A PWM Harmonics Elimination Method in Simultaneous Estimation of Magnetic Field and Displacements in Bearingless Induction Motors

This paper introduces a novel method for simultaneous estimation of the revolving magnetic field and rotor radial displacements in bearingless induction motors. Four auxiliary windings, i.e., search coils, are wound around the stator teeth in an induction-type bearingless motor. The coils are connected to a high-frequency voltage source. The middle-point voltages between two search coils are processed with simple analog circuits to obtain radial rotor displacements. These middle-point voltages are affected by harmonic voltages generated by a general-purpose PWM inverter connected to motor terminals. Two methods of noise elimination are proposed and tested. The theoretical analysis and the successful experimental results validate the proposed approach.