A Novel Multi-Unit Out-Rotor Homopolar Inductor Machine for Flywheel Energy Storage System

Homopolar inductor machine (HIM) has been widely applied in the field of flywheel energy storage system (FESS). However, conventional HIM suffers from the low power and torque density due to its unipolar air-gap flux density. To solve this problem, a novel multi-unit out-rotor HIM (MOHIM) with bipolar air-gap flux density is proposed. First, the structure and operation principle of MOHIM are illustrated. To simplify analysis, MOHIM is divided into unit machines (UMs) based on the equivalent magnetic circuit. The back-EMF, flux density and output torque of MOHIM and UM are respectively compared, which proves the effectiveness of UM. Finally, the performance of MOHIM is investigated by three-dimension finite element analysis (3-D FEA), which indicates that this machine is a good candidate for the application of FESS. Keywords-Homopolar inductor machine (HIM); multi-unit out-rotor HIM (MOHIM); bipolar air-gap flux density; unit machine (UM).