Motor cooling method using flow boiling of two-phase refrigerant and its analysis with lumped parameter thermal model

To improve the stability and efficiency of electric motors, an advanced cooling method that can replace the existing water-cooling method is required. Even though various studies utilized a liquid refrigerant in the cooling system, none of studies explicitly presented the cooling performance enhancement of motor by the superior heat transfer of flow boiling. In this study, a flow boiling cooling method using R134a is applied with a conventional water-cooled motor design, and its cooling performance is compared to that of a water-cooled motor. A transient three-dimensional lumped parameter thermal model based on cylindrical coordinates is developed. This model can calculate the heat loss of an electric motor, and determine the heat transfer and temperature change inside a motor under a given driving condition. The results indicate that the flow boiling cooling method maintains the winding at a temperature lower than the conventional water-cooling method. The power consumption of the pump required for motor cooling is reduced reliably by applying flow boiling cooling. However, it is necessary to use the appropriate refrigerant flow rate by considering the heat lost from the motor. If not, the two-phase heat transfer stops owing to complete vaporization of the refrigerant inside the motor. To realize efficient flow boiling cooling, the minimum required volume flow rate of R134a considering the motor driving condition is suggested in this study.