I was using the schematic from here to make my PWM-to-10V converter. After changing the PWM frequency from the default 5Hz to something around 200Hz, I was able to somehow control the VFD input. Still got some issues but currently using a workaround.
- Converter not giving full 10V at maximum frequency. At full speed, the converter only provides around 8.5V thus limiting my top speed to around 20000rpm. Workaround: Set maximum operating frequency on VFD to around 30000rpm thus at 8.5V, I was achieving 24000rpm.
- Frequency control not linear. I notice that the accuracy of the PWM control is around 5%. This is within my acceptable 'error rate' :-)
Also found out that the aquarium water pump I have does not have enough pressure to push the water up the tube (about 2.5 feet vertical) when tube is empty. Need to get a bigger pump soon...