Effect of unbalanced and inclined air-gap in double-stator inner-rotor axial flux permanent magnet machine

Due to its novelty in structure, the appealing force between rotor and stator disks, axial flux (AF) permanent magnet (PM) machine are easily affected by air-gap asymmetries. In the inner-rotor double-stator AF machine, the rotor disk is assembled between two outer stators. It is usually assumed that the air-gap clearance is ideal condition in electromagnetic analysis. However, manufacturing imperfections in the machine or any perturbation to rotor disk during operation would results in an unbalanced and inclined air-gap clearance, which would generate unbalanced force acting on the structure and further increase the asymmetry extent. Cogging torque, which is imperative requirements for high performance applications, is directly related to modulating effect of the air-gap and slot opening permeance. In this paper, the cogging torque under un-normal air-gap condition of AF machines is first calculated using a combined analytical and numerical methods, followed by a structure analysis to study the effect to mechanical stress, deformation and bending force on bearings. Results and conclusions given in this paper are instructive for successful development of AFPM machines.