When should I use a Hard Start Relay- Capacitor in a Hermetic Compressor? And How it works? Thanks.

You usually use a start capacitor on old compressors having a high voltage draw. Best bet is a hard start kit. It has a solid state relay built in. Its connected in parallel with the start wire on the compressor. Hope this helps

Well, the voltage across a capacitor cannot change instantaneously, so the capacitor initially acts as a short which activates the start winding. The capacitor has a parameter called ESR or effective series resistance. Now suppose you do this starting electronically, so your not limited by the ESR.

Hard start kits seem to be necessary because of the use of TXV's without the ability to equilize pressures when the system turns off this the sysen starts with high head pressure.. Also TXV;s are sometimes required by the manufacturer when the vertical distance between the evaporator and condenser is exceeded. It's also used to extend the life of the compressor.