Analytical Calculation of Surface-Inset PM In-Wheel Motors and Reduction of Torque Ripple

Accurate calculation of magnetic field distribution is a prerequisite for motor design and optimization. This article develops an analytical subdomain model of external rotor in-wheel motor with surface-inset magnets (ERIWMSIMs). Besides, to weaken its torque ripple, an improved topology by integrating magnetic flux modulation slots (MFMSs) into tooth tips is proposed. First, the entire field problem is divided into five subdomains, viz., magnet rotor slots, air gap, stator slots, stator slot openings, and MFMSs. Then, this study solves Laplace's or Poisson's equation of magnetic vector potential in each subdomain according to the type of excitation source and use the boundary conditions of adjacent subdomains to determine the magnetic field solution. Consequently, the electromagnetic performances such as air gap flux density, back electromotive (EMF), cogging torque, and on-load output torque are accurately predicted. The analytical results are shown in line with those obtained from finite-element analysis (FEA) and experimental results. Furthermore, the influence of MFMS parameters on the torque is further investigated. When double MFMSs are integrated into each tooth tip, the depth of the MFMS is 0.7 mm, and the width is 1.05 times the slot opening; the results demonstrate that the torque ripple can be essentially weakened compared with other topologies.