4-20mA signal plus VFD motor cables in shared conduit
4-20mA signal plus VFD motor cables in shared conduit
(OP)
I know it's considered bad practice, question is practicality & likely worst case trouble arising . . .
We retrofit small (5-15hp 240V) ag drainwater pumps with VFD's (PWM, V/hz) & 4-20mA/24VDC 2 wire level transmitters (pressure membrane piezoelectric transducers), run PID for constant level control. The typical setup is a pump panel, short underground run (10-40ft) of PVC conduit to the sump location containing motor conductors plus existing LL conductors (to float switch or induction relay probes). We've done 5 so far, pulled shielded sensor cable through the existing conduit, seem to be working OK.
I see quite a spectrum of practice/suggestions on this, from 6" minimum separation/all steel conduits/shielded motor power conductors all the way to a suggestion that 4-20mA signals are bullet proof, don't care about higher frequency interference. Our experience tends towards the latter, but I definitely haven't put a scope to it, also don't camp out with these installations or log them for hiccups - I figure components are inexpensive, consequences of inadvertent operation are minimal, what the heck, let's see what happens.
Now though, we might ramp up, do lots of these and also some new complete installations. I'd hate to have to go back to 100 sites for warranty rework (especially if the transducers suffer damage somehow, most expensive part of the whole setup), but also don't want to add (significant) cost by insisting on separate conduits, different material, etc., if in fact it isn't important for this application.
I'd be grateful for any suggestions from those of you with experience in similar applications.
We retrofit small (5-15hp 240V) ag drainwater pumps with VFD's (PWM, V/hz) & 4-20mA/24VDC 2 wire level transmitters (pressure membrane piezoelectric transducers), run PID for constant level control. The typical setup is a pump panel, short underground run (10-40ft) of PVC conduit to the sump location containing motor conductors plus existing LL conductors (to float switch or induction relay probes). We've done 5 so far, pulled shielded sensor cable through the existing conduit, seem to be working OK.
I see quite a spectrum of practice/suggestions on this, from 6" minimum separation/all steel conduits/shielded motor power conductors all the way to a suggestion that 4-20mA signals are bullet proof, don't care about higher frequency interference. Our experience tends towards the latter, but I definitely haven't put a scope to it, also don't camp out with these installations or log them for hiccups - I figure components are inexpensive, consequences of inadvertent operation are minimal, what the heck, let's see what happens.
Now though, we might ramp up, do lots of these and also some new complete installations. I'd hate to have to go back to 100 sites for warranty rework (especially if the transducers suffer damage somehow, most expensive part of the whole setup), but also don't want to add (significant) cost by insisting on separate conduits, different material, etc., if in fact it isn't important for this application.
I'd be grateful for any suggestions from those of you with experience in similar applications.





RE: 4-20mA signal plus VFD motor cables in shared conduit
In addition, we generally recommend special shielded power cable for the VFD output circuit as well as use of steel conduit for this circuit to reduce radiated noise.
It sounds like you are not having issues to date, so you are probably unconvinced, but I would not do what you are doing.
Cheers,
Dave
RE: 4-20mA signal plus VFD motor cables in shared conduit
The 4-20mA wiring voltage rating must be adaquate for running in conduit with the motor line voltage.
The 4-20mA wiring (and the VFD-motor wiring if you want to be extra cautious) should be shielded.
In applications ranging from fractional to 100HP VFD's powered from 120-600VAC I've never encountered enough interference to measureably affect a 4-20mA signal level. I can't say the same about 0(2)-10VDC signals. However I have witnessed a few cases where the 4-20mA wiring was not shielded and the interference caused a 4-20mA controller to experience faults and other erratic operation due to the interference.
Sean
RE: 4-20mA signal plus VFD motor cables in shared conduit
rmw
RE: 4-20mA signal plus VFD motor cables in shared conduit
If your customers have any electrical smarts at all your company will lose much technical integrity as they will initially read the VFD's manual, which will certainly have in bold letters near the front, "DO NOT ROUTE CONTROL WIRES NEAR THE MOTOR LEADS!" Of course it will all get much more serious when one of their employees are injured because the drive does something unexpected...
