Additional cogging torque components due to asymmetry in stator back iron of PM synchronous motors

Purpose - The purpose of this paper is to present the dependence of the level and harmonic structure of the cogging torque in permanent magnet synchronous motors (PMSM) to interlocks and notches in a stator back iron, which are standard methods for stator lamination stacking in mass-production. Design/methodology/approach - Methods of stacking up the lamination like welding or interlocking are introducing magnetic asymmetries in stator back iron which causes additional harmonic components (AHC). A finite element method and Fast Fourier transform were used to calculate cogging torque harmonic components (HC) with regard to the applied number and positions of interlocks and notches. All simulation results were verified by laboratory tests. Findings - It has been established and proved that technologies for stacking lamination packs cause local saturation peaks in back iron which give rise to additional cogging torque AHC and consequently increase the total cogging torque. It is also shown that the magnetic properties of interlocks cannot be simply considered as air regions but adequate relative permeability of such affected soft magnetic material must be determined to improve the accuracy of FEM calculations. Practical implications - Considering presented results, it is possible to foresee which AHC will include the cogging torque of mass-produced PMSMs due to the stator lamination stacking methods. In this way, the optimal stacking method can be selected in order to minimize the effect of AHC. Originality/value - So far, authors dealing with the cogging torque have not taken into account the influence of the stator lamination stacking method on the level of torque oscillations.