PM Synchronous Motor Induced Vibration

Abstract

Permanent magnet synchronous motors (PMSMs) are widely used in the industry. Radial dynamic forces arise when the rotor is eccentrically located relative to the geometric center of the stator due to electromagnetic (EM) fields in the motor. A possibly deleterious effect of this may be unwanted vibrations. A second effect of the EM field is a negative stiffness which tends to pull the rotor away from the stator center. This stiffness can shift rotor natural frequencies, especially in systems that have relatively low bearing or shaft stiffness, i.e. in some magnetic bearing applications. Resonance may occur when a natural frequency is shifted into an excitation frequency, i.e. from imbalance or the dynamic forces resulting from the eccentric motor. Most prior studies modeled the eccentric force with a simplified analytical model neglecting iron saturation and accounting only for the fundamental component of the radial air gap flux density. An improved approach is presented here utilizing the finite element method (FEM) to perform dynamic, electromagnetic field analysis of PMSMs with rotor eccentricity. Both radial and tangential airgap flux densities and eccentric forces are considered. The example illustrates how a rotor resonance can occur due to its negative stiffness shifted natural frequency coinciding with a forcing frequency produced by the eccentrically operated PMSM.