VFD cable

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
Pump installer called me about a VFD and a different installer. 3rd hand service call? Question is about VFD cable being used with a submersible. The installer did not use on load side as instructions suggested/required and the VFD has failed. Is that a typical fail or is it more inclined to the motor or cable?

I suggested the installer was SOL for failing to install per instructions, regardless.
 

Jraef

Moderator
Staff member
Location
San Francisco Bay Area, CA, USA
Occupation
Electrical Engineer
More info today.
The drive maker wants them to use plain old UF cable out to the well head. Not drive cable.
Where is the VFD? At the well head, that would make sense. Nobody cares too much how the line side of a VFD is wired, it’s the load side that matters.

VFD cable is not a “scam”, but it is largely unnecessary if you are using steel conduit and only one conduit per VFD output. That will not be the case on a submersible, so I can see a mfr requiring it to help reduce the cable capacitance. Cable capacitance charging current, because it is basically instantaneous, can damage the transistors in the VFD if it is excessive.
 

paulengr

Senior Member
Pump installer called me about a VFD and a different installer. 3rd hand service call? Question is about VFD cable being used with a submersible. The installer did not use on load side as instructions suggested/required and the VFD has failed. Is that a typical fail or is it more inclined to the motor or cable?

I suggested the installer was SOL for failing to install per instructions, regardless.

NO drive requires VFD cable. None of that cable is rated for submersible use. That’s a complete load of crap.

I’ve seen others used but the most common submersible cable is type THW. Some people, mostly mining, use type W. It is surge tested to 2.2 kV by spec. We’ve used it for years, VFD or not. There are major reasons NOT to use a VFD with a submersible having to do with bearings and cooling design issues.

Hands down if a VFD failed due to non-use of a VFD cable, it’s time to rotate manufacturers. It really is quite that simple. First thing to check is your available short circuit current and the VFD rating. Also realize that VFDs are not that hardy, don’t like dirt/dust/humidity/heat/frost, and are very intolerant of crappy power.

But given that THHN which is thinner tests to 2850 V as per NEMA testing, I suspect this is very low. Given that the motor will be rated 1000-1400 V and surge test below the cable surge test, it seems like the VFD cable spec is a load of crap. The other argument is that somehow a rounded cable in a jacket will develop less asymmetrical shunt capacitance, enough to make a difference. I routinely measure motor performance, dozens of motors per month. I look for current imbalances like that as well as common mode currents. Guess why I often see common mode currents? Maybe because a VFD by nature outputs common mode currents!! Far more than you will get from even some pretty bad wiring jobs.

Those are the major claims. The rest are such a joke it doesn’t bear repeating. And yes the purveyors of this load of crap are companies like Allen Bradley, Belden, Alpha, and more. This is one of those things where if someone brings it up, escort them off the plant site with security and contact legal and purchasing.

At around 100+ feet there is a problem with VFDs called reflected waves. The distance is slightly affected by shielded wiring. When I say shielded wiring I mean surrounded by a metal covering such as a conduit. It does not require VFD cableThis is a real issue. The solution is to use a dv/dt filter which is fairly small and cheap. This increases the limit to over 1000 feet. But if you ignore this issue it destroys motors, not VFDs.
 

garbo

Senior Member
Pump installer called me about a VFD and a different installer. 3rd hand service call? Question is about VFD cable being used with a submersible. The installer did not use on load side as instructions suggested/required and the VFD has failed. Is that a typical fail or is it more inclined to the motor or cable?

I suggested the installer was SOL for failing to install per instructions, regardless.
The large hospital/research center that I retired from had over 500 VFD'S from 2 to 1,750 HP of which not one had overpriced special VFD cable. At least 75 drives replaced starters from the 1970's along with plain Jane old NEMA frame motors. Some of these motors were over 30 years old and never rewound. They were smart in placing over 90% of the drives within 25' of the motors. All the drives used standard THHN/THWN building wire in EMT conduit. Control wires were always in a separate conduit. So in my opion I think that the VDF cable is not usually needed. We always paid a vendor a small fee to perform start up when we installed or replaced drives. They never questioned the use of normal building wire. Another good thing about having a vendor perform the start up they extended the warranty ( free parts & labor ) out to 3 years.
 

augie47

Moderator
Staff member
Location
Tennessee
Occupation
State Electrical Inspector (Retired)
FWIW: Nassau Cable shows a VFD Cable "suitable for submersible pump applications"
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The large hospital/research center that I retired from had over 500 VFD'S from 2 to 1,750 HP of which not one had overpriced special VFD cable. At least 75 drives replaced starters from the 1970's along with plain Jane old NEMA frame motors. Some of these motors were over 30 years old and never rewound. They were smart in placing over 90% of the drives within 25' of the motors. All the drives used standard THHN/THWN building wire in EMT conduit. Control wires were always in a separate conduit. So in my opion I think that the VDF cable is not usually needed. We always paid a vendor a small fee to perform start up when we installed or replaced drives. They never questioned the use of normal building wire. Another good thing about having a vendor perform the start up they extended the warranty ( free parts & labor ) out to 3 years.
Had pretty much the same experience at a plastics manufacturing plant, except the runs between the drives and the motors always exceeded 100'.
 

