I'm trying to install a plasma cutter on a CNC machine, but the problem is that the cutter uses a high-power HF start to initiate the cutting arc, and this causes horrible interference with my control electronics.
The main source of the EMI is the cable from the cutter's inverter to the torch (also the torch itself). With the torch cable disconnected, I measured only some tiny amount of EMI. The cut metal is ground-clamped back into the inverter (the ground clamp is positive in respect to the torch), and this also creates some EMI, but not as much as the torch itself.
By probing a free floating control wire (not connected to anything at either end), I've been able to make some measurements. The cutter creates periodic oscillation bursts every 2.62 ms, and each burst rings at around 2-6 MHz with peak to peak amplitude of above 100V (limit of my scope). When I repeated the measurements at different points in my system (computer output port, machine control board, various wiring and shielding), I measured the same results.
I have already wrapped all cables (most of all, the torch cable), the computer case and other electronics with aluminium shielding tape, and connected the individual shields single-ended into a star pattern. I have to admit though, I'm not sure where to actually ground the shields to. I tried to connected the shields to my mains safety ground (yellow/green). I'm using stranded wires everywhere, but the mains ground is solid core, so I didn't expected great results, however, grounding this way actually made it even worse, I measured less interference when the probed cable's shield was left floating. I think the mains ground line itself is picking up interference and not doing a good job of grounding it.
I triple checked the conductivity of the shields, and I'm getting a clear path to the ground everywhere.
I tried to ground the shield to the cutter's ground clamp terminal, but this also didn't appear to help at all, if not also make it even worse.
I also added ferrite cores on every cable, and it helps a little bit, but not nearly enough. Also, all the electronics and control cables are of course as far from the cutter as possible, and there is no physical connection between the CNC machine and the plasma cutter, other then the shared mains power grid.
I also made some other interesting observations. On a control board for the CNC machine, there are several LEDs that pick up enough interference to very slightly light up when the cutter (HF generator) is turned on. I did a lot of experimenting with this and it appears that when the shields are just grounded to my mains ground (described above), the effect is quite random. Sometimes, it made the LEDs light up more, and sometimes it made them light up less. I was not able to find any mechanism behind this, and the effect was very small, so it was probably just some minor details such as cables shifting slightly. However, I noticed a significant change when I added a ferrite ring to the grounding wire where it connects to the shield. It made the LEDs dim almost completely, when connected to the mains ground (compared to when the shield was left floating), but only at a certain number of turns, but it would have no effect with more or less turns around the ferrite ring. I replicated the same results with a ferrite sleeve. One sleeve would dim the LEDs a lot, while adding more than one sleeve would negate the effect.
I'm not really sure how to tackle this whole issue, I ran out of ideas. Any help is greatly appreciated.