cable" is a stretch in that it has almost ALL of the main salient points one would look for in something marketed as "VFD cable". XLP 1000V insulation, full copper foil shield, separate ground wire inside of the bundle, consistent geometry. But also keep in mind, there is no official designation of what is or is not "VFD cable". That's why I keep putting it in quotes, it is just a marketing term at this point.

The only things that are potentially less than idea (and would be part of most "VFD cable" design specs) are;
[ul]
[li]The ground conductor should be insulated,[/li]
[li]The ground should be the same size as the power conductors (likely irrelevant being that it is so over sized anyway), and[/li]
[li]It is highly unlikely that you can safely terminate #8 conductors on the terminals of a 2HP VFD. For example on our drives, the largest conductor that you can terminate is #10 for that frame size, others are likely the same. So having to add another splice connector / reducer can end up adding a potential failure point. That could also be made irrelevant if you can find a crimp-on fork terminal that will fit under the VFD terminal, but generally fork terminals stop at #10 also. That means you will have to "jury rig" some sort of termination method, not good.[/li]
[/ul]

Download this document and read the section on cable recommendations, starting on page 11. It doesn't go into all of the "why's", but is a good compendium of the important points.

"Will work for (the memory of) salami"

 

[dpc]

Gunnar,

I think there is not really any disagreement. I agree that good quality shielded cable is the right approach - that is what I specify. I was just addressing his specific question regarding Code requirements. The National Electrical Code in the US is mainly concerned with fire hazards and to a lesser extent shock hazards. It doesn't always address proper design considerations in all circumstances.

Installing a system that meets the NEC does not guarantee that the system will work as expected.

Cheers,

Dave

 

[jraef]

... It doesn't always address proper design considerations in all circumstances.

Installing a system that meets the NEC does not guarantee that the system will work as expected.

Such basic truths and yet so poorly understood...

Not a week goes by where I don't have a conversation, usually over some sort of problem or another that could have EASILY been avoided, that doesn't include my saying one of the following two corollaries to that:

"The NEC is a collection of MINIMUM standards for not killing or injuring people or property, nothing more. Just like "Don't point that at your sister!"

And...

"Just because you CAN, doesn't mean you SHOULD, and just because you are NOT FORCED, doesn't mean you SHOULDN'T!".



"Will work for (the memory of) salami"

 

[bdn2004]

Jraef,
Thanks for the excellent link. This is a Rockwell Automation drive also.

We brought up the oversized wire but that got overruled. The only explanation I can think is that they(engr firm) standardized on a that size so that we could purchase a larger amount and feed all the various sizes with the same wire. We indeed had to change out all the motor termination boxes.

The Vfd cable specified is difficult to find. That's why we got the cable we did.
Will a bare copper ground really hurt anything? What?

 

[dpc]

The bare copper is not going to hurt anything, but it doesn't help with EMI. If the circuit is run in a steel conduit, that will certainly help. Okonite C-L-X cable has interstitial ground wires plus the continuous aluminum sheath, so that is a good option as well.

 

[bdn2004]

Just for the record I'm attaching a copy of the portion of the publication Jaerf linked to with my markups on how well the cable complied with that spec. for our application (damp area, sensitive equipment)

I see 6 specific requirements: 1) Copper wire, 2)XLPE insulation, 3)insulated ground wire the same size as the phase conductors, 4)insulation thickness at least 15mil, 5)Copper tape shield 6) Braided shield and ground wire

This cable doesn't comply with 2) or 6)

I called Rockwell Automation technical support (1-440-646-3434) and they looked at it and said this cable could not be recommended.

We're pulling it out and replacing it with the recommended cable. What else can you do?

 

[Padlock01]

Hi bdn2004
In regard to your query on cabling for VSD applications I have attached some literature on types of cabling.
Here in SA we have used 3core PVCSWA + a PVC coated earth conductor since the entry of VSD's.
Not all manufacturers "mourn" about noise pollution and yes one can use 4core screened cable which is imported (costly)

Good luck

Paddy

 

[Padlock01]

bdn2004
See attachment
PADDY

 

[bdn2004]

Padlock01,
Thanks for this link. That cable is similar to what was specified.

Except it has one fatal flaw in our Clients eye....it doesn't have a LSZH low smoke zero halogen outer jacket. And that's what makes it hard to get, where minimum quantities and long lead times are the norm.

 

[bdn2004]

Paddy,
One more thing I read through the article. The way I read it it says you need 3 insulated wires and a centrally located insulated ground wire surrounded by a braided shield. Or a 3/C armored or shielded cable and an additional insulated external ground wire run between Vfd and the motor.

Makes me wonder why we can't keep what we have in place, cut the internal bare copper ground wire off at each end and run an external insulated ground wire?

Also, the cable you link to doesn't look like it meets the spec of that article.... In particular the shield is not braided and the ground wire doesn't look to be in the center of the cable. Or is that your point?