Keith Cress
kcress - http://www.flaminsystems.com
RE: 4-20mA signal plus VFD motor cables in shared conduit
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: 4-20mA signal plus VFD motor cables in shared conduit
When VFD controlled motor would start speed reference was all over the place and would eventually level off. But not before causing chaos.
Also if control cable in same conduit as line voltage could be a NEC problem if cable does not have proper rating.
RE: 4-20mA signal plus VFD motor cables in shared conduit
Mike Halloran
Pembroke Pines, FL, USA
RE: 4-20mA signal plus VFD motor cables in shared conduit
If the wire insulation values are adequate then they can share the conduit.
Keith Cress
kcress - http://www.flaminsystems.com
RE: 4-20mA signal plus VFD motor cables in shared conduit
As Keith said, NEC allows mixing of any voltage levels up to 1000 V as long as all conductors are insulated for the maximum voltage present in the conduit. In years past, we used to run 120 V discrete control wiring in the same conduit with 480 V motor leads on constant speed motors (clarifier drive motors). The motors were small and at 120 V control, we never had a problem. Analog signals in same pipe as PWM output is another story. Still legal, but not a good practice.
RE: 4-20mA signal plus VFD motor cables in shared conduit
Of the hundreds and hundreds of VFD trouble calls I have done in my career, I'd venture to say that 75% have been from signal corruption of some sort. People do it all the time, and most of those people regret it later or (ironically) complain that VFDs are more trouble than they are worth. Keith's earlier allusion to "Murphy's Law" is absolutely correct.
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RE: 4-20mA signal plus VFD motor cables in shared conduit
1st thing that happened was the Estop signal bulb just glowed during run. Normal running state was bulb off. Estop the bulb would be on.
2nd thing that happened was that the system would stop on its own.
Other things happened that could not be explained but you get the picture.
Only thing we could do at the end besides ripping out the wiring was to put MOVs to knock off the noise that the VFDs created on the Estop wiring.
I would listen to what Keith is saying, I experienced some of things he is describing.
RE: 4-20mA signal plus VFD motor cables in shared conduit
This could describe most of the projects I've worked on.
RE: 4-20mA signal plus VFD motor cables in shared conduit
Some complain vociferously to the VFD manufacturer and others that they have been sold a piece of junk, rip out the VFDs, and buy a competitors product which they quietly install correctly.
CJC
http://www.vfdriveinfo.com
RE: 4-20mA signal plus VFD motor cables in shared conduit
RE: 4-20mA signal plus VFD motor cables in shared conduit
RE: 4-20mA signal plus VFD motor cables in shared conduit
I have seen this time and again. If it doesn't destroy the board, you get spurious problems on the VFD; the VFD will bring up a fault or warning message and then the blame is then left with the VFD as the 'messenger' of bad practice.
Your VFD instruction manual, whatever the make, will advise against this. If it doesn't, then there is a problem with the manual or the company who writes it.
RE: 4-20mA signal plus VFD motor cables in shared conduit
RE: 4-20mA signal plus VFD motor cables in shared conduit
It is essential for proper drive operation to have the motor at the same ground potential as the drive.
RE: 4-20mA signal plus VFD motor cables in shared conduit
RE: 4-20mA signal plus VFD motor cables in shared conduit
Shielded cable does a pretty good job of suppressing capacitively coupled interference. It has limited impact on magnetically coupled interference. At a point 'remote' from the cable the magnetic fields of the conductors interact and largely cancel each other, but 'near' the conductors (i.e. touching or close to the cable sheath) the local magnetic fields do not fully cancel in the immediate vicinity of the conductors. Having a signal cable in the area 'near' the cable is inviting problems. The best antidote to magnetically coupled interference is distance, while other options such as mu-metal screening are effective but expensive, or just impractical.
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