Centroid Community CNC Support Forum

Hi all,

I am bench testing my Acorn board and have been succesful for the most part.
I have completed bench/comms test - no issues.
I have completed spindle output bench test - voltage steady and within +/- 5%

I am using a Fuling (chinese) 5.5kw VFD and an Air cooled 4.5Kw spindle - Max speed 18,000rpm, Max frequency 300hz.

Report is attached.

Power supply for Acorn and VFD are on separate circuits, both with filters on the AC inlets

I currently have it wired so Acorn Output 8 is set to 'VFD Enable', this is wired to the 'Enable' pin on the VFD, which puts it in RUN mode when active.
I am then using the +/-10V control from the Acorn to control the spindle speed. This works perfectly, until I drop below about 12500rpm spindle speed/220hz on vfd/7.2V on spindle output from Acorn - The spindle speed will fluctuate up and down about 500rpm.

I'm sending a simple 'M3 Sxxxxx' in the MDI to control speed

When I put a voltmeter on the +/-10V spindle speed supply, the voltage is fluctuating with the speed change in the spindle, indicating that the VFD is just doing what It's told.

With spindle power supply disconnected, the voltage remains steady on the spindle control voltage at all speeds - indicating it may be interference from the VFD


I then tried putting a jumper across the 'Enable' pins on the VFD, and disconnected the 'VFD enable' wiring. The issue disappeared and spindle rpm & control voltage was very steady at all speeds - this narrows it down to something happening when the Output 8 relay is used to close the 'VFD Enable' pins. The speed control is exactly as one would expect

I have also tried Output 1 on the Acorn with no change



Not sure quite where to go from here, I need something to send the VFD a spindle enable to use the VFD properly, but can't/won't run it with such a speed fluctuation


I have also attached a photo of my wiring layout - I am very aware that there are no E-stops wired in, and that it's not tidy as yet.


Thank you,

Stuart

Make sure the analog out lines are shielded and properly grounded using a separate twisted pair cable (two wires, nothing else). You can then try a 0.1 microFarad capacitor (100pF) across the analog output if the problem is still there.

Ok thats good info, thank you

Can you clarify your wording around the grounding and twisted pair cable?

I have some shielded cable, which has two separately shielded cables (image attached)

Do I ground both ends of the shielding, or just one end, using twisted pair cable?

Appreciate the help,

Stu

Something like this: https://www.digikey.com/en/products/det ... 00/7009257 I normally try to ground the shield on the end where the signal originates; in this case the Acorn end. Oftentimes this is overridden by which end has a suitable ground point nearby. I don't like to go more than a few inches to find a shield ground so will adjust which end I ground accordingly. You also want to maintain the twist right up to the terminations. Don't strip them back 6" and untwist them, and don't run them through terminal blocks in between.

I had the same problem when I hooked up my VFD. I used a single piece of 4 conductor shielded stepper motor cable for the analog input and the enable. The VFD didn’t like that. My cure was to run a separate cable for the digital and analog inputs. I made sure that I grounded the shields at both ends. It works perfectly now.

FWIW: I used a single cable on my old setup and it worked fine. I’d love to see the output of the relays on an oscilloscope.

tblough wrote: ↑Fri Feb 05, 2021 7:20 am You can then try a 0.1 microFarad capacitor (100pF) across the analog output if the problem is still there.

Not a biggie but 0.1uF is actually 100nF or 100,000pF. A 100pF cap wouldn't have any effect that you'd notice.

I'm old. Senior moment.

Thanks a lot guys, I will replace the unshielded 4 conductor cable with two separate shielded, earthed cables

The only controller I've built prior to his was a Gecko G540 and Mesa 5i25, fairly different setups but apparently quite robust against EMI

Loving the Acorn so far though, simple to setup and seems like a decent 'light industrial' control

Ok, so first move was to replace cable with shielded, grounded cable. This improved the issue but did not fix it completely. Best results were when shielding was grounded at both ends.

I then put a 0.1uF capacitor across the analog outputs on the Acorn end, which fixed the problem completely. Steady voltage and RPM at all speeds

Thank you all for your help

That’s great news. I added a 1mf tantalum cap to my analog output too.