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My 4'x8' machine motor wire length recommendations

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Re: My 4'x8' machine motor wire length recommendations

Postby servant74 » Mon Dec 27, 2010 3:23 am

Even for limit switches, stranded is best. Most Ethernet 'patch cables' are stranded.

BTW, for stepper power I found a seller on eBay that sells 18ga 6 conductor stranded in multiples of 10' length pretty reasonable. If you order 6 10' cables, you get one 60' chunk unless you request otherwise. - I am just a happy customer... When I purchased it was just under $.60/ft plus $8S&H

http://cgi.ebay.com/CNC-Stepper-Motor-Hook-UP-WIRE-18-6Sh-Buy10-increments-/330507884761?pt=LH_DefaultDomain_0&hash=item4cf3ce18d9#ht_500wt_922
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Re: My 4'x8' machine motor wire length recommendations

Postby airnocker » Fri Jan 21, 2011 6:11 am

I agree, for these machines one should only use stranded copper wire.

For those who want to know why, Google "wire skin effect", but in essence alternating current signals tend to flow only in the outer edges of a wire. A single strand of copper wire yields the worst case scenario while stranded wire helps to greatly minimize this issue.

I know that most tend to place their stepper drivers long distances from the steppers themselves. This increases cabling costs considerably due to the need to run them or the BOB signal cables within shielded cables.

My stepper motor cables are only as long as they came originally because I put the drivers within that distance.

Believe me, you will spend far less money on rubber grommets to mount the drivers near by and provide a modicum of vibration isolation than all the cable costs needed to do otherwise. If anyone would like more insight to my strategy let me know.

So now my two cents have weighed.
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Re: My 4'x8' machine motor wire length recommendations

Postby Awesomeness » Fri Jan 21, 2011 7:55 am

I'd like to hear more about your strategy, but I think some of your assumptions to conclude that it's far cheaper to keep the drivers closer to the the motors aren't totally correct. I got 4-conductor, 18ga, unshielded stranded copper wire for $0.50/ft, and used about $25 worth. So at maximum, if I incurred no other costs by switching methods, the most I could save is $25. But you're still going to use some wire (less than $25 worth), and then you'll need a longer parallel cable, and certainly some other things such as the rubber grommets you mentioned. Especially in the context of a $5000+ machine, even the full $25 is of trivial impact.

On the other hand, the argument that it's a smarter design to have them closer probably holds much more water. Keeping the wires from the driver to the motor as short as possible maximizes the power provided [across the analog signal]. Since the signal across the parallel cable is digital, communications received at any power level that is decipherable is sufficient. The driver-to-motor power loss can be overcome with larger cabling, and 18ga is already overkill for 3A@36V (wire spec for 4-conductor 18ga is 6A capacity). So then it becomes the design trade off between the "convenient" design of having all the drivers far from the motors but conveniently located, or the "more efficient" design of getting the drivers as close as possible to the motors.

Personally, more importantly than that issue, I wish I'd have put some connectors where the cables meet the motors, instead of soldering them. I actually looked around for some good ones, but couldn't really find anything I liked. Molex 4-pin connectors, like those used in a PC to power the CD-ROM drive, are something I considered. In the end I got impatient and just soldered them so I could play, :D .

On a side note, I've had no issues with signals, that required I use shielded wire. I know some others have, but I haven't personally.
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Re: My 4'x8' machine motor wire length recommendations

Postby Beermkr » Fri Jan 21, 2011 2:30 pm

I soldered the first machine but after changing one of the motors out and having to deal with all that, the second machine uses a "European Terminal Black" from Radio shack. They have 12 positions and can be cut down to any size you need. Each pair of connections has a mounting hole through it that a #4 wood screw fits.

R/
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Re: My 4'x8' machine motor wire length recommendations

Postby barskey » Fri Jan 21, 2011 4:58 pm

About connectors: I used some Molex 4-pin connectors from Fry's. I put a male connector on the motor, and a female on the controller. This way I could test my motors/controllers on the bench (*REPORT ME I'M A SPAMMER* table) by plugging them in directly. Then I just made a long "patch" cable consisting of wires in a split-loom tube with molex connectors on each end.

