Modelling magnetic noise in induction machines

The paper presents an analytical technique for modelling the magnetic noise in induction machines, which allows the air gap radial magnetic forces to be expressed as functions of the space harmonics due to eccentricity, saturation, slot geometry, and the winding arrangement. A study of the effects of rotor eccentricity is used to practically evaluate the model. It is shown experimentally that the model successfully identifies the harmful forces for noise. It is also shown how eccentricity is responsible for introduction of harmful low-mode forces. A comparison of noise test results between a standard 4-pole induction motor, running with conventional bearings and one fitted with Active Magnetic Bearings not only shows the influence of rotor eccentricity on noise but also demonstrates the importance of the mechanical coupling between the stator and the rotor.