A coreless permanent-magnet machine for a magnetically levitated shaft-less flywheel

This paper proposes a framework for the design of a coreless permanent magnet (PM) machine for a 100 kWh shaft-less high strength steel flywheel energy storage system (SHFES). The PM motor/generator is designed to meet the required specs in terms of torque-speed and power-speed characteristics given by the application. The design challenges of a motor/generator for this architecture include: the poor flux paths due to a large scale solid carbon steel rotor and zero-thermal convection of the airgap due to operation of the machine in vacuum. In order to tackle these challenges, several other parameters such as a proper number of magnets and slots combination, wire thickness, number of turns in each coil, magnets with high saturated flux density and magnets size are carefully considered in the proposed design framework. Magnetic levitation allows the use of a coreless stator that is placed on a supporting structure. The motor/generator is designed and simulated using a finite element analysis (FEA) package. The torque, power, and speed determined by the FEA electromagnetic analysis are met by the application design requirements and constraints for both the charging and discharging modes. Finally, the design and analysis of the coil holding structures are also discussed.