Bundling excess servomotor cable

[cokeguy]

Which is the correct way to bundle excess servomotor power cable? We are installing a machine with 35 small (less than 1KW) servomotors, each one has about 10 or 20 meters excess cable, and we prefer not to cut it. I know that we shouldn´t coil it, so, for example, a possible solution would be to place a Tee rather than a an elbow somewhere in the trajectory, and have the cables go through an extra 5 or 10 linear meters of cable tray back and forth (one or two times?), before routing it back through the Tee to its final destination. Can somebody recommend other alternatives? How many times can we "back-and-forth" power cable through its raceways to absorb excess length without running into the same issues as having a coil? We don´t have much space to maneuver, by the way, and the cable trajectories are pretty much defined by now and cannot change them much because of mechanincal interference with piping and such. Thanks everyone.

P.S. any link to guidelines regarding this issue would be very welcome.

 

[Skogsgurra]

Why should you not coil it?

(I know the answer - you and many others think that you create an inductor by doing that. You do not. At least not one that will influence your servos)

Put your cables where you find suitable - and in any shape you like. The reason is that the cables are "bifilar" or rather "polyfilar" so that any current going to the motor returns in the same cable - no net magnetic field - no extra inductivity if you coil them.

Actually, the only thing that happens is that stray currents will see a slight increase in common-mode impedance. And that is a positive thing. It reduces bearing current problems somewhat.

Gunnar Englund

http://www.gke.org

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

 

[cokeguy]

Thanks skogsgurra, actually I´m not concerned about the servos themselves, but about servocable-generated EMI or similar interferences that could affect nearby resolver or control cables form the same or other machines (at least that´s what I´ve been told by the field service guy from the machine supplier, and I have seen similar recommendation in other servo-driven machinery). What do you think about that, does it sound reasonable, or is it simply some sort of non-fundamented "urban-legend" ? Thanks...

BTW, these are brushless AC servos...

 

[Skogsgurra]

I think that you hit the nail's head - dead on! It is an urban legend. Supported by the fact that inductors are coils. But they are monofilar coils. And the servo cables are not.

What a semi-illetarate service guy says is not the whole truth. It is usually everything but the truth.

If this were an important issue, it would be mentioned in the installation manual. Do you find anything about it there? No - you don't.

Gunnar Englund

http://www.gke.org

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

 

[Skogsgurra]

Ain't this great!? Spelling illiterate wrong!

Gunnar Englund

http://www.gke.org

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

 

[itsmoked]

skogs; I thought you did it on porpoise!

Cokeguy; If you are truly concerned about radiated noise the less the cable the better. Cut the cord. I mean 20 extra meters !!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

Absolutely! The EMI thing is well worth considering. But, on the other hand, your supplier (talking EU and similar regions now) shall have declared some conformity with EMC standards with the existing cables, which he shall have tested regarding EMI. And, if in the US, there is the FCC 15.

So your cable lengths should be fine. Sometimes, the connectors are so intricate that you will make things worse if you cut and re-connect. A good, low-impedance ground connection is sometimes difficult to get without special tooling.

Then, there is the VERY interesting question about total EMI from the machine. I had a machine where many servos worked together and the total conducted radiation was way over any existing limit. But every single servo was under the limit. A case for court to decide. I think.


Gunnar Englund

http://www.gke.org

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

 

[cokeguy]

Thanks for your comments, and you´re right skogs, no mention about that issue in the installation manual. Anyway, I would prefer not to cut the cables because they give us some margin to cut out damaged (burned) cable sections, which is a real psosibility in our process, no matter how well we protect tha cables and raceways. Is it OK to splice and rejoin servomotor power and resolver cables midway through the cable (without undoing the connectors)? Thanks again...

 

[Skogsgurra]

With good craftmanship and care - especially with the screens - I would not hesitate to splice the cables midway.

Gunnar Englund

http://www.gke.org

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

 

[cokeguy]

Skogsgurra, have you seen any reference in the web (besides eng-tips of course) which mentions this things you´ve said about the servomotor cables, not being monofilar and therefore not causing any problems if coiled? Something that specifically addresses servo cables and why there´s no problem with coiling them. Just in case somebody questions the installation, either in-house personnel or field service guys. I´m too bad a teacher (and very rusted in EE theory) to take them through a basic EE textbook and explain it to them theoretically.

Also, what about the recommendation of separating power cables from resolver and control cables on long runs? This particular machine calls for at least 30 cm of separation, and I remember reading somewhere a rule-of-thumb recommending at least 10cm of separation for every 10 meters of cabling. Would appreciate very much your comments on these issue also, is it an urban legend also?... Thanks..

