Why star ground????

[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