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

Abstract

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.

A Design Method for the Cogging Torque Minimization of Permanent Magnet Machines with a Segmented Stator Core Based on ANN Surrogate Models

“…Finally, note that in the last two harmonic spectra, unexpected weak harmonic components occur. These harmonics can be caused by slightly asymmetries in the model [21]. Nevertheless, the obtained results are overall in good agreement with the expectations.…”

“…The cogging torque of PMMs with a one-piece stator core can be suppressed by using rotor skewing techniques [10], "dummy" stator slots or notches [14][15][16], different slot openings [17], or by choosing different stator slots and teeth widths pairing [18,19]. While rotor skewing approaches are ineffective in case of SSC machines due to the additional lower order harmonics introduced by stator core segmentations and stator asymmetries [10][11][12][13][20][21][22], the effectiveness of the other techniques has not been proved in the presence of SSCs.…”

Cogging torque sensitivity to permanent magnet tolerance combinations

“…It is due to the fact that the step-skew angle θ was not exactly 6.66°, as we found out later by precise measurements. In measured T cog can be seen some other harmonic components N AHC i = 6, 12, 18 that are correlated to asymmetries in a stator lamination stack like interlocks and notches [12,15]. Another parameter, which was not considered in presented study but certainly exists, is a discrepancy in PM magnetization [16], anyway this effect has origin on the rotor side, therefore it contributes T AHC components of already known orders N AHC i = 9, 18, 27.…”

Cogging Torque Suppression of Modular Permanent Magnet Machines Using a Semi-Analytical Approach and Artificial Intelligence

“…Note that these errors affect both AHCs and IHCs of the cogging torque and may potentially compromise the effectiveness of the optimization. It is also worth noticing that manufacturing and assembling tolerances are sources of other AHCs of the cogging torque, as illustrated in [39] and [40].…”