I’m afraid I’m not very competent when it comes to providing sound advice to keep the EMI gremlins away, when every precaution has already been taken, especially since I was lucky to never have such issues in mine, back when I used a third-party VFD.
You may have seen this thread and others from folks who struggled with EMI on thrid-party VFD setups. What I see being discussed often is adding a standalone EMI filter box either at the VFD power input (mains) or between the VFD and the spindle, and I suspect that the C3D VFD embeds one.
For the PWM itself I was referring to an opto-isolation adapter, I’m sure I have seen a few references on the forum over time but I can’t seem to find one right now
Anyway, let us know how troubleshooting goes, I’m happy to support any way I can, but the help will more likely come from someone in the community who actually struggled with EMI in their third-party VFD setup and finally found a way to address it.