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Stepper Motor Cables - Shielded or Not?
« on: October 30, 2012, 06:53:44 AM »
I started a previous thread about limit switches and interference - still haven't solved that one, but when I do I'll update with what I've found.

My questions now are:

1.  Do you use shielded or unshielded cable to supply your stepper motors?
2.  If shielded, where do you ground your shields?

IIRC, I have seen a few cnc hardware suppliers that sell 4 core unshielded cable, pressumably for stepper motors.  And from my current experience :) if I used unshielded stepper motor cable, then I may be blissfully unaware of my likely earth loop problem.

Thanks.

Ian B
 
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Ian B

Offline Hood

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Re: Stepper Motor Cables - Shielded or Not?
« Reply #1 on: October 30, 2012, 07:26:27 AM »
I always used shielded when I had steppers and still  do now I have servos on my machines.
Shielding of all wires should be taken to one single point and that point connected to Earth or at least that is the way it is done on all industrial lathes/mills etc that I have seen and it is the way I do it.
I use 24v on my machines for signal wires so that means they are noise resistant and any 5v signal wires such as encoders or step/dir are differential signals so again more noise immune.

Hood
Re: Stepper Motor Cables - Shielded or Not?
« Reply #2 on: October 30, 2012, 08:04:53 AM »
Ian,
I use unshielded and twisted wiring, with no problems.  I always use 0.1 ceramic caps on all inputs (switches, zero plates) to my controllers, and never have false triggering.

 Maybe I'm just a lucky exception :).

Regards,
John Champlain
www.picengrave.com
Re: Stepper Motor Cables - Shielded or Not?
« Reply #3 on: October 30, 2012, 09:11:57 AM »
Thanks for the replies.  I just wish an earthing/grounding problem would leak a benign purple die to help identify the exact location of the problem!

Hood,

I have two questions for you:

1.  When you say shielding of "all wires" do you mean "only the stepper motor wires" or "absolutely all wires" i.e. stepper supply cables, cable between VFD and spindle, limit switch cables, home switch cables, earthing wire to case of PC, earthing jumper wire to VFD body, earthing wire to body of machine?

2.  If you were to put an "X" at the location where all these wires meet, would it be on the frame of the machine, PC case, controller case or elsehwere.

Picengraver,

Could you post a photo or two, or a specific description, of exactly where your capacitors are installed?  I have 4 wire stepper motors, so I assume the capacitors would be soldered between the supply wires to each phase.  The limit switch circuit has only two wires to the controller, but my opto home switches have three wires. 

When I do resolve my problem, I'm going to post about lessons learned and include a schematic of my solution to hopefully help others on this topic.   
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Ian B

Offline Hood

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Re: Stepper Motor Cables - Shielded or Not?
« Reply #4 on: October 30, 2012, 10:49:31 AM »
Now that I use 24v I dont shield most of the cables. Cables that have high voltages in them such as motor power and servo drive cables are shielded. When I was using 5v for signals then I had absolutely everything shielded.
I have control cabinets on all my machines and that is where the shields are connected.
Hood

Offline RICH

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Re: Stepper Motor Cables - Shielded or Not?
« Reply #5 on: October 30, 2012, 11:08:22 AM »
I use shielded cable for the anything outside of the controller. ie; 4 conductor wire with a 100% shield from the controller
to the stepper and do what Hood mentioned.

I am no expert and will only say that most people just don't understand what the shielding is doing.
The shield keeps noise ( noise used loosely and in general ) inside from leaking out or keeps noise from leaking in.
Like water running in a pipe, the shield acts as a barrier to keep the water inside the pipe or takes what is happening outside
from going inside. The shield can be 100% or less, thus like a strainer water could flow through it, but the noodles don't go down the drain.......... if 100% then it's like a pipe but if more open it can restrict what can enter or exit and that depends on what the water is made up. If you have water inside the shield you need to get rid of or minimise the amount of water, wherever it is comming from, and the same goes for outside the shield. Sometimes the water is mixer with oil and you just have a mess on your hands and you are not going to separate the two.

How much water water exists inside or outside the pipe matters and depending on the rate of water flow you would do
things differently to get rid of it. If it only a drip and causes no problems you may be able to live with that condition but if it is a lot then eventualy it gets on whatever is inside or outside the pipe . So anything connected to the pipe sees the water and may cause a problem.

So much for the water analogy and maybe gives a very high level understanding to better understand shielding.

In most cases if one follows the general recomendations for using shielded wiring they will not have a problem.

Now for the experts before they get into the theory of dealing with the water ............
Just what do you do when the roof is new, installed properly , and water is driping into a room and can't find the leak?
Sometimes water / noise  problems are not simple and defies logic........

FWIW,
 RICH
Re: Stepper Motor Cables - Shielded or Not?
« Reply #6 on: October 30, 2012, 12:06:16 PM »
Ian,
Capacitors are not needed on motor wiring (outputs).  They do help on controller inputs such as limit/home switches.  Place a 0.1 ceramic capacitor between each used input connection and a ground connection on your controller/breakout board.

Capacitors on inputs will not resolve a ground loop problem.  It is also possible to get noise into your system through the power mains connections.  An AC input filter device, and/or a UPS system may help if this is an issue.  AC filters are available from Jeff Birt at Soigeneris (www.soigeneris.com).

Regards,
John Champlain
www.picengrave.com
Re: Stepper Motor Cables - Shielded or Not?
« Reply #7 on: October 31, 2012, 01:11:15 PM »
Its worth using shielded wire, with the shield attached on one end only, typically at the controller end at a ground point that is also attached to the machine frame.

mcmaster.com sells shielded multi-conductor cable specifically for cnc use, ie flexible and oil resistant and you can buy it by the foot.

Paul T.
www.springtest.com