Mechanical-magnetic coupling vibration instability of an annular rotor subjected to synchronous load in axial-flux permanent magnet motors

Abstract

This work examines the vibration instability regarding the out-of-plane bending of an annular rotor of axial-flux permanent magnet (PM) motors. An analytical model is built up in magnetic-load-synchronous frame, where the magnetic load is rotating with the rotor at the same speed, and the rotor-fixed support and rotation stiff of the annular and web are included. By using the synchronous frame, a governing equation of motion without time-variant coefficients is developed. Based on this model, the eigenvalues are formulated through conventional vibration theory. The dependence of mechanical-magnetic parameters such as the vibration wavenumber, magnet count and rotation speed on the vibration instability is examined. The stable and unstable regions are estimated by the closed-form transition curves, and further the position, shape and size of unstable regions are predicted by the transition curves expressed by mechanical-magnetic parameters. The results verify the divergent instability always exists regardless of the combinations of the wavenumber and magnet count, but the flutter instability can occur or be eliminated. The rules governing the occurrence or elimination are presented. The damping effect on vibration instability is also examined. Main results are demonstrated by numerical calculations.