Rotor mechanical stress and deformation analysis of coreless stator axial-flux permanentmagnet machines

Axial-flux permanent-magnet (AFPM) machine with its compact structure, flat shape and high torque-to-weight ratio is increasingly being popularized and applied in various applications, such as wheel motor drives and portable generator sets. The coreless stator design of AFPM machines reduces the weight and eliminates core losses, which can operate at higher efficiency than conventional machines. The coreless AFPM machine has two outer rotor discs and one inner coreless stator disc. The rotor discs play a role both in magnetic circuit and mechanical support. For the coreless AFPM machine, the axial and circumferential electromagnetic force in the rotors will be generated by air-gap magnetic field. The circumferential electromagnetic force leads to electromagnetic torque, which is necessary for the motor rotation. The axial electromagnetic force may cause the rotor's deflection and affect the machine's reliability. Several analysis methods of the axial electromagnetic force and structure deformation for the AFPM machine have been reported. In [1], a simplified two-dimensional (2-D) finite element method (FEM) and three-dimensional (3-D) FEM are used to analyze and design the rotor discs of a high-speed AFPM generators. In [2], the combination method of numerical calculation and FEM is used to study the stiffness of the rotors of an AFPM machine. But there still remain some problems. 3-D FEM is time consuming and 2-D FEM is not accurate enough.