Why star ground????

[glynster]

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]

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.

[glynster]

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]

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]

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]

Great info, thanks guys

[Coco737]

MyCNC.pdf
This is my setup everything was working fine I had even cut a couple of jobs with no problems. Then all hell broke loose with the stepper motors going mental when I hit RUN on the VFD. So with looking online as you have guessed it is an EMI problem. The Spindle cable and the Stepper Motors cable are not shielded or grounded. So I have ordered 4 core shielded cable and will change all cables when it arrives, this is no bother to do. But I really could do with some help with the grounding, the more I read online the more confused and scared of making a ground loop or any other mistake that can be made. So if anyone who can help please look at the attached file and tell me exactly where to attach grounding wires. I would be very grateful.... Thanks in advance..

Coco

[Kitwn]

Coco,
The accepted wisdom is to connect cable shields to ground only at one end, usualy in the control box, and to connect them all to a single point if possible. The problem with earthing at several points is that you can create earth circuit loops with a current flow, and where there's a current there's a magnetic field which can induce interference. The origin of all this thinking is keeping mains hum out of audio circuits. Don't get too wound up about it, you will not break anything by doing it differently and the screening will still be better than nothing. It sounds as if the noise from the VFD is getting into the input control circuitry of the stepper drivers but this could be coming in through the output wiring so screening the outputs to the motors is a good plan.


I've been investigating spurious limit trips on my machine recently and with the help of my trusty oscilloscope am reasonably sure the problem is interference from the VFD feeding back into the mains supply affecting the break out board and computer running LinuxCNC. I've ordered a mains filter from RS components which I'm hoping will stop the interference escaping from the VFD and fix the problem. I see you have a similar filter in your diagram. These filters are only effective for the live and neutral connections and it may be necessary to add filtering in the form of a ferrite ring on the earth connection as well. I have yet to try this but have ordered some ferrites as well.

VFDs produce interference well up into the radio frequency range, I've measured frquencies up to about 6MHz recently which are very difficult to control. Once the bits arrive (RS are sending them from the UK!!!) I will put some results on the forum.

[Coco737]

QUOTE=Kitwn;123348]Coco,
Using screened cable for the VFD and stepper drive outputs is definately a good idea. Noise being fed back into the driver outputs can easily find it's way into the more sensitive input circuits as well.

I've been investigating spurious limit trips on my machine recently and with the help of my trusty oscilloscope am reasonably sure the problem is interference from the VFD feeding back into the mains supply affecting the break out board and computer running LinuxCNC. I've ordered a mains filter from RS components which I'm hoping will stop the interference escaping from the VFD and fix the problem. I see you have a similar filter in your diagram. These filters are only effective for the live and neutral connections and it may be necessary to add filtering in the form of a ferrite ring on the earth connection as well. I have yet to try this.

The noise from the VFD has components up to several MHz which can be difficult to get rid of. Standard types of earthing don't always work for these frequencies and it is present all over the earthed metal case of my machine controller which is connected to mains earth. No amount of screening on the VFD to spindle wiring or the BOB to motor drivers (both of which I have on my machine) is going to keep out interference which is getting in through the power supplies, especially the "earth" connection!

RS are sending the filter to me in Australia from the UK so I'm not sure how soon it will arrive but I'll put something on the forum about my results.

Kit[/QUOTE]

Thanks Kit, I'll look forward to see your results. I've got ages to wait anyhow. 16/4 shielded cable can't get in the UK so coming from the states, M542T stepper motor drivers out of stock in UK Stepper online so god knows when I'll be up and running again March hopefully

[Muzzer]

"Conventional wisdom" is split. Some recommend only grounding one end of a screen, others recommend both. In practice, there's no right or wrong answer and I've found cases either way where one works and the other doesn't when I've done EMC testing on professional VFD installations.

Most VFDs are sold without a mains filter yet they need one. The reason they don't usually come with one is because it's not possible to define one that will meet the requirements unless you know the exact details of the installation, not least how the motor will be connected up. So many people just connect them up without a filter, then have the devil of a job trying to figure out why they have noise problems.

Here's the recommended filter for the 4kW Yaskawa VFD. It's probably a bit OTT for a 3kW VFD but gives you an idea what to expect. The VFD itself mounts on top of the filter using the 4 threaded holes - very neat - and the wiring provided is even the right length for the model of VFD it's intended for - in this case the 4kW V1000. There's no guarantee this will meet the requirements out of the box but it's likely to get you close if it doesn't.
https://inverterdrive.com/group/EMC-...8-40-07-V1000/

For noise in the 1-10MHz region, the clip on lossy ferrite clamps are great. Even better, they don't require you to disturb the wiring and can be used on both mains and signal wiring. This sort of thing. Genuine TDK ones would be good:
https://www.ebay.co.uk/b/clip-on-ferrite/bn_7024808499