Why star ground????

**glynster** · 02-02-2016

Thank you, that's cleared it up perfectly.

I was going to run my VFD on a separate socket outside of my electronics enclosure - so I can access it and because I assume it will generate heat and suck up a lot of current. The only part of the spindle system I was going to run through my control box is the connection between the VFD and the PC (Using a RS232 to RS485 Converter and grounding the cable shield inside the box - to the star ground). Are you suggesting that I should still bring over the earth wire from the VFD into my control box even though it's on a different plug? Should I do that instead of or as well as grounding it at its own plug? I thought I had to use a 15amp RCD breaker between the RFD and the power?

Also, since you've answered so clearly and generously, one more thing - I am guessing the spindle height touch plate I intend to use needs to be connected to one of the BOB inputs and the spindle earthed to the GND on the BOB? - even though it is also going to be Earthed via the star ground to the control box through the earth to the machine? Does that mean I should only connect the BOB ground temporarily while I do the setup? Otherwise the touchplate would be grounded (through my machine) to the star ground AND to the ground on my BOB. Hope that makes sense.

**cropwell** · 02-02-2016

What you need to avoid is a ground loop, where there are two or more paths that any current in the ground circuit can take. This is because AC mains and high frequency currents from VFDs, inductive electronic components like relays and transformers generate alternating magnetic fields. This can induce stray currents in signal conductors of sufficient magnitude to cause false signals where a ground loop allows a current to flow.

Star grounding is the easy way to avoid this problem. Your backplane can act as a star point.

Shielding should be grounded at one end only.

The AC and DC grounds need not be separated if the DC generating devices are not earthed on the AC side. Sometimes (if they have an earth wire) they are internally connected, but not often.

I have the Machine and control box grounds connected and the BOB ground connected to the machine frame and spindle, so that the touch plate returns a 0v signal. I have no problems with that arrangement.

Cheers,

Rob

**JAZZCNC** · 02-02-2016

Originally Posted by glynster

Are you suggesting that I should still bring over the earth wire from the VFD into my control box even though it's on a different plug? Should I do that instead of or as well as grounding it at its own plug? I thought I had to use a 15amp RCD breaker between the RFD and the power?

No if it's on it's own Plug then just use the Earth in Plug. Regards the Cable from VFD to Spindle only connect one end of shield and then just connect shield to Earth on VFD. No need take to Star point.

Originally Posted by glynster

Does that mean I should only connect the BOB ground temporarily while I do the setup? Otherwise the touchplate would be grounded (through my machine) to the star ground AND to the ground on my BOB. Hope that makes sense.

In theory the Bob Input Gnd will be tied to earth thru the PSU providing power so what you say is correct and will work but due to nature of BOB's being unreliable then it's best if you have Direct connection to the Gnd Input.