Besoeker3

Senior Member
Location
UK
Occupation
Retired Electrical Engineer
I think all VFDs these days are PWM.

What difference does the frequency make?

I kind of agree if you are going to run cable it should be a VFD cable, but running THHN in metal conduit is functionally about the same thing.
We don't use THHN. We normally use steel wired armour, usually three phase with earth a half sized earth wire.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
We don't use THHN. We normally use steel wired armour, usually three phase with earth a half sized earth wire.
SWA would not have the EMF containing ability of either VFD cables or metal raceways.

It is basically what we would call tray cable with steel wire spiral wound around the conductors under a plastic jacket.
 

Besoeker3

Senior Member
Location
UK
Occupation
Retired Electrical Engineer
SWA would not have the EMF containing ability of either VFD cables or metal raceways.

It is basically what we would call tray cable with steel wire spiral wound around the conductors under a plastic jacket.
It isn't a cable tray. No sable tray involved. I don't know how to describe it other that STWA.
 

paulengr

Senior Member
SWA would not have the EMF containing ability of either VFD cables or metal raceways.

It is basically what we would call tray cable with steel wire spiral wound around the conductors under a plastic jacket.

Coaxial cable is a central conductor surrounded by another metal conductor. It is not required for it to be continuous. In fact with CAT 5 or 6 the shielded version is called ScTP, short for screened or mesh. In coaxial cable charges on the conductor create a mirror charge which perfectly cancels the electrical field of the inner conductors. The shield is very thin and cannot possibly “block” the high frequency signal because the thickness is much smaller compared to the wavelength of the contained signals. SWA or type MC in North American terms is a spiral wrap of steel which does the same thing as conduit. The RF signals travel along the surface owing to the skin effect.

Regardless since all 3+ phase conductors are tightly bundled together shunt capacitance is very small and their fields will overlap unless there are ground paths other than down the cable which could create a loop antenna radiating a magnetic field.
 

Besoeker3

Senior Member
Location
UK
Occupation
Retired Electrical Engineer
Coaxial cable is a central conductor surrounded by another metal conductor. It is not required for it to be continuous. In fact with CAT 5 or 6 the shielded version is called ScTP, short for screened or mesh. In coaxial cable charges on the conductor create a mirror charge which perfectly cancels the electrical field of the inner conductors. The shield is very thin and cannot possibly “block” the high frequency signal because the thickness is much smaller compared to the wavelength of the contained signals. SWA or type MC in North American terms is a spiral wrap of steel which does the same thing as conduit. The RF signals travel along the surface owing to the skin effect.

Regardless since all 3+ phase conductors are tightly bundled together shunt capacitance is very small and their fields will overlap unless there are ground paths other than down the cable which could create a loop antenna radiating a magnetic field.
We wouldn't use coax conductors for VSD drives.
 

Phil Timmons

Senior Member
Location
DFW
Occupation
Depends on the pay and the day
Conversation has sort of drifted to Best Practices -- but back towards the start and WHY to follow the Manufacture (Warranty) Requirements -- it is generally to ensure that the Owner / Customer can enforce a Warranty Claim, and that was they hired us (Licensed Contractor / Electrician) to install, which is typically part of the warranty requirements, as well. Some require that we install Line Filters, as well. Sort of becomes a go-along-get-along thing for compliance. We just take pictures, write the invoice to match the warranty requirements, and tell the owner to file it all with the drive manual.

Only really bad thing I have noticed in cable selections is from the amount of cross-talk (really power line "talk" to a signal line) that VFD cables can create. Since as someone above noted these are (all?) or at least mostly Pulse Width Modulated (PWM) from high(er) frequency sources -- even though they present typically something like 60 hz (full speed) down to 20 hz or so . . . there are really pulsing freqs in the 10 kHz range, and those can radiate enough signal to create noise on other control or signal lines. Shielded VFD cable is supposed to reduce this noise.

So what I have observed from Allen-Bradley E-Stop circuits (24V) that are run along with the VFD feeders -- say 100 or more feet -- from Hazardous Areas, back to safe areas, where the VFD is located -- the VFD cable "noise" can be so severe on the 24V E-stop circuit that the opto-isolator LED in the VFD control may still think there is signal (from the E-Stop) even when the E-Stop is hit (open) -- so the VFD and Motor keep running. Bad Thing.

Fix for that was shielded (and grounded on VFD end) 24V E-Stop cable, and routing the E-stop cable away from the cable tray with the VFD power cable line(s).
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
It isn't a cable tray. No sable tray involved. I don't know how to describe it other that STWA.
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.
 
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