April 17, 2021

Grounding improvements after adding VFD?

2 min read
I have an old CNC machine (K2 KG3925) I’ve been running for years with a...

I have an old CNC machine (K2 KG3925) I’ve been running for years with a router, but having recently upgraded to a 3 phase spindle + VFD I’d like to take the time to improve the wiring (specifically grounding and noise suppression).

The not-very-good diagram attached below shows the basic layout of the system as it is now.

The CNC controller PCB is driven by a parallel port connection from the PC, and is powered by a 5VDC line taken from one of the PC’s USB ports (ugly, but that’s how it was delivered).

The VFD is a Huanyang 2.2kW unit, that was supplied with the spindle.

In terms of earthing paths, the PC, Controller and VFD are all connected to UK 240V mains (live, earth, and neutral). The controller enclosure is earthed via its mains connection.

On the CNC machine, there is continuity between the spindle can, all servo motor cans, and the CNC body itself; by virtue of metal-on-metal contact for all parts.

The first thing I need to do is wire the 4th pin on the spindle connector to the spindle can (as it’s not connected to anything on these cheap Chinese spindles), and then run that line to the earth on the VFD.

I understand the correct way to do it is to have a 4 core shielded cable (U, V, W plus E), where the E core would go to the spindle 4th pin, and also connect the cable shield to that earth core in the cable at both ends. However, I have some suitably rated 3 core shielded cable to hand. Would it be acceptable to connect the shield to the 4th pin on the spindle, and the earth connector on the VFD, rather than use 4 core cable?

The above would mean the CNC machine body itself is now earthed, but only by virtue of its continuity to the spindle body. Should I also add a separate earth line between the CNC machine body at the earth point on the VFD? I’m aware that with the spindle installed in the CNC machine that would then create a loop of earth wiring.

I notice there is no continuity between the three earth (E) points on the VFD, so should I wire the spindle earth (and possible CNC machine body earth) to the left earth point in the VFD on my diagram; so they are definitely connected to the mains earth?

Finally, what other changes should I make? The encoder and limit switch connections between the CNC machine and the controller use Cat (Ethernet) cables; which I assume will have some degree of protection against noise. None of the mains power leads I’m using have any ferrite beads, and I have no EMI filters – would it be advisable to add some?

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