 

[Skogsgurra]

Your last question first: No. That is definitely not an urban legend. Your cables may be very well screened and having good grounds - it is still advisable not to put encoder and other "high-speed" signal cables close to the PWM cables (motor cables).

Certain machine builders do that (in Cable Chains) but they have made tests and are using cables that have shields that allow that kind of "rule breaking". If there is room for separate cable ways, do put them in separate trays or conduits. Mostly to avoid discussions - and probably also trouble.

The first question: I cannot remember any text saying that explicitly. But it follows from basic physical laws, and practical installations verify that it is not a problem.

Gunnar Englund

http://www.gke.org

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

 

[itsmoked]

cokeguy; It is just fundamental electrical theory that if you have both directions of current flow in the same cable the magnetic fields are contained/cancelled within the cable and so no transformer/solenoid/inductor effect is going to occur. Because of this I don't think you are going to find this or any "stated" rule, kind of statement. (now watch someone will)

If you put two arc welding cables on the ground next to each other and strike an arc they will move. This is because they are cables with current flowing only one way in each of them and they react with each other (they usually snap together). If they were in a single outer jacket the 'cord' would not move.

You normally should not leave arc welding cables coiled when welding as then the effect you suspect does occur. But with your drive cables the current is much lower and both directions are contained closely in the same cord.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Skogsgurra]

Now, that's good teaching!

Gunnar Englund

http://www.gke.org

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

 

[cokeguy]

We just started up the 35 servomotor machine which inspired this thread, and unfortunately we began to have some problems with some proportional valves (4-20ma) cabled alongside the servomotors. Of course, some people (including the field service guy from the machine manufacturer) pointed towards the excess cable coiled below the control room´s floor as a possible culprit, since each of the machine´s 120+ cables has about 10 to 20 meters excess length.

Fortunately, the problem turned out to be a software configuration issue and has pretty much been taken care of by now. I had trimmed the 35 power cables during installation, but not the resolver and other assorted control cables. Disregarding my protests, they made me trim 10 multiconductor cables carrying 4-20 ma signals to eliminate coiling, before the real problem was corrected.

In order to better defend myself from other similar situations, I still have a question regarding the previous postings: if there is no net magnetic field sorrounding a servomotor cable, and therefore no inductor effect if coiled, why is it still recommended to keep them separate from resolver or control cables? If there is indeed some small residual noise/interference from the cables, wouldn´t coiling them make it worse, not from the coiling itself but from having more cable bundled within a smaller volume? And finally, please pardon my ignorance again, could coiling of low level signal cables (like resolver cables) make them more susceptible to picking up noise? Again, I´m just asking myself questions that somebody could shoot at me and which I can´t technically respond. I still have a bunch of resolver, 4-20 ma signal, and other assorted digital I/O cable all coiled together in the control room, but trimming them up would be pretty challenging and difficult, because they are multiconductors (30+ wires on some of them) with special connectors on both ends.

p.s. I almost forgot another issue: I have an ethernet cable running in in its own 1" conduit, and for about 20 meters this conduit runs parallel to a tray carrying 5 servomotor cables, about 1/2 meter from the closest of them. Communication is really bad, almost null, so we had to run another ethernet cable to get things going. The cable is physically OK. Could the closeness to the motor cables be causing problems, or is 1/2 meter far enough? These are small motors, less than 1kW, from the same machine mentioned above.

Thanks a lot again in advance for your comments to this novel... I mean post.

 

[Skogsgurra]

From my first answer: "Why should you not coil it? (I know the answer - you and many others think that you create an inductor by doing that. You do not. At least not one that will influence your servos)"

This still holds. The important words are "You do not. At least not one that will influence your servos".

That is not to say that you do not have ANY stray fields from a motor cable. There are three fields; the E-field, the H-field and the electromagnetic field, which is H and E combined and exists only some distance from the cable - and then attenuated by distance. All of these can influence nearby conductors and more so the longer the common stretch is. That is why it is recommended not to put motor cables close to signal cables. It is still being done and that proves that the recommendation is overly pessimistic. But worth following to avoid unnecessary discussions like the one you had recently about coiling cables.

It is only the H-field that is influenced by the coiling. The others are not.


Re the ethernet cable and bad communication: I do not think that any fields stop your communication. An HF potential difference between cable ends is more likely. The potential difference is usually created by stray currents (motor capacitance is the culprit) from motor back to servos. It is sometimes possible to get better communication if you run a heavy copper wire (I have used 16 mm2 and also 35 mm2) along your ethernet cable and clamp it heavily to cable ends/connectors (bare metal, of course).

It is good to have your feed-back. We all learn from this.



Gunnar Englund

http://www.gke.org

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