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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.

In the last three decades, different vector control methods [field-oriented control (FOC), field acceleration method (FAM), universal field orientation (UFO), direct self control (DSC) and Takahashi method among others) have been proposed. It is difficult for students and nonspecialists to understand the drawbacks and advantages of each one. With this in mind, the objective of this paper is to propose a clear classification and comparison of them.