Methods for calculation of skewed permanent magnet motors for short and highly saturated motors

Abstract:
Skewing of either the rotor or the stator stack is a widely applied method for reducing cogging torque and torque ripple in permanent magnet motors. The basic idea behind skewing is to influence the interaction between the rotor magnets and the stator slots. In many theory books and publications, skewed motors are calculated under the assumption of linear addition along the axial direction which is known as multislice modeling. This neglects the effects of the stray fields and field fringing at both ends of the machine and an uneven flux density distribution along the axial coordinate. These simplifications can be made for motors with a high length-to-diameter ratio and low flux densities. For shorter motors with significant saturation, there may be a relevant difference between the linear calculation and the actual cogging torque and torque ripple. Thus, a skewing angle optimization based on the linear superposition may not yield the expected reduction of cogging torque or torque ripple. Under these circumstances, an improved calculation method for skewed rotors is desirable. To assess the influence of the field fringing in skewed motors with small axial lengths, an exemplary motor with short axial length is calculated by a magnetostatic 3D FEA simulation and a multislice model is created that can account for the influences of field fringing near the motor end sides. Thus, the influence of the field fringing on the multislice model's accuracy can be determined. However, even with the inclusion of field fringing in the multislice model, this technique reaches its limitations when skewed motors with short axial lengths are calculated.