VFD cable

Besoeker3

Senior Member
Location
UK
Occupation
Retired Electrical Engineer
I know what it is. I've used it before. Not for domestic use though. It's not used in the US.

I was trying to describe it to people who might not have run across it. It is similar to what we call tray cable except under the outer jacket there is a spiral wound wire that serves as both an armor and I think is an equipment grounding wire, or whatever they call it over on the other side of the pond.

We don't really have a cable like it here.
My field was/is industrial. Paper mills, cement mills, petrochem and the likes.
 

garbo

Senior Member
We don't use THHN. We normally use steel wired armour, usually three phase with earth a half sized earth wire.
Worked on hundreds of older Danfoss VLT drives & newer model FC 200'S. Both had IGBT'S that could produce up to 5,000 hz switching. Never had a problem using standard THHN/THWN building wire in steel EMT conduit and always either Greenfield or steel jacket seal tight flexible conduit last few feet motor. So firmly believe that frequency has no effect on running condition, motor or drive length. Often got over 15 years on VFD'S wired that way. Some times had to adjust think it was called switching frequencies on motors making objectionable noise. I was always concerned the most on locations & environments where drives operated. Most of our drives were located in clean fairly dust free locations and we blow them out with a portable air compressors 1 to 4 times a year during PM'S . When contractors were doing demo work we made them tape on filter media on drive intake & even the exhaust of cooling fans on drives and made them replace it at least once a week. A clean cool drive is a happy drive !
 

garbo

Senior Member
At the large hospital/research center that I retired from had over 500 drives from 3 to 1,750 HP and none had the expensive shielded cable. Some of these drives replaced motor starters and plain Jane basic NEMA motors from early 70's with no problems. Always ran basic THHN/THWN basic building wire inside of steel EMT conduit and a full dise copper ground wire. Most VFD'S were located within 100' of motor. I asked the extremely talented drive tech about using the expensive shielded cable and he thought that it just about every case it was a waste of money. Our newer drives above 40 HP were 18 pulse who both drive company techs did not like them. Always wondered if the 18 pulse drives affected use of shielded cable.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
Always wondered if the 18 pulse drives affected use of shielded cable.

No, that would have no bearing on it. The 18 pulse is on the front end (input) rectifier of the drive, the need for shielded cables is based on issues with the output from the high speed switching of the transistors.

Anecdotal evidence is widespread, but is often based on the early decades of VFD use, when a 50Hp VFD was the size of a refrigerator and the old BJT transistors had a turn-on time that was 200 times slower than modern IGBT transistors have now. You can no longer apply those anecdotes to what happens in those output circuits today.

That said, the fact that MOST installations in this country use steel conduit does mitigate most of the risks involving EMI/RFI radiation from the cables, so long as you stick to having one VFD output cable set per conduit. PVC conduit and cable tray is a whole different story. But still, that leaves the distance /standing wave issue to contend with and that involves other extenuating circumstances, so luck can play a part in it appearing to not be an issue.
 
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