People always think a start capacitor sits there and builds up a huge charge, and that is what starts the compressor. While it does do that to a certain extent, that really isn't its purpose in motor starting- rather it is to change the phase a bit- delay the cycle. Think about trying to start pedaling a bicycle with the pedal at the very top of the travel. It's much easier to start if it is halfway down. That is what the start capacitor does.
We have used them in the 12 volt side- on old ferroresonant converters they were used to eliminate the battery.
