Efficiency Optimization of a 100-W, 500 000-rpm Permanent-Magnet Machine Including Air Friction Losses

This paper proposes a method for the efficiency optimization of ultra-high-speed permanent-magnet machines. Analytical methods are applied for the modeling of the machine that is equipped with a diametrically magnetized rotor and a slotless stator. The outer dimensions of the machine are design constraints, and the internal dimensioning is optimized for minimum losses. The air friction losses are taken into account in addition to the usual iron losses, copper losses, and eddy current losses. Laminated silicon iron or laminated amorphous iron is used as the stator core material. The results show that air friction losses influence the optimum design considerably, leading to a small rotor diameter at high speeds. The loss minimization and the amorphous iron core make it possible to reduce the calculated losses by 63% as compared to a machine design not considering air friction losses. The resulting efficiency is 95% for a 100-W, 500 000-rpm machine excluding bearing losses.