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My friend (a Mechanical Engineering student) is building machine that uses an array of motors in an application where they are under almost no load. He showed me the thin, old, ribbon cable he intends to use to power them, and it seemed like a bad idea to me.

The thing is, although I am an Electrical and Computer Engineering student, I have very little background in power systems and electromagnetism. It seems like using flimsy and thin ribbon cable to supply power to a motor is a bad choice, but aside from it possibly breaking from being a little brittle, I can't justify why. It just seems wrong.

So my question is, what are the limitations that these little ribbon cables are putting on the motors? Am I just being snobbish or is there a compelling reason beyond mechanical concerns why they are a bad choice?

Edit: The first answer to this post pointed out some of the possible advantages to ribbon cable. Thanks! In the specific case I was taking about, my friend is just using two lines on the cable for the positive and ground of each motor, but using several lines on each cable makes a lot of sense and had not occurred to me before.

Sebastian Immel
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    It depends on motor current and length of the cables. Fundamentally, there's nothing wrong with ribbon cables. – Nick Alexeev Mar 06 '13 at 07:45
  • In this specific case, I'm sure it's an okay solution to his problem. However, I was hoping for a little insight into the concerns that come into play with cables in power electronics. For example, if the motor were drawing an amp and separated from the power supply by 10 meters, would a thin ribbon cable run into trouble? – Sebastian Immel Mar 06 '13 at 07:54
  • Although I'm happy with the accept, it is bad practice to accept answer so quickly. If you wait accepting an answer, you'll probably get more people to take a look at it and likely get different view. Say after a day (this is a website visited from all over the globe) you select the best answer. – jippie Mar 06 '13 at 08:01
  • Thank you for explaining the etiquette to me. I hope you don't mind that I un-selected your answer. I've used the stack exchange engine a lot, but never as an active participant. Your answer was really helpful though! – Sebastian Immel Mar 06 '13 at 08:08
  • I don't mind the unselect. I think it is important that people get as many good quality answers as possible (and in turn I may learn from them myself). If an answer is helpful, you can upvote it. If (say after a couple of days) an answer is really good, than you can accept it. – jippie Mar 06 '13 at 08:18
  • As soon as I have 15 reputation I will certainly upvote your answer. It seems obvious now, but it genuinely didn't occur to me that you could supply power in parallel along a ribbon cable like that. Very cool. – Sebastian Immel Mar 06 '13 at 08:21
  • I stand corrected. I immediately upvoted your question. – jippie Mar 06 '13 at 08:36
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    Ribbon cable tends to have mechanically delicate conductors when they are not supported by the outer. Clamp ends so the bare wires never flex. Provide some form of strain relief if they provide a moving connection. – Russell McMahon Mar 06 '13 at 09:31

2 Answers2

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array of motors ... under almost no load

Depending on the motor, even under low load current draw can be substantial. The current draw will of course increase under load, more so in stall conditions.

using flimsy old ribbon cable to supply power to a motor is a bad choice

Not necessarily true: Inside your CD ROM drive you may well find motors with a full load current flowing through FPC / FFC connections along the way, even flimsier than a ribbon cable. It depends on the current drawn and the rotation and PWM frequencies (where applicable).

Also, hard disc drives (not SSDs) have motors with substantial spin-up current requirements, and their supply current comes in via the same ribbon cables (IDE cables, for instance).

is there a compelling reason beyond mechanical concerns

There is, if the motor is being controlled via a very high frequency PWM signal, or is itself a high speed motor: The signals would radiate EMI, if not shielded - The recommendation in such cases would be shielded twisted pairs, at the extreme. However, for a hobby project such a consideration is most likely maximum overkill.

Conclusion: If the ribbon can handle the current, nothing wrong with using it.

Anindo Ghosh
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  • Ribbon cables are often arranged as signal/ground/signal/ground alternating - would that reduce the effective radiated EMI? – pjc50 Mar 06 '13 at 09:34
  • @pjc50 If the signal and ground lines were alternated, this would reduce EMI. Using shielded wire would reduce emission almost completely (except from the junctions), and using twisted pair would reduce risk of interference (common mode) from _inward_ EMI due to other sources. – Anindo Ghosh Mar 06 '13 at 09:39
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Ribbon cable can be a good choice:

  • If current isn't too high (as @NickAlexeev states), check the AWG number on manufacturers website. (eg. AWG28 => ~ 1A and 0.2\$\Omega\$/m)
  • You can use several conductors in parallel to increase maximum current and lower loss in the cable.

Advantage:

  • You can use even conductors for powering the motor and the odd conductors for return current. This lowers electromagnetic emission, which means lower interference with other electronics.
  • The cabling looks tidy.
jippie
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