The way I do my testing is this.
1. Connect a dc power supply in the range if 6v to 15V in place of the supply coming from the VFD.
2. Initially set the max spindle speed to the supply voltage times 100. so if you are using a 9V battery set it to 900
3. Now in Mach3 enable the spindle and set the speed to the maximum spindle setting. In out case 900
4. Ensure that the DC-03 multi-turn attenuation potentiometer is turned fully clockwise. (Max output, no attenuation)
5. Measure the voltage being generated by the Digispeed. It will be less than 9V by about 0.7V as this is "stolen by the semiconductor junctions.
6. Then set the Max pulley setting to match this measured voltage level, say 830. This would equate to 8.3V
7. Now, you can check the linearity of the generated voltage against the commanded voltage. If you want to see 5.4V at the output , enter a speed of 540rpm. You can then check a range of values.
Then, once you hook it up to the VFD set the Max pulley speed to match the actual measured maximum speed. Note that if you supply 10V from the VFD to the DigiSpeed, you won't get 10V back, but a little less. This will consequently reduce the maximum speed of you spindle. If this is a problem, you have two options;
1. Some VFDs have parameter settings that allow you to scale the max speed of the spindle. That is, you can set up the VFD to produce 400Hz (max speed) when it sees 9.3V rather than 10V at the control voltage input.
2. Use an on-board DC/DC converter in the DC-03. With this the DC-03 will not rely on the 10V supply from the VFD, but generate the voltage it self.
Cheers,
Peter.