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-   -   Connecting Shielded Cable to motors (http://www.mechmate.com/forums/showthread.php?t=1308)

KevinL Wed 14 January 2009 16:22

Connecting Shielded Cable to motors
 
Is there a preferred approach to connecting the shielded cable to the motors? It appears that Gerald has plugs, but I can't tell how the plugs are tied into the motor. The options seem to be 1) open up the motor case and run the shielded cable all the way inside the motor, or fashion a plug that runs all the way into the motor and somehow attach that plug to the motor. 2) Cut the motor wires relatively short and splice the shielded cable to those wires. or 3) Splice the shielded cable to the motor wires as is.

Does it matter? Are we going to pick up any interference from 6-8" of unshielded wire?

Marc Shlaes Wed 14 January 2009 18:30

Stop! Don't open the motor case! That just about destroys steppers. I'm sure that Gerald and Mike Richards can tell you much more about why / how that is really bad.

Richards Wed 14 January 2009 18:49

I would connect the shield to a common point where all the grounds are connected. That means that I could leave the shield unconnected at the motor. (You only connect one end of a shield.)

The important thing is that you need to have a plan. The theory behind shielding and grounding is that you should have one and only one point where all the grounds are connected. That is sometimes not very practical. What I have done, when building many controllers for various process control computers is to connect all of the grounds and shields within a controller to a common point (usually a block of grounded DIN rail connectors), then I would run ONE and only ONE line from the controller to a common stud where all of the other grounds, shields were connected.

If you use my approach, use heavy wire (14 gauge or larger) to connect the control box to the common ground stud. Use heavy wire (14 gauge or larger) to connect a ground on each axis to a common ground stud. Use a heavy wire (14 gauge or larger) to connect the case of the computer to the common ground stud.

In effect, the common ground stud becomes the grounding point for everything on the machine.

Keep in mind that ALL shields should only be grounded on one end (the end that connects to the common ground stud). All axes should have one end connected to bare metal and the other end connected to the common ground stud. All power supplies should have a ground wire going directly to the common ground stud.

Sometimes that design is not possible. When it is not possible, then use a heavy ground wire (14 gauge or larger) and securely connect any device that can't be directly connected to the common ground stud to the ground stud with that heavy wire.

The goal is to have zero resistance between all "grounded" parts of the machine. Any grounded part that is not zero resistance will impede the flow of electricity and possibly "might" cause a problem sometime in the future.

As you are grounding, remember that paint acts as an insulator, anodizing acts as an insulator, loose or improperly crimped connectors act as an insulator, thin wire acts as an insulator. So, avoid anything that acts as an insulator and you should be fine.

KevinL Thu 15 January 2009 06:31

Thanks Marc and Mike. So it sounds like having a short length of unshielded cable not only will not hurt anything, but is preferred anyway.

Gerald D Thu 15 January 2009 06:45

That short unshielded cable is at least 4" [100mm] away from any other unshielded wires, and that is the main reason that it is technically okay.


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