Whether it is frequency control or DC speed regulation technology, it is impossible for the heat pump system to reduce the outdoor air temperature corresponding to the coefficient of performance to a temperature lower than that achieved by a fixed speed heat pump system with the same rated heating coefficient. In other words, the speed control technology cannot change the outdoor air temperature constraints on the economics of the heat pump system. Therefore, for a heat pump system operating at low outdoor temperature conditions, the main significance of variable frequency speed regulation or DC speed regulation is that no additional electric heating means is needed, and the efficiency of the heat pump system is sacrificed by changing the operating parameters of the system. You can get a larger heat supply. In the larger operating range, the performance coefficient of the heat pump system with the variable speed capacity compressor can still be maintained above the level, which makes it have the advantage of saving electricity relative to the electric heating system. Cooling of the compressor motor and return of the lubricating oil are the main factors that limit the lower limit of the evaporation temperature of the heat pump system. In order to simplify the understanding of this problem, the consequences of changes in system evaporation temperature due to changes in outdoor air temperature are theoretically analyzed. In fact, the system mass flow is generally attenuated by the evaporation temperature drop. Accordingly, in the case where the condensing temperature is constant, the volume flow rate of the high-pressure side gaseous refrigerant in the system will decrease by the same amount with the mass flow rate due to the change in the evaporation temperature. From the standpoint of motor cooling conditions, compressors that use high-pressure refrigerant to cool the motor, such as rolling-rotor compressors, reduce the flow of refrigerant across the surface of the motor windings, which can result in deterioration of the cooling conditions of the motor.
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