Design and Evaluation of Interior Permanent-Magnet Compressor Motors for Commercial Transcritical (R-744) Heat Pump Water Heaters

This paper describes the design and experimental evaluation of two high-efficiency interior permanent-magnet (IPM) motors for use in a commercial transcritical CO₂ (R-744) heat pump water heater (HPWH). Two IPM topologies, a 36-slot 6-pole topology and a 9-slot 6-pole topology, were selected, optimized, and prototyped using a common flat-bar IPM rotor. The experimental analysis includes standard measurements of electrical machine parameters, and dynamometer tests. The initial measurements of the CO₂ compressor performance with a stock line-fed induction machine and the 36-slot 6-pole IPM configuration have been completed. This paper finds a small isentropic compressor efficiency improvement from using the 36-slot 6-pole IPM motor compared to the stock line-fed induction machine at rated operating conditions (essentially constant speed). This paper also expands the open literature on the impact of high-efficiency motor drives on CO₂ compressor operation and CO₂ HPWH systems.