A Coreless Permanent-Magnet Machine for a Magnetically Levitated Shaft-Less Flywheel

This paper proposes a framework for the design and analysis of a coreless permanent magnet (PM) machine for a 100 kWh shaft-less high strength steel flywheel energy storage system. The PM motor/generator is designed to meet the required specifications 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 the operation of the machine in a vacuum. The magnetic flux in this architecture tends to be 3-D rather than constrained due to lack of a core in the stator. 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 magnet 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 proposed PM motor/generator comprehensive geometry, electromagnetic design, and mechanical dimensioning are followed by detailed 3-D finite-element analysis (FEA). The torque, power, and speed determined by the FEA electromagnetic analysis meet the application design requirements and constraints for both the charging and discharging modes of operation. Furthermore, the design and analysis of the coil holding structures are discussed. Finally, the motor/generator static thermal analysis is discussed in order to validate the proposed cooling system functionality.