Do you know if there are any refrigerant check valves in that 40 ton machine or is it just 4 compressors piped in parallel?
Doing more reading on these electronic expansion valves. I really like this idea because a system can be built with the proper sensors and EXV and then the rest of the playing is in the software (the easy part for me).
There is some good info in here.
http://web.ornl.gov/~jacksonwl/hpdm/...Orlando_V7.pdf
My brain gets a little out of control, but you could even use one VFD drive to run 3 different 110V compressors. The VFD I have converts 220v single phase to 220V 3 phase. As I understand each phase is 110V to ground giving 3 different 110V controlled sources. Add relays to control on/off to each compressor. One relay on and ramp up to needed capacity. If max capacity is reached on the single compressor, ramp down, switch a second relay and ramp back up to needed capacity, etc. Add in the EXV and it sounds like fun!