I recently got 2 cheap 6040 machines, and I've run into a weird issue. At any speed other than max speed, the RPM kind of wanders up and down over the course of half a minute or so. If I hold any button on the control panel for the VFD it'll do the same thing but fluctuate faster. I'm not sure what the problem is. As well, the RPM on the display reads up to 12000RPM at 400Hz, but I have an 800W spindle that's supposedly rated up to 24000RPM?

I did some looking around, and I couldn't actually find the exact VFD I have. Model number says "AT100-S2-0R7G", no branding. It looks almost identical to this one: https://www.alibaba.com/product-deta...017975889.html

As for what's controlling it, It has one of these USB MACH3 boards that reads as RNRMOTION and it worked with just copying over the proper .dll files: https://www.aliexpress.com/item/32967913484.html
It looks like it's just sending out PWM for speed control.