OT: VFD cables - should I consider them?

Is it worth it to use VFD cable (such as Belden 29503)?

What are the advantages or disadvantages compared to just using individual THHN or eqiv. conductors.

I'm running three VFDs through the same 2" conduit (under concrete floor) along with conductors for one motor not on a VFD. Conduit is 19' (6M) to a junction box that breaks out to the four motors.

I just used them on an install of 9 powerflex 70's. I had an 8x8 cable tray to work with so it was much easier to lay the bundles in the tray vs running individual conductors.


There's alot of theory on the destructiveness of IGBT generated waveforms and the laying of cables and their interference with each other. Also in consideration are the lengths and whether you need load reactors. There's a good whitepaper on the some of that:

http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/drives-wp019_-en-p.pdf

In PF applications I usually follow these guidelines, and I've not had an install fail yet (knock on wood, I have one with 20 drives coming up next week):

http://literature.rockwellautomation.com/idc/groups/literature/documents/in/drives-in001_-en-p.pdf

Pay special attention to Chapter 5 where it goes into detail about why NOT to use THHN.

Thanks Robert. Heres one on me

That was an eye opener - some of the things described are exactly what I have in this installation. I'll be placing an order for the cable today.

I like the whitepaper Robert beat me to posting the second article

We pretty much use them exclusively, but actually for the primary reason of eliminated raidiated noise. We use cables that are 100% copper shielded. Then again, we also make that cable

Shielded cable really only reduces radiated noise. So, if your installation has no sensitive equipment anywhere close and the motor leads are in conduit, you don't gain much.

And, shielded cable can make common mode noise (noise in your ground system) a lot worse due to the added capacitance from the leads to ground. If you have a good stiff grounded wye distribution network and a low impedance ground from motor and drive back to the distribution transformer, you will likely not have any problems.

But, if you have one of those blasted floating delta systems or a high resistance grounded wye or a high impedance ground path, watch out! You will live to regret the shielded cable. You will find yourself installing a grounded wye secondary drive isolation transformer to deal with the ground pulses.

In one of the plants i work at they have 4 VFD's in the same conduit. One thing that has come up is the amount of inductance we see in the wires. I can shut one drive down to change the motoe and still get a pretty good blast from the supply leads, a volt meter usualy shows anywhere from 50-95 volts...This coupled with the amount of water on the floor makes changing a motor a "Shocking" experiance!. to get around this we have to twist all wires together with the ground wire...

Usualy now i run a seperate conduit for each motor..

darrenj said:

Usualy now i run a seperate conduit for each motor..

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My preference as well, but out of the question here. I'm stuck with an existing under a 12" thick concrete floor.

LOL..you do what you can..

I agree with DickDv. The one time I gained from using shielded cable was an installation on a bunch of tire machines in which the wiring was required to be routed along-side numerous 0-10vdc signals, RIO and DH+ communications and lots of digital PLC I/O.

The shields paid off by capturing and draining the interference. During the startup of about the fourth upgrade including new VFDs, an electrician forgot to land the shield on ground at the main panel terminal strip. I had problems with a pressure transducer signal (0-10vdc) until we discovered it and connected it. He asked me if it was okay to hook it up with the machine running, I said sure, it is jsut a drain, right? He already had it wrapped with heat shrink tubing and it was just laying there uner the terminal strip. Surprisingly, when he landed the wire it arc'ed as if it were carrying quite a high voltage. It must have been a 1/2 inch long blue spark. We both thought for a split second that we had just shorted a motor lead, but the drive never faltered, and then my analog input value immediately calmed down.

My current employer has much longer motor lead runs, and dozens of VFDs packed in the same control rooms, and none of them are shielded. (Some of them aren't even very well grounded). They have taken the less expensive approach and isolated and shielded most of the sensitive signals, but even when I run across some poorly done repairs, and many of the "rules" broken with regard to isolation and shielding, we have not had any problems.

If you have to run parallel to low level analog or comms signals for some distance, then it is good insurance, otherwise a waste of money.

I am not a drive expert but we have used THHN for years without any problems. However in all our installations the motor is usually within 5 to 10 feet of the drive. We do use a common mode choke per the advice of Rockwell. Proper grounding is probably the most important issue.

Well DickDV, reducing or eliminating radiated noise is of prime concern here. We have to protect the ultra-sensitive test department from any and all electrical noise, down to the microvolt level. PITA.

Also, the size of the lines we deal with usually requires very long runs of the motor leads, so the cable quality is important.

rta53 said:

I am not a drive expert but we have used THHN for years...

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Same here. I still feel that I have a lot to learn about drives. But you might want to read the material Robert included in the second post. It opened my eyes (does that mean I can't be a Democrat anymore? )

Having read that, if I weren't using a VFD cable I would at least consider XPLE insulation.

The second issue I should have addressed is that using shielded motor leads does absolutely nothing to protect the motor. On long leads, the ringing and reflected wave problems are just as bad as when using THHN or similar individual leads in conduit.

To protect the motor, you need, first, an MG1 Part 31 motor, second, the shortest leads you can run, and third, a lead reactor, dv/dt filter, or sine filter depending upon the severity of the problem.

If you have an MG1 Part 31 motor, a drive carrier frequency of 8KHz or less, and 460V service, a 100hp motor can go out to 250 feet without risking motor damage. A 10 hp motor under the same conditions can go out only 60 feet. You can extrapolate between those two points. Beyond those lead lengths, you need motor lead filters as listed above.

Alaric said:

(does that mean I can't be a Democrat anymore? )

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I knew there was something wrong with you, but for years I just have not been able to figure it out.... but know I got it

Just kidding....