The VFD controls the speed of the motor driving the compressor. You can tell the VFD what speed run the motor at through several different ways.
1. An analog output from a DAQ device or a compact Fieldpoint module that goes into an analog input on the VFD.
2. Serial communication between the PC or compact Fieldpoint controller and a serial communication port on the VFD.
I've used both of these methods with a couple different brands of VFD.
Look at the manual for the VFD to see what options you have available. Even if you use the serial option, you may still have to use some external hardware such as switches or relays to enable the operation of the VFD. Actually, it is a good idea to always have a hard wired E-stop set up so that you can disable the VFD and have the motor shutdown in the event that something in your software control fails in trying to communicate with VFD.
