Analytical Prediction of Torque Ripple in Surface-Mounted Permanent Magnet Motors Due to Manufacturing Variations

In the production of surface-mounted permanent magnet machines, manufacturing tolerances also contribute to torque ripple. Additional harmonics of low electrical orders are created in the torque spectrum. Slight manufacturing variations develop asymmetry conditions in the rotor as well as in the stator. Finite element models can be used to analyze the magnitude of pulsating torque generated due to these asymmetries. However, due to its random nature, lack of periodicity, and symmetry, it is time consuming to study the problem with numerical methods for its uneven geometry that does not allow reduced geometrical models. Moreover, the experimental verification of all possible variations would be very challenging as it would require part dimensioning and part sorting a large quantity of motor builds and finally testing of all samples. Therefore, in this paper an analytical model is presented to study the effect of such imbalance on the torque ripple. The model shows an agreement against finite element models and also with experimental results when predicting torque harmonics that come from asymmetries.