I just run across this on Youtube. He is speed controlling a Grundfos circulation pump with a microprocessor pulse width modulating a solid state relay. Watch the whole video as he explains the process and even shows the signal results on a scope. I did not know an AC motor could be controlled this way. Wonder if this results in energy savings. He even has the PCB and schematics available.
This could likely also be used on blower fans too, but what about compressors?