Equivalent Thermal Conductivity Prediction of Form-Wound Windings With Litz Wire Including Transposition Effects

Litz wire has been widely used in high-frequency electrical machines and transformers to minimize induced current loss and maintain high efficiency. Its heat dissipation capability can be a key design factor for high-power-density, high-frequency electrical machines. Although litz wire equivalent thermal conductivity has been studied, its transposition arrangement on heat dissipation enhancement is usually neglected. This article focuses on developing an analytical model to predict litz wire equivalent thermal conductivity including transposition effects. 3-D finite element models and hardware experiments are used to validate the proposed analytical model. Fast and accurate litz wire thermal conductivity prediction values can be obtained. The transposed and parallel arrangement effects on litz wire equivalent thermal conductivity are compared and discussed. This study shows that the transposition arrangement could improve litz wire heat dissipation capability by 10% to 30%, demonstrating that electrical machine power density can be potentially increased by up to 14%. The proposed method can advance winding design to push the boundaries of electrical machine power density.