Issues in reducing the cogging torque of mass-produced permanent magnet brushless DC motor

Abstract

A variety of techniques are available to reduce cogging torque in permanent magnet brushless DC motors. Theoretically, all the techniques are quite effective for minimizing the cogging torque. This research presents the efficacy of these methods in mass production subject to manufacturing tolerances/variations. The cogging torque minimization becomes a challenging task when the requirement is very stringent in applications such as electric power steering and robotics. Some of the known techniques for reducing the cogging torque are the magnet pole arc design, skewing, step skewing, and dummy slots in the stator lamination. They are discussed in this paper considering manufacturing tolerances/variations when used in mass production. Finite element analysis is carried out to determine the worst-case scenarios. The research demonstrates that the cogging torque-amplitude and -frequency are highly sensitive to magnet shapes, dimensions, locations and magnetization pattern as well as slot/pole combination. In reality, the cogging torque may not be eliminated completely but minimized to a satisfactory level depending on the application requirements.