Torque quality assessment and sizing optimization for surface mounted permanent magnet machines

Torque quality assessment and optimization methods are presented in this paper. General purpose sizing equations are obtained and sizing optimization of the slotless TORUS machine is shown for maximum power density or efficiency points. For pulsating torque analysis, general instantaneous electromagnetic torque equation and torque ripple factor (TRF) analysis are carried out. Besides, stator winding distribution factor and rotor PM skew factor, excitation field form factor to indicate the excitation field harmonics, open slot factor to indicate the slot effect based on the calculation of airgap permeance are introduced for nonslotted and slotted axial flux surface mounted PM machines. Based on the above analysis, optimum design can be achieved for minimum ripple torque. On the other hand, cogging torque can be canceled using nonslotted structure. A slotless TORUS machine is used as a sample application for the torque ripple minimization in the paper. Furthermore, 2D and 3D finite element analysis (FEA) of both nonslotted and slotted surface mounted PM TORUS type (axial flux external rotor internal stator) structure, AFIR type (axial flux internal rotor external stator) structure and RFSM (radial flux internal rotor external stator) type structures are investigated for an insight into pulsating torques, ripple torques and cogging torques. The results obtained from both TRF analysis and 2D/3D FEA are illustrated in the paper.