In-slot Cooling Enabled Heavy Rare-Earth Free High Power Density Electric Motor for EV Application

This paper presents a 20,000 rpm 72-slot/12-pole heavy rare-earth-free interior permanent magnet synchronous motor (IPMSM) design integrated with novel in-slot evaporative cooling and single-phase shaft cooling techniques. The in-slot cooling design replaces traditional slot-liner paper with a micropin fin-enhanced liner to facilitate coolant flow between the active winding and slot-liner paper, allowing for direct heat extraction from the winding via thin-film evaporation. Additionally, single-phase shaft cooling has been implemented to regulate magnet temperatures below the threshold limit at high speeds. The motor topology has been systematically optimized to enhance power density and prevent premature demagnetization of heavy rare- earth-free magnets. An extensive electro-thermal analysis was conducted to evaluate the performance of the optimized motor configuration. Results showed that the proposed motor design can provide a 10 sec. peak power density of ~86.59 kW/L and a continuous power of ~78 kW without demagnetization, which are ~73% and 41.82% higher than the Department of Energy’s 2025 target for electric motor for electric vehicle (EV) applications.