Design of Experiments to Address Manufacturing Tolerances and Process Variations Influencing Cogging Torque and Back EMF in the Mass Production of the Permanent-Magnet Synchronous Motors

A number of manufacturing challenges exist in creating a robust permanent-magnet synchronous motor (PMSM) design to meet critical requirements for cogging torque and torque ripple in specific applications. In addition, reduction of the back electromotive force (BEMF) harmonics that lead to torque ripple is also required during design. Due to manufacturing tolerances, maintaining capability with these requirements is very difficult, particularly for mass production of units. A design of experiments (DOE) that combines manufacturing tolerances with assembly process variations is proposed here to address the sensitivity of cogging torque and harmonics. The limits for the parts and the process variations are extracted from an actual production environment. The DOE is laid out according to one of the robust engineering matrices chosen from Taguchi´s method. The DOE results are then utilized to set the tolerance limits for manufacturing the rotor magnets and to set the allowance for the rotor assembly process variations. The method helps to limit the cogging torque and harmonics within predefined bands, thereby producing a robust motor. This paper also investigates the capability of maintaining the tolerances of magnets for a surface-mounted PMSM. The analysis is done using a finite-element method and verified with experimental results.