Research On Outer-rotor High-speed Permanent Magnet Synchronous Machine Used On Flywheel Storage System

Outer-rotor permanent magnet synchronous machine (PMSM) used on flywheelstorage system is high-speed machine working in vacuum environment. Accordingto its specific structure, working environment and the key problems, this paperpresents a systematic research and analysis respectively from the aspects of themachine design method, the strength of rotor, motor loss and temperatureperformance, etc, which can provide reference for outer-rotor flywheel machine andthe related research work for general high-speed machine.According to the machine design requirements, the structure of an outer-rotorPMSM with a rated power100kW, rated speed20000rpm is designed. Consideringboth the mechanical and electromagnetic performance requirements, the designmethod of an outer rotor high-speed machine is given. And the machine is optimizedfrom the perspective of loss and temperature rise.Considering the centrifugal force of the outer-rotor due to the high-speed, avariety of rotor structures are discussed. First, the rotor strength of generalsurface-mounted PMSM is analyzed and the effect of the additional contact pressureis discussed. Then to overcome the shortcomings of general surface-mounted PMSM,three improved structures are put forward and compared. Last, the influence ofstructure parameters on the rotor stress is analyzed, including the influence of thematerial filled in the inner-pole gap, the influence of the sleeve material andthickness and the influence of magnet segment. This work can provide a referencefor outer-rotor structure design.In view of the high speed rotor loss and temperature rise, analytic calculationof the rotor radiation heat transfer model in vacuum is established. Thecorresponding relationship of the rotor temperature and loss is given. And from theangle of the rotor temperature rise limit, the rotor loss permission is presented. Thenthe motor structure parameters (slot width, length of air gap, sleeve material andthickness) are optimized to minimize the rotor loss. The effect of magnet segment onthe reduction of rotor eddy current loss is performed, and a new method of magnetinhomogeneous segment is put forward, which is more practical and effective.Due to the magnetic levitation bearings and the heavy flywheel outside, the air-gapeccentricity phenomenon of the outer-rotor machine is easily caused. Hence themotor loss under this circumstance is analyzed. First, the analytical solution ofadditional magnetic field and the resulting additional permanent magnet eddycurrent loss caused by the eccentricity is deduced. Then the effect of different typesof eccentricity and eccentric angle on the rotor loss and the eddy current distribution are respectively discussed. And the relationship of eccentricity ratio and rotor loss isperformed.