VFD Cable

Since VFD's are becoming so popular with modifying existing equipment, the installation requirements are sometimes on the back burner. I have a 3 Hp motor and only have single phase (230vac) at my home shop. I am installing a VFD to power the motor. I do not have sensitive equipment that would be impacted by emf noise, other than a laptop. How critical is using VFD cable for an install. The cable run is about 20' from the VFD to the motor. Line reactors?

Well....20 feet is not much. That said, that doesn't mean you can ignore all the requirements that the VFD manual will set forth. For example, keep the VFD feed lines to the motor away from other cables...don't run them in the same conduit. Use shielded cable for the VFD control leads, if you are controlling the VFD from someplace other than the VFD face panel. Etc.

A line reactor? Not likely needed.

i have several vfds. first i followed "the instructions". lately i dont. no shielded wires, nothing. never noticed anything bad. but then again my shop is free standing and i dont listen to the radio. use notebooks regularly.

I have 3 VFD’sin my shop. No shielding, conduit, line reactors etc. I do use a laptop, iPhone, and iPad routinely in the shop with them running. Never had an issue. Just my experience.

Denis

I use equipment that is sensitive to it so my VFD's for my inhouse builds are installed in steel enclosures with galvanized backplane, VFD cable, EMC cable glands, shielded signal conductors, and my attempt at proper wiring practices. Doing this has fixed the issues I've had in the past with VFD's.

VFD motor cable can be bought at fair price from automationdirect.com cut to length and delivered to your door in 2 days.

I've done some without any of that that didn't bite me, but now if I can I install it as best as I'm capable of and it solves future me problems.

If you bundle the wires, or use multi-wire cable, and include a ground in the wire bundle that is connected from the VFD to the motor, you will have a lot less EMI emitted.

Shielding is nice, but in many cases is not the cure-all that it is often hyped as. Some installs with shielded wire can blast out more EMI than a good install with a wire bundle or multi-wire cable.

JST said:

If you bundle the wires, or use multi-wire cable, and include a ground in the wire bundle that is connected from the VFD to the motor, you will have a lot less EMI emitted.

Shielding is nice, but in many cases is not the cure-all that it is often hyped as. Some installs with shielded wire can blast out more EMI than a good install with a wire bundle or multi-wire cable.

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I know a bit about VFD cabling. I helped develop the VFD cable that Service Wire sells. I will try to keep it short and sweet - condensed version.

There are 3 sources of stray currents associated with motors and VFDS. The first is circulating currents caused by distributed capacitance between the stator and rotor. Fix is a properly insulated bearing on opposite drive end of the motors. 3 hp motor - don't worry about it.

Second is stator to rotor capacitance current flow. Danger is taking out the bearings in the motor (fluting) and potentially getting stray current in your equipment. Fix is a ground brush on drive end of motor. 3hp motor - ignore it.

The last and most significant is stator to frame capacitive current flow. This is generally the most disruptive and can corrupt your grounds, cause noise in surrounding electronic equipment and wipe out bearings in motor and other equipment. Solution is a shielded motor lead cable with shield properly bonded to drive backplane and motor frame. The shield is a low impedance return path for the stray currents back to the drive and also prevents crosstalk. Even for a 3 hp motor, it is important.

In testing, I fried both motor bearings on a 240 volt 3 phase 1 hp motor in 70 hours. Lots of energy there.

For 3 hp motor, buying cheap VFD cable is best, but you can put it in metallic conduit and just twist 6 leads together (3 phase leads and 3 ground leads is best - concentric grounds are best but not critical) and put them in the conduit. Or at least use MC (metal clad) cable that you can buy at Lowes. In your case should suffice. Just make sure the shield/conduit/MC is bonded to the drive backplane and motor frame. I use VFD cable.

All of those are forms of "common mode" current. That typically IS the current that makes the most noise in an installation.

That's the reason I suggested the ground wire. The "common mode" (CM) current does not go out on one of the three wires and come back on another, the way 3 phase is supposed to. It essentially goes out on all three wires, and has to find another way back. Since it is often due to very small differences in timing of the VFD pulses, it tends to be high frequency.

If there is no ground wire in the bundle, then the current has to come back in some other path, which has typically got a good deal of "loop area" since the path will be along the surface of the machine, or down a ground wire, etc, and not necessarily close to the wire bundle. Loops make good antennas.

If you include a ground wire, and connect it at both ends, then that is the "lowest energy" path for the current to return on. If it is included in the bundle (or multi-conductor cable), then the loop area is very small, and the emission of noise will be small.

The ground wire can be just a wire, or it can be the shield on the cable. Either works well.

A few dollars a foot is not much money. Total cost would probably be more than your VFD. That is what hurts the most.

The generated RF will have something to do with your carrier frequency. And the quality of the import components in the drive.

