usb485 to huanyang vfd

PD14, PD16, PD18 and PD20 are all accel times, but not particularly well documented. Maybe try changing all of them?

Still the same when I change all the accel times. When the speed approaches 3000rpm I do a flashing warning A.oL but it still very slowly is increasing speed. After 3000rpm it isn't very long until I get a fault E.oLA According to the manual it is inverter overload. Something to note is I ran this off a digital control circuit and used the potentiometer on the face of the VFD to manual change the speed. I just set up a relay between my old BOB the VFD. Running in that mode I never saw this kind of behavior or any faults. Ran for probably 2 years like this before the computer went out and I decided to upgrade from the parallel port BOB. I also tested changing the number of poles of the motor listed in the VFD. It defaults to 4 but I think these 24000rpm spindles are 2 pole if I am not mistaken.

Oh one more thing I checked was to set the display to show target frequency and it was bang on every time. So I pretty confident this is all in the inverter unless there is some parameter for spindle ramp time in linuxcnc. When I was reading in the documentation section, it looks like you have hand code that in if you want to use, which I don't,

I reset to default settings and set it up so I can use the Start/Stop on the faceplate of the VFD. It does the same thing as the rs485 connection. Which confirms there isn't any issue with the communication. But still having limited speed even with all the settings set to 400Hz. I even set PD072 to 400Hz, which is a analog frequency limit. It made it speed up faster, but not improvement on RPM of 2,500rpm. If I crank the potentiometer to the max it does rapids to about 2500rpm and slowly climbs from there. Gives me a A.ol warning just after 3000rpm and then eventually stops with E.oLa

Just a thought have you set the param. for the number of poles on the motor I have seen them set for 2 and 4

That seems like VFD/motor issue.
Are you sure the VFD is set properly for that motor, the main thing is motor poles.
www.google.com/search?source=hp&ei=oDPAX...sKDXwQ4dUDCAY&uact=5
Check if the motor starts heating after a minute or two of use, but stop the VFD after that minute, wait for another 2 minutes and check the motor, if it is still cold, let it run for another minute, stop, check temperature after 2 minutes.