4-20mA signal plus VFD motor cables in shared conduit 13

[bentov]

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.

 

[dpc]

The cable from the VFD to motor can be a strong source of EMI, due to the PWM output waveform. We would never put a 4-20 mA signal, or any other analog signal in the same 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

 

[Mobius44]

I agree it's not recommended to share the same conduit, but know many people mix these types of wiring over short runs without problems.

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

 

[rmw]

If you have any unused conductors in the conduit you can ground one end (not both) of the (each) unused conductor and that will assist with shielding. That is, of course, AFTER doing the methods already recommended; use of shielded signal cable. I have used this method when fortunate enough to have the unused cables present and where I was compelled to run the signal cabling in the same conduits and/or cable trays as the power cables.

rmw

 

[itsmoked]

You have to answer the question of what occasional disasters are going to cost you and your company. Note also they will appear at the worst time possible. They will also be intermittent, non-obvious, and time consuming to find and fix.
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

 

[Skogsgurra]

DO NOT DO IT!

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[powersoff]

I would not do it. I have had bad experience with a shielded control cable(4/20ma) secured with cable ties to PVC conduit with motor leads in it.
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.

 

[MikeHalloran]

I thought it was a flat-out NEC violation regardless of the insulation.




Mike Halloran
Pembroke Pines, FL, USA

 

[itsmoked]

No Mike. NEC couldn't care less if an installation functions correctly or not. They are there to prevent inconvenient fires and to a lessor extent electrocution.

If the wire insulation values are adequate then they can share the conduit.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[dpc]

Mike,

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.

 

[jraef]

That's correct. As long as the control cable is rated for as high or higher than the motor leads, it's allowable. Not advisable, but not illegal.

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.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[controlsdude]

This reminds me of a story of an installation where the electricians thought they were smarter than the engineers and also did not comply with the installation procedures and routed the 480vac VFD leads next to 120vac emergency stop signal wire. Not even close to the 4-20mA signal above which btw is much more sensitive.

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.

 

[dpc]

an installation where the electricians thought they were smarter than the engineers

This could describe most of the projects I've worked on.

 

[CJCPE]

jraef: "...most of those people regret it later or (ironically) complain that VFDs are more trouble than they are worth...."

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

 

[resqcapt19]

The use of a an analog control cable in the same conduit as the motor circuit conductors is not always permitted by the NEC even if the insulation on both have the same voltage rating. Many analog control circuits are NEC Class 2 circuits and these circuits cannot be installed in the same raceway as power conductors no matter what the insulation rating is. 725.136(A)

 

[bartb102]

Definitely not a good idea to mix VFD leads with 4-20ma for that long of a run. What is cost of failure to operate correctly vs. one-time cost of installing second conduit? If you are using PVC conduit motor wires should be shielded too.

 

[ozmosis]

The overwhelming feedback is not to do it. Having such noise coming back on the 4-20mA signals will also have the potential of destroying the control board of the VFD.
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.

 

[MikiBg]

I think it's more useful to spread subject here than start a new thread. In my surrounding signal cables are shielded almost by default. The question is when to use shielded power cables from VFD to motor as well. Specially for large drives, with high currents and big cables,where price difference between shielded and unshielded isn't negligible. Furthermore, there are cables with copper and lead shield. The latter is for mechanical protection, but conducting too, weaker electrical characteristic, but smaller price than copper one. When the shield is applied, how is it need to be earthed?

 

[DickDV]

Shielded motor cable has a shield and internal ground conductor that is sized to carry current. Ground BOTH ends, one at the drive frame and the other at the motor frame thus forming a ground bond.

It is essential for proper drive operation to have the motor at the same ground potential as the drive.

 

[MikiBg]

Here is a good document about the subject. There are some practical advices besides laboratory tests.