When compressor is in operation, the outlet of compressor always has lots of refrigerant oil. We need to install an oil separator between compressor and condenser to separate the refrigerant oil and refrigerants in order to prevent the compressor not lose too much refrigerant oil and prevent the refrigerant oil flow to system with refrigerants to interfere the condenser’s heat function and evaporator cooling function. Then the oil will flow back to the compressor crankcase. Oil-return device operates on the ball valve moving on the bottom of oil separator and the high-low voltage theory. The oil separator can ensure the sufficiency of oil in compressor and let the compressor reach its best cooling performance.