Thermal Analysis of Potted Litz Wire for High-Power-Density Aerospace Electric Machines

Increasing the power density and efficiency of electric machines (motors and generators) is integral to bringing electrified aircraft (EA) to commercial realization. To accomplish that, an effort to create a high-efficiency megawatt motor (HEMM) with a goal of exceeding 98% efficiency and 1.46 MW of power has been undertaken at the NASA Glenn Research Center. Of the motor components, the resistive losses in the stator windings are by far the largest contributor (34%) to total loss in the motor. The challenge is the linear relationship between resistivity and temperature, making machine operation sensitive to temperature increases. In order to accurately predict the thermal behavior of the stator, the thermal conductivity of the Litz wire-potting-electrical insulation system must be known. Unfortunately, this multimaterial system has a wide range of thermal conductivities (0.1 - 400 W/m·K) and a high anisotropy (axial vs. transverse), making the prediction of the transverse thermal conductivity, and thus the hot-spot temperatures in the windings, difficult. To do this, a device that simulates the thermal environment found in the HEMM stator was designed. This paper discusses the design, thermal heat conjugate analysis (thermal model), manufacturing, and testing of HEMM's statorette. Results are analyzed by thermal resistance network modeling and microthermal modeling and are compared to analytical predictions of thermal conductivity of the insulated and potted Litz wire system.