Cogging torque minimization technique for multiple-rotor, axial-flux, surface-mounted-PM motors: alternating magnet pole-arcs in facing rotors

A variety of techniques exist for reducing the cogging torque of conventional radial flux PM machines. Even though some of these techniques can be applied to axial flux machines, manufacturing cost is especially high due to the unique construction of the axial flux machine stator. Consequently, new low cost techniques are desirable for use with axial flux PM machines. This paper introduces a new cogging torque minimization technique for axial flux multiple rotor surface magnet PM motors. First, basic principles of the new technique are explored in this paper. A 3-kW, 8-pole axial flux surface-magnet disc type machine with double-rotor-single-stator is then designed and optimized in order to apply the proposed new method. Optimization of the adjacent magnet pole-arc which results in minimum cogging torque as well as assessment of the effect on the maximum available torque using 3D finite element analysis (FEA) is investigated. The minimized cogging torque is compared with several existing actual machine data and some important conclusions are drawn.