Electromagnetic forces caused by eccentricities in permanent magnet machines

Abstract

Eccentricities cause forces and excitations in electrical machines. These forces result in excessive wear and a reduction in the operational lifetime of the machine. Thus, it is necessary to model them to prevent these conditions and reduce the effects. This thesis was done at Yaskawa Lappeenranta. The scope of this thesis was to model forces caused by different eccentricities and verify the usability of 3D-FEM modelling for the company’s purposes. Furthermore, a simple analytical model was created to calculate axial forces. The FEM models made during this thesis were the first 3D models used to calculate the electrical properties of machines within the company, thus providing new knowledge of their machines’ operation. Yaskawa Lappeenranta uses Altair Flux software for their electromagnetic calculations. Therefore, it was selected as the program to model the machines. Two machine types were selected for this study, PMM500 and PMM1500, where the former represents an embedded magnet machine and the latter a surface magnet machine. Selected machines were modelled with radial 2D and 3D models. Actual machine geometries were followed closely, and the materials were selected from Flux’s and Yaskawa’s libraries. Due to technical restrictions, complete machines could not be fully modelled, and only models with a few partial stacks were created. It was found that some of the forces do not follow a linear increase concerning the number of stator stacks, which is a severe flaw in the practical usability of the program. High calculation times also pose a significant drawback for the practical usability of 3D modelling to calculate the electromagnetic properties of the machines. Radial forces were modelled only with 2D-FEM models. Radial eccentricity’s main result is the increase of the DC component of radial force due to a change in permeance across the machine’s air gap. The DC-component increases linearly, concerning eccentricity, and as the force increases, harmonic components from two different sources could be seen. Harmonic components caused by stator slotting could be seen on both the radial forces and the cogging torques. Eccentricity creates harmonic components whose orders are multiples of pole numbers for both machines’ cogging torques. Additionally, eccentricity induced a low order harmonic component to the PMM500’s radial force. No significant result can be concluded for eccentricity’s effect on cogging torque’s amplitude. The amplitude decreased on the PMM500 or increased on the PMM1500 with increasing eccentricity. Axial forces were modelled with 3D models. Axial eccentricity mainly increased axial force’s DC component. This increment was not linear but instead started to saturate with higher eccentricities. Axial eccentricity also decreased the radial force as the decreasing axial length decreased the overall magnetic flux density. The results of the created analytical calculation tool compared to the axial force of the PMM500 machine were promising. The overall difference between the force calculated by the analytical tool and the measurements of the whole machine was only 7.37%. The difference was higher when compared to 3D-FEM results. This is due to axial force not increasing linearly with increasing eccentricity, which is the basis for calculation with an analytical model. However, the model could not be compared to other machine types. Thus, more verification is required before the credibility of this model can be summarised.