Investigation of Manufacturing Tolerances on Cogging Torque and Torque Ripple in Permanent Magnet Synchronous Machines

This thesis investigates the influence of manufacturing tolerances on cogging torque and torque ripple in permanent magnet (PM) synchronous machines by phasor analysis, Taguchi method, and finite element method (FEM). The investigated manufacturing tolerances include PM tolerances, rotor eccentricities, tooth bulges, and additional air gaps for single-type tolerances, and interaction effects of PM tolerances or tooth bulges and rotor static/dynamic eccentricities for multiple-type tolerances. All the obtained results are validated by the 2-dimensional FEM and experiments on the prototypes.
First, the worst-case scenario of single-type tolerances in multiple PMs or teeth with the highest cogging torque is identified by using the Taguchi method and/or phasor analysis, considering PM tolerances, rotor eccentricities, tooth bulges, and additional air gaps. Compared to the tolerance in one PM or tooth, the cogging torque resulted from the tolerances in different PMs/teeth is much more complex and serious.
Second, the interaction effects of multiple-type tolerances, including PM tolerances or tooth bulges and rotor static and dynamic eccentricities, on cogging torque are investigated. Both weakening and strengthening effects are revealed, depending on the relative locations of multiple-type tolerances. By detecting these relative locations during manufacturing process, it is possible to mitigate the worst scenario and minimise the influence of manufacturing tolerances.
Third, the influence of slot and pole number combinations on cogging torque is studied in PM machines with multiple-type tolerances, i.e., tooth bulges and rotor static/dynamic eccentricities.
Finally, a robust design strategy based on the Taguchi method is proposed to reduce the torque ripple caused by the real distribution of additional air gaps in mass production. It shows that the asymmetric circumferential positions of teeth can significantly reduce the torque ripple in mass production.