Rotordynamic Assessment for an Inside Out, High Speed Permanent Magnet Synchronous Motor

摘要

High speed and high rotor tip-speed, in particular, combined with high magnetic and electrical loading, are key enablers for obtaining high specific power in electric machines. The rotor described in this paper pushes the tip speed to about 80% of the speed of sound. An outer rotor design with a carbon-fiber retaining ring on the outer diameter (OD) was chosen to maintain high magnetic loading in the airgap while ensuring structural integrity at high speed. To achieve manufactural feasibility, a cantilevered rotor structure was proposed to suspend the rotor around the stator. This architecture introduces significant rotordynamic challenges, one of which is the ”trunnion effect,” a rotordynamic effect uncommon in traditional electric machines. This effect was identified when a discrepancy was observed during a rotor test and a root-cause analysis was performed to explain it. The original rotor dynamic model was modified to include this effect, and verified through a ping test. The trunnion effect, which significantly reduces the critical speeds, can be offset by selecting appropriate dimensions for the rotor end plate using the revised model. Thus, even in the presence of this effect, the proposed unconventional motor topology is confirmed to function reliably.