OT: When is VFD cable recommended/required?

[seth350]

It sounds obvious, I know. The cost of good VFD cable for many motors over a modest distance is quite pricey.

The manuals say to always use a type of shielded cable rated for VFD applications.

In your experience, what requirements would an application have that require VFD type cable when using an induction motor and a VFD?

Always?
When motor HP is greater than X?
Cable length greater than X?

I ask because it seems to be excessively expensive and labor intensive to run VFD rated cable for many motors under 1HP, no more than 70’ from the cabinet.

[James Mcquade]

Before we go there, what voltage is the motors?
if 240v , no line / load reactors required.

if 440,460,480, then you need to install line and load reactor for motor leads longer then 25 ft - based on specs I read. the reflected power wave can go back into the plant power bus and create issues.

when using vfd's, you should use vfd rated motors. the motors are designed to work with vfd drives. standard motors can work, but will burn up faster.

you use shielded vfd cable to eliminate electrical noise, especially in control cabinets where the plc I/o are.

I will let other users chime in, please correct me if I an wrong.

james

[bulletin blues]

Yesterday I downloaded a motors e-book included in a rockwell newsletter that had info on why to use vfd cable. You have to enter user info, but I don't think you need a tech connect.

https://info.plantservices.com/ebook...trical-systems

[seth350]

Supply voltage we use is 480VAC and we do always use VFD rated motors with class F insulation with VFDs. I also have a line filter feeding the VFDs, no load reactors.

My question is generally when a project is spec’d, what makes you use VFD shielded cable versus the lesser?

We have OEM machines that have come in without VFD shielded cable on this project. I have spec’d a conveyor system using Belden 1000V VFD shielded cable for 24 0.5HP motors. The conveyors will transport product to and from all the other OEM machines.

Seeing their machines without the VFD cable made me doubt my decision.
In terms of dollars, was I correct to use the VFD cable over non shielded? Would I have cut a corner if I had not used the VFD cable?

[bulletin blues]

Well, in my experience, we had a motivated electrician who decided to replace about 20 standard overloads, all motors off of one drive, with the new and improved... I can't think of the term right now... "motor circuit protectors" over a weekend. They are all easily over 100 feet away, the furthest over 200 feet away. They are approximately 0.5 HP. Standard THHN through conduit. They were installed in the 1980's. The reason was that the existing overloads are located at the bottom of the cabinet and it is difficult to see which one is tripped, difficult to isolate etc. After replacement and testing, they all tripped randomly. Poor guy had to go back and re-install the originals. He's not as motivated any more.

I did some research and it appears that placing the overloads closer to the motor might have helped. Now that we're on the subject, maybe running shielded cable may help, but that will likely never get re-visited

That's my experience with NOT using VFD cable.

So, from my experience, be careful with the Motor Circuit Protectors, as they seem to be sensitive to longer distances.

[dogleg43]

If the motor and or the VFD manufacturer recommends using VFD cable then I would use it. Otherwise, if tech support is required they will probably want to use the lack of VFD cable as being part of the problem.

Speaking from experience.

[rupej]

Rockwell has a paper that talks about it. IIRC, it clearly stated that it wasn't necessary unless noise would cause an issue to neaeby devices. It did not suggest any risk to drive or motor by not using VFD cable.

[jraef]

IMHO and experience you do not need shielded cable IF you use STEEL conduit and rigorously ground it (use grounding bushings, etc. etc.) and you use ONE CONDUIT RUN FOR ONE MOTOR (i.e. don't run the wires for 4 motors inside of one conduit run).

ANY other way of running the conductors will require shielded cable; PVC conduit, cable tray, duct banks, gutter etc. I've seen an installation where just 25ft of a 300ft motor run was coming out of the steel conduit, laying in cable tray (next to the input cables) and back into steel conduit to the motor, but in that 25ft of exposure it picked up enough induced voltage to cause damage to the windings of an inverter duty motor.

If the mfrs tell it to you in this way however, they can catch blame for a failure if you were not fastidious about your conduit run and grounding issues, so they just do a CYA statement telling you to always use shielded cable because although expensive and redundant, there is nothing wrong with shielded cable inside of steel conduit.

Also understand that for the VFD mfrs, the products are going everywhere in the world and in most of the world, they do NOT use steel conduit as much as we do here in North America, so for them, it is more important.

[dogleg43]

Jref, I agree with all of your points. As a system integrator we often just supply the VFD, controls, and do the programming (and are responsible for the VFD system working correctly). Usually the motor and field wiring are done by an EC who is subcontracted to the GC. So it gets tricky trying to convince them what type of field wiring to run.

We’ll normally forward to them in writing what the VFD manufacturer requires and tell them we can’t be responsible if things don’t work because they (EC) did not follow the suggested requirements.

[seth350]

Thank you jraef, I was hoping you would give your personal experience. I opted to use a steel wire basket with a separator for motor cable, I/O, and comms. Using the wire tray, I felt it was necessary to use a shielded cable for the motors. The IO and comms are also shielded.

So maybe I did a half decent job after all.

[PLC Pie Guy]

Steel conduit or teck at very least. But around here, I always have the guys use VFD cable if there is a drive involved. Just to many headaches over the years and replacing of motors, drives and other sensitive equipment in the areas... due to people using WTF ever is laying around to connect motors. Usually what they grab isn't even rated for our wet environment. The VFD cable is rated for our environment and I have yet to see a piece fail.

Also, its likely that whatever other cable you would use is likely only rated for 600V..... Not even close to the possible output you could see from today's drives! 600VAC in does not equal 600VAC out on a 3 phase drive!

We don't even have this conversation anymore. Its a no brainier. Especially if somebody else is footing the bill!

[Rson]

Maybe it is just my industry, but I use cables / receptacles for every motor under 10 HP.

The Turck cables (I'll have to look up part numbers later) are pretty awesome and not super expensive. I think I pay about $150 for a cable, a receptacle for the panel, and a receptacle for the motor. Then you just plug-and-play.

Most of the time though, my motors are within 2-4 meters of my enclosure. Prices definitely get more expensive the further away the motor is.

Also, many times my enclosure ships separate from the machine, and I don't always want to leave wiring the motor to the end user. With the cable / receptacle combo, I can at the very least plug in the motor for testing before it leaves our facility. The $150 pays for itself in this convenience.

And, if it needs to be relocated, just buy a longer cable!

[seth350]

Thank you guys for sharing your experience. I feel much more confident.

VFD Cable = No Brainer

[jraef]

Originally Posted by PLC Pie Guy

...
Also, its likely that whatever other cable you would use is likely only rated for 600V..... Not even close to the possible output you could see from today's drives! 600VAC in does not equal 600VAC out on a 3 phase drive! ...

Good point and I should have brought that up. Even if you use steel conduit, I now recommend using XHHW conductors, not THHN or THWN (on the OUTPUT side, input side doesn’t matter). You want Cross Linked Polyethylene (XLPE) insulation, not PVC (as is used in THHN) because PVC van have microscopic bubbles in the plastic that weaken the insulation capability in the face of higher voltage spikes in VFD outputs due to reflected waves. XLPE insulation is made differently, like heat shrink, so there are no bubbles or weak spots. That’s why you usually see it with 1000V or 2000V insulation ratings. It’s more expensive, but less expensive than replacing the cables later. Most good VFD cable will use XHHW for the conductor insulation, only the outer jacket might be PVC where it doesn’t matter.

[seth350]

Thank you jraef, I will remember that.