Effect of manufacturing variations on cogging torque in surface-mounted permanent magnet motors

During the mass production of surface-mounted permanent magnet motors the rotor and stator geometrical dimensions usually deviate from their nominal values due to manufacturing tolerances which impact machine performance. Radial misalignment in the placement of the permanent magnets on the rotor surface as well as slot opening variations are common deviations observables in the final product. As a consequence, new harmonic orders of cogging torque are created. Finite element models can be used to analyze the cogging generated due to these asymmetries. However, it is extremely time consuming to study this uneven geometry numerically since periodicity and symmetry conditions cannot be used to reduce the model. Therefore, in this paper an analytical model is presented to study the effect of such imbalance on the cogging torque. The model is validated against a finite element model and also with experimental results.