Lets Talk Wiring And EMI

So how do you know if you have EMI problems. I did not use any shielded cables in my build and everything seams to work peachy. I’m running a 30XP so no Hz start. Also my THCAD10 is in my main control cabinet.

If you have EMI problems things won't work correctly. You might fire the torch and the computer will reboot in a more obvious example, but you can have any number of other odd symptoms. They will pretty much all happen only when the torch is cutting - often when it is first fired.

Shielding is not needed for every external wire - I have never seen a problem with unshielded wires from between drives and stepper motors. Those are high current, and it would take a lot of noise to cause any problems there. Shielding on those wires might be good in cases to reduce noise created by the high-power step currents from radiating out. I'm not sure if you might be better off with ferrite beads though - PCW would know a lot better than I about that.

Now for an HF start high-power machine, you do have to be more careful and aggressive with grounding/shielding/ferrites etc. I have little experience with that myself though.

I think a lot about EMI/RFI has been written for HF start machines. Running a 24 volt logic system as we do with Mesa cards is a big step up from the lower voltages of the Mach3 community

Designing for noise starts with good control box layout

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Mains power segregated from the rest on the top right corner (and top left of door)
48 Volt power to steppers routed around the top to stepper drivers at the top left
Signal wires routed seperately above the 7i76e
External wiring exiting from the bottom with steppers fairly isolated from the rest.

As far as external wiring goes and ignoring plasma for a moment, the motor wiring is the most significant source of noise. Its expensive but purchase shielded wire with thick enough cores to carry the required current. We elected to use 5 pin connectors on the case for stepper motor cables and attached the ground/shield to one of them. Inside the case, we used more of the same shielded cable to run to the controllers, and attached a spade connector to the shield right at the controller and folded this back and secured with shrinkwrap. We connected these to one of the copper earthing points provided on the industrial case we used. Subsequently, I upgraded the IEC connector on the mains inlet to a IEC RFI filtered one. They only cost a few dollars so its cheap insurance. Do not connect the shield at the motor end.

All wiring to the gantry is via 9 core shielded cable and I have breakout boards and terminal strips to distribute these signals. I used DB9 connectors and in the control box, I used DB9 connectors with breakout boards and terminals on them which made it easy to wire up.

Then I upgraded the wiring to include a star ground attached to the table and followed the Hypertherm document. I think this is a good Idea. I resisted for a while but if nothing else it provides a clear ground path around the linear rails back to the table frame. It would have been pretty easy if it was done when I first wired everything up.

Currently, I have not attached the star ground to any external earth so I guess it depends on the mains earth wire. When the machine was at home it was very close to the main building earth so I earthed the table and the plasma cutter back to it by seperate wires. This definitely reduced noise when I prodded with an Oscilloscope.

I debated for a long time whether I should mount the control box to the table or not as a seperate controlbox would also reduce the impact of noise but in the end I did and have not had any dramas.

JTknives wrote: So how do you know if you have EMI problems. I did not use any shielded cables in my build and everything seams to work peachy. I’m running a 30XP so no Hz start. Also my THCAD10 is in my main control cabinet.

I'm sure you will be fine with your machine. If you get Spurious random errors, its likely noise.

So for plasma, I think what I've outlined is all you need to do for a blow back style torch (eg, Hypertherm, Thermal Dynamcs, Everlast).
If you have a HF start, I think you need to add the ground rod and also be aware of the potential for noise spuriously triggeirng the probe signal from the Ohmic sensing wire. With the Islander/Plasmac probe design, this should not be an issue as it is physically disconnected unless probing. With other designs, if you have spurious Ohmic triggers, my HF consultant reckons the way to go is to fit a Feed through power line filter where it enters the control box.

mozmck wrote: Shielding is not needed for every external wire - I have never seen a problem with unshielded wires from between drives and stepper motors. Those are high current, and it would take a lot of noise to cause any problems there. Shielding on those wires might be good in cases to reduce noise created by the high-power step currents from radiating out. I'm not sure if you might be better off with ferrite beads though - PCW would know a lot better than I about that.

Yes, the risk is noise affecting other electronics. Plus its just another antenna out there in plasma world. to conduct the nasties back into the control box. Thats why I was advised to continue the shield right through into the control box right to the stepper controller. I don't think many people go that far.

A few hours extra work building your machine at the beginning can save hours of fruitless trobleshooting down the track.

This should be a sticky!

Also
Position the plasma source as far from the computer and control box as you can.
Do not use cheap keyboard and mouse for plasma machines, find older ones as they tend to have proper shielding. New ones do work, but some new ones will make a lot of trouble, like a lot.
Metal control box, with ground screw on it, all grounds connected to that same screw.
Side info, plasma cutter ground is at a positive level, while the torch is negative.

Well if its to be a sticky, here are some pics.
My star ground is a donated brass relic. I think I'd modify a brass buss bar if I was not donated this. Make sure its ground back for good electrical connection to the table.


This shows ground wires to Z axis stage and floating mount. This is the only place I shared a ground wire as I figured for 50mm, it would be fine and made for a neater install. You can see there is a thick cable bolted between the brass and the table hanging down. This would be your cable to the earth stake if you need it.

In the following photos ground wires are black with a red crimped on lug.



I have not run an earth wire from this to the control box as its secured to the table via several bolts and is there adequately earthed already.

Y axis ground wire one on each motor (yeh, I know Nema 34 is too big Tommy but I upgraded because I found a cool prebuilt 5:1 reduction pinion drive)


Z axis motor ground


So you can see this is not very hard to do.

I use a cheap wireless keyboard and mouse which seems to work reliably but with a 7i76e, I'm only connected via ethernet..