Question about grounding stepper motors

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22 Nov 2022 16:29 #257347 by dlv
Pardon the potentially dumb question but I'm a little confused about one thing; Grounding stepper motors.   I read this is important, so I've done it, but I'm not sure why and I'm worried about ground loops.

I have been testing my system without yet having connected the plasma machine.  All is working very well.  I have all my stepper motors grounded with a 5th conductor that runs in my cables from to the motors.  These attach to the body of the stepper motors.  Those cables are also shielded, with shields only connected at the control box, not at the table.   Grounds and shields are star connected at the control box.

My concern is that is by grounding the motor bodies this way I am effectively providing multiple ground paths between the plasma table and my control box, through those stepper cable ground wires.  Isn't that a possible source of ground loops?  The metal control box will be (not yet) mounted on the side of the table, effectively giving yet another ground path to the table too.  I could use isolation mounts if that's bad.

Also concerned what that means given those motor grounds are then also electrically connected to the plasma torch clamp (through the slats, table etc. to the motor mounts). There is no continuity between the clamp and the safety ground as I expected because I assumed the plasma arc is an isolated supply.   If there's any potential between the plasma clamp and earth ground then that's going to go through those ground wires of my stepper cables.   I don't know if this is bad or not.  Perhaps that's even the reason for grounding the motors?  Should I be earth grounding my table?

Or maybe I am worrying about nothing?

FWIW I am in the US and there are earth ground rods at my service box of my shop. 

Cheers,

-dave

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22 Nov 2022 18:56 #257355 by tommylight
To much worry, probably.
Reading about anything on the net has become a minefield, way to much wrong informations floating around.
From what you wrote, everything seems fine, but here are some tips about testing stuff in a "safer" way:
-do not connect the plasma to the machine
-have the machine run some gcode,
-while the machine is running, fire the plasma far from the machine
-fire it on air
-do some cuts
-press the nozzle against the scrap metal and fire the torch
If all is well, the machine will run it's course without stutters or stops.
Repeat all above but close to the machine control box, 2-3 feet far is ok.
Wire the THC, repeat same tests.
Wire the plasma, repeat.
Arange the plasma cables, repeat.
That way if there is any issue, you know where to look for it.
--
Plasma machine working in a fully metal encosed shop, it shuts off a monitor on the other shop. Several other monitors in both shops, including the one on the machine work OK, but that one blanks the screen every time the torch fires.
The reality is, plasma is a nasty interference machine, you might never have issues with basic grounding, but you might also have issues with everything done properly.
Do some tests.
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22 Nov 2022 19:38 #257362 by dlv
Tommy as always thank you so much. Those are great tips.

I must say, I wonder a tiny bit about my neighbors and if I'll start to hear some talk about strange interference on their TVs at certain hours of the day. I'll have to play dumb. LOL
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22 Nov 2022 19:58 #257367 by tommylight
LOL, nice one! :)
Then again, TV's are made to work with an antena so have good protections against interference.
Wait, do new TV's have antena inputs?
Oh hell, i know a 40" Telefunken has one, no idea if the 55" LG has one!
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22 Nov 2022 20:25 #257372 by rodw
I don't think your 5th wire shoud connect to the motor. Leave it disconnected both ends.
If you find the hypertherm tech note on plasma table grounding, they say to add  a seperate (can be unshielded) wire from each motor frame back to a star ground point on the table frame. Other wires should be added that bridge any motion slide from the torch clamp back. The intent is to provide a good path for any EMI to drain away without it having to pass through motion slides etc. The star ground should connect to the machine ground, the mains ground, the work clamp, the control box ground and any grounding stake (not required for non HV blowack).
Also its critical to have an EMI filter on your control box mains connection. I found most EMI entered the control box via the mains earth connection. 
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22 Nov 2022 22:55 #257391 by dlv
Hmmm,  Interesting.  I found the PDF you're referring to Rod.  Seems that HT best practice of grounding motors & rails to the table is when the table is grounded to earth, so EMI can drain away.   *BUT* it also says not required if using a non-HF blowback start.  I do have a non-HF blowback start machine.  

And yes I also do have an EMI filter where the mains connection enters my control box.

That search for the PDF also led me to more discussions on the subject and as Tommy said, there's a lot of conflicting information out there.  A number of folks (plasma spider) say they don't bother earth grounding the table at all and run just fine.  

Well, I guess it's clear as mud.  It is easy to disconnect the ground wires from the motors so I can try it both ways using the fault isolation process Tommy suggested and see what works and what doesn't.   I think (hope?) it won't fry anything if there is EMI, just cause errors and things stop.    

Still working on actually welding up the table frame.  Taking longer than I anticipated and now we've got the US Thanksgiving holiday upon us that'll distract me with family visiting and cooking turkeys.  I won't be able to test any of this until that's done.    
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