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View Full Version : Why star ground????



glynster
01-02-2016, 08:19 PM
I am assembling my DIY electronics enclosure and wiring everything up and of course I have heard that star grounds are the best in this case. My case/box is perspex with a aluminium plate as a base to which all components (drivers, BOB, PSUs, etc) are connected.

It has me wondering, why trace all grounds back to a single star ground point when I could simply drill and ground into this aluminium plate multiple times locally (aluminium is high conductive after all) and then connect the earth lead to the ground plate also? The entire base of the case would then be one common ground which in turn is then connected to Earth?

Also, I am confused about what components need grounding to the chassis/star point since some seem to be acting as the neutral although they are labelled as ground? For example, I have the KK01 BOB from CNC4YOU - do I ground that?

Sorry if this is a rookie question - I couldn't find a specific answer in search.

JAZZCNC
01-02-2016, 09:39 PM
It has me wondering, why trace all grounds back to a single star ground point when I could simply drill and ground into this aluminium plate multiple times locally (aluminium is high conductive after all) and then connect the earth lead to the ground plate also? The entire base of the case would then be one common ground which in turn is then connected to Earth?

No simple or easy answer to this because Noise and grounding is black art in it's self. Basicly what you trying to do is provide a path for any stray voltages to reach Ground and not feed back into the system. If you have multiple grounds then you provide alternative routes which can feed back into more senstive cables like Step & direction etc.

While you may get away with out noise issues using the ground setup you describe then for the little effort it takes to create a Single STAR point it's not worth taking the risk. I promise if you do get affected from noise it will take 200 times longer to track down than put in the STAR point, it will also drive you mental trying to find.!!

Regards grounding/Earthing BOB etc then you need to know the difference between AC & DC and how they work.
In your system you have AC devices like PSU's the input side should be connected to Mains Earth often called Ground. These PSU will then Give DC output with voltage and 0V connections. The 0v is often called Gnd. This is completely different to Mains AC Ground and is local to that Voltage source.

Your BOB, DRIVES and any other DC powered device use there own Local power source so don't need grounding. The source that creates the DC is connected to Mains Earth.

So the only things you want going to STAR point are from AC devices like PSU's and Any shields etc.

Edit: Here's a rough layout of how it would go with Star Ground.

17483

glynster
02-02-2016, 02:09 AM
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, 01:33 PM
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, 04:56 PM
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.


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.

glynster
03-02-2016, 02:42 PM
Great info, thanks guys