Effects of saturation on eccentricity force in permanent magnet synchronous machines

Permanent magnets synchronous machines are nowadays widely employed. In case of symmetrical construction, aligned mounting of the rotor and balanced supply, the distributed surface magnetic actions on the rotor feature a null resultant. Anyway, in case of defective assembly or when bearings exhibit a reduced or variable stiffness, the eccentricity of rotor determines a resultant force depending on angular position and currents. This paper investigates such phenomenon for a surface-magnets machine, using FEM analysis to permit keeping into account saturation. Simulation results are reported and commented, highlighting that such force may be affected from stator currents in a counter-intuitive way when magnetic saturation of the core becomes appreciable: in such event, the worst-case scenario required for design purposes must be carefully singled out.