Requirements in my VFD manual:

- In all cases, long parallel runs must be avoided.
- Do not use cable with an insulation thickness less than 15mil.
- Use copper wire only.
- Do not reduce wire gauge when using higher temperature wire.

Then there are 2 pages on Unshielded/Shielded/Armored Cable.

One thing in there:
For many installations, unshielded cable is adequate, provided it can be separated from sensitive circuits.
As an approximate guide, allow a spacing of 0.3 meters for ever 10 meters of length.

“My car runs fine with bald tires. I have driven 80MPH on bald tires and not had a wreck. Tire treads are over rated.”

Jraef said:

“My car runs fine with bald tires. I have driven 80MPH on bald tires and not had a wreck. Tire treads are over rated.”

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Hmmmmmm.

Well if the steering is bad, or the brakes don't work, I;d say the tires are not your issue.

Yes, if not installed right, VFD cable is over-rated, and not a cure-all. And open wiring, or unshielded multi-core, if properly installed, will beat a crappy install of shielded cable.

I've done a LOT of EMI mitigation. Everything from fixing total disasters to ground-up designs for very low emission. Low power, higher power, etc.

We made a 150 MHz clock processor board that when laying on the table and working, was only 3 dB over the emission limit, with no enclosure, no shield, nothing. We were a little disappointed in the 3 dB, but it was actually very good. That took only some correct usage of the normal components, just the bare PCB, loaded up and working.

Good design works well. The other kind is commonly a bit different.

I disagree with the idea that under 3hp you don't need to mitigate common mode current to prevent bearing damage, because I have seen numerous failures of small 40watt HVAC inverter fan motors with fluted 608 bearings.

Some details on those motors: the steel stamped shell of the motor which holds one of the bearings is typically grounded. The other bearing is pressed into plastic, and so there is no direct conduction path. Typically the magnets are ceramic and are insulated from the shaft. Sometimes the magnets are potted in a soft rubber, other times ceramic magnets are glued onto the face of the steel rotor stampings.

I have also started to see embedded magnets inserted under the surface of steel rotors... what this means is, neodymium magnets plus taking advantage of saliency is now cheaper than ferrite magnets. I'm not sure this is a good thing with regard to how fast we're going to run out of neodymium, but that is an environmental topic.

The inverter circuit board and the windings are encased in injection molded plastic.

What I have not seen is fluted bearings on the concentrated pole inverter fan motors. i find this to be interesting. proportionally they have less surface area of windings exposed to the rotor. the rotor is ceramic magnets glued on the face of the steel rotor. These motors can fit 1hp at 1250 rpm into the same 56C frame that you can only reach 1hp at 3600 rpm via standard induction motor.. and can do so at like 80% efficiency.

photo of a 4 pole 6 coil concentrated winding in this paper:
Noise Reduction of Swing Compressors with Concentrated Winding Motors | Semantic Scholar

Jraef said:

“My car runs fine with bald tires. I have driven 80MPH on bald tires and not had a wreck. Tire treads are over rated.”

Click to expand...

The sky is falling! The polar caps are melting.
It's been 20 years since my first VFD went in without consideration for anything but proper grounds and length.
Builder's wire for everything. No issues... I'll die a happy man.

BETTER, is the mortal enemy of good enough.

johansen said:

I disagree with the idea that under 3hp you don't need to mitigate common mode current to prevent bearing damage, because I have seen numerous failures of small 40watt HVAC inverter fan motors with fluted 608 bearings.

..................................

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Motor power has less to do with the problem than supply voltage and carrier frequency. A 480V motor has considerably more issue than a 230V motor.

Some 230V seem not to have issues, some do. Pretty much any 480V will run into problems. The higher voltage just pushes more CM current.

Lower carrier frequency will reduce problems..... fewer pulses per second means more time until a problem becomes evident.

Scruffy1949 said:

I do not have sensitive equipment that would be impacted by emf noise, other than a laptop. How critical is using VFD cable for an install.

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I think of the millions of cells in my body as sensitive to emf. I got a good surplus buy on some of this:

https://www.lapptannehill.com/lapp-...multiconductor-cable-14-awg-4-conductor-black

so do you ground one end of shielded cable or both?

dian said:

so do you ground one end of shielded cable or both?

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Conventional approach has been to ground only one end, to avoid ground loops.

A section from a Allen Bradley VFD manual:
(The Shield terminal is on the metal plate where all wiring leaves/enters the drive box)

Shield Termination

The Shield terminal provides a grounding point for the motor cable shield.
The motor cable shield should be connected to this terminal on the drive (drive end)
and the motor frame (motor end). A shield terminating cable gland may also be used.

When shielded cable is used for control and signal wiring, the shield should be
grounded at the source end only, not at the drive end.

I work with large equipment in a bunch of different fields. I dont really see any "requirements" being upheld on any equipment except for Drives controlling servo motors. VFDs, without extreme controlling, with minor thing, dont get any special treatment.

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