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3D finite element analyses and design optimization of AFPM for flywheel energy storage system

This paper presents the optimization design and analysis of axial flux permanent-magnet (AFPM) machine (internal stator external rotor) used in flywheel energy storage system (FESS). The main idea of design is reduce the pressure and friction acting on the lower bearing system, thus reducing the bearing losses and, therefore, the self-discharge of the stored energy during the standby mode. Due to the unconventional flux path distribution of this machine, a 3-D finite element (Maxwell 12) method was used to analyses the design, of course including its electromagnetic torque and axial force performances. The effects of the rotor permanent magnet (PM) and slots skew angles on the cogging torque and the axial force have been studied. It is found that an optimum skew angle is effective in reducing the overall cogging torque with negligible effect on the static axial force. The latter is essential as it can be utilized to minimize the axial bearing pressure in FESS application.