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Electrification of traditional combustion power units has been a major trend. The low emissions, low noise and high efficiency characteristic of electrified power, fit the vision of a low carbon emission future. The development of high power density electric motors is key to facilitating large scale, heavy duty applications. The demand for dense power leads to significant heat flux, causing thermal management to become one of the main obstacles in developing high power density electric motors. Multiple components in the motor generate heat. For example, the motor of interest in this paper is a 1 MW, high power density, surface mounted permanent magnet motor, with a segmented and laminated stator on the outside, and a laminated rotor on the inside. Heat is generated in the stator winding, stator core, magnets, rotor core as well as the motor drive. For high speed motors, windage loss could also be significant in the air gap. Among the heat-generating components, the stator winding is the primary heat source.

For this study, a comprehensive thermal management solution was developed. The power density of the motor, based on active mass, exceeded 22 kW/kg and majority of the loss came from the stator windings. Thus, a dedicated direct winding cooling combined with an integrated cooling jacket were deployed. Multiple winding cooling schemes were explored, such as investment-casted cooling channels in potting, hollow conductors, flooded slots and Litz-wire-wrapped cooling tubes. The flooded slots with scaffolding-shaped spacer were chosen in the end, which demonstrate good thermal performance, low pumping power, pressure requirements and low risk of partial discharge as the dielectric coolant also served as liquid insulation. A cooling jacket with integrated power module cooling was designed to cool the stator core and power modules. The cooling jacket included a compression sleeve, which served as the mechanical support to hold the stator segments as well as the cooling surface for the stator cores, and nine cold plates, hosting 18 power modules on top, placed around the curved outer surface of the motor. The cooling concepts were designed, simulated and validated by testing. A functioning prototype was constructed and in the process of testing.