Compressor not starting - Refrigerators

Hard Start Capacitor
A hard start capacitor, its only purpose is to add additional starting torque when the compressor is starting and for just a few milliseconds.
It is wired thru a normally closed set of points in a potential relay and in parallel with the run capacitor, when the compressor is starting the start capacitor is in the circuit adding additional starting torque to the compressor, when the compressor reaches a certain speed it generates enough back EMF (voltage) to energize the potential relay coil and open the normally closed set of points and take the start capacitor out of the circuit and then will stay open, keeping the start capacitor out of the circuit as long as the compressor is running. The hard start capacitor is only in the circuit or working for a few milliseconds, it’s not doing anything once the compressor s back EMF opens the potential relay coil.
These potential relays have different cut in and cut out voltages, so the same relay won’t work with any compressor and may actually damage the compressor or shorten its life if you don't use the right potential relay. The compressor manufacturer and the local parts store list the correct capacitor start relay and start capacitor for every compressor and now they are putting the information on the compressor on the nameplate with the model and serial number.
As a rule of thumb you can use a capacitor or start capacitor within 10% of its rating if you can’t get the correct one.