A PWM harmonics elimination method in simultaneous estimation of magnetic field and displacements in bearingless induction motors

In this paper, a novel estimation method of both revolving magnetic field and rotor radial displacements is proposed. 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. Thus, two methods of noise elimination are proposed and tested. The theoretical analysis and the successful experimental results validate the proposed approach.