If I ever need to disconnect my motors for trouble-shooting or replacing a part (think upgrades), it is simple.
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Re: My 4'x8' machine motor wire length recommendations

Postby airnocker » Fri Jan 21, 2011 5:32 pm

I think some of your assumptions to conclude that it's far cheaper to keep the drivers closer to the the motors aren't totally correct.


I apologize for not making my point more clear. It is not nearly so much about cost, but it has everything to do with low-power and high-power signal transmission management in a way that minimizes radiated or induced electro-magnetic noise and keeps it out of low-power control lines. A side benefit is that this will typically keep cabling and cable management costs lower since ALL signal paths are lower.

All my control electronics are mounted on the table. The only cables that go to or leave from the table are power cables or the parallel cable:

Stationary cables
1. A standard length parallel cable
2. USB 5V regulated DC cable to opto-isolated BOB (computer-side)
3. 5v regulated DC cable from wall transformer to opto-isolated BOB (control electronic-side)

Cables that move with X-gantry travel and plug into power a few feet from table
4. 6ft. 110VAC power cable to 36V driver supply (also mounted on table on X-gantry
5. 6ft. 110VAC power cable to router or router speed controller


This idea or concept isn't necessarily everyone's "cup of tea" and I'm not trying to convert anyone but rather put some sound engineering concepts out there for consideration.
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Re: My 4'x8' machine motor wire length recommendations

Postby Awesomeness » Fri Jan 21, 2011 11:35 pm

Those are all good points, from a scientific standpoint. I wonder if there is actually cause for concern though. I've got about 15' of cable between the motors and drivers on my system, and all the cables are bundled together. Realistically speaking, all my functionality is fine, despite the "issues" that technically exist. So I'm curious how much impact there really is, and how severe it would have to get before it was a real problem, not just a theoretical one.

The radio antenna on my truck creates drag and makes it go slower than it could without one. Yes, that's a totally ridiculous analogy, but sometimes I feel like people are trying to fix problems that may be trivial, or at least the impact is unclear without further testing/study.

According to my pocket reference, 18ga wire has a resistance of 1 ohm for 153.6 ft. That's pretty low.
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Re: My 4'x8' machine motor wire length recommendations

Postby airnocker » Sat Jan 22, 2011 12:20 am

You are talking about "if there is actually cause for concern", I'm not raising concerns.

I'm merely talking about best practices and their benefits.

There are many, many ways to skin a cat and wind up with perfectly good results.
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Ethernet cable 'power connector' ?

Postby servant74 » Sat Feb 19, 2011 12:14 am

I have been thinking of making a PCB that would have screw terminals on one side, and places to plug in up to 3 RJ45 sockets for ethernet cables to allow using them for sending power over longer inexpensive cables. Would that be of interest to anyone?

I totally agree that in general sending stepper type of power over ethernet cables isn't a good idea. But if you calculate the 'circular mills' of the wire used in ethernet cables, using two wires together is a bit conservative for the power normally carried. And putting 3 cables in parallel rather than just 2 would add more current carrying capability.

Not that ethernet cable (the flexable, stranded wire cable) is best, but it could be an inexpensive easy option. Still, keeping the wires as short as reasonable is a good idea ( 2, 2.5 and 3 meter, roughly 6, 7.5, 9' cables are pretty cheap on ebay, and I like the pre-molded ends :) .. )
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Re: My 4'x8' machine motor wire length recommendations

Postby airnocker » Sat Feb 19, 2011 4:19 am

It's feasible, but will you save money in the process when you factor in the RJ45 jacks, plugs and terminal strips, and end up with solid power connections. But it is a nice idea that you should explore and give your results.

Low voltage, low current control lines will work great over RJ45 connections.

14 AWG Silicon Wire from Robot Marketplace runs 99 cents per foot and is extremely flexible. 16 gauge runs 60 cents per foot. I'd vote for solid terminal-to-terminal connections for stepper driver power.

BTW: you can buy BOBs with RJ45 and terminals from CNC4PC in addition to RJ45 terminal boards that mate right up the the BOB terminals.
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