Equivalent two-phase mutual inductance model and measurement algorithm of three-phase bearingless motor

For a bearingless motor that has two sets of stator windings, i.e. torque windings and suspension windings, the mutual inductance between the two sets of stator windings is a critical parameter; it is the basis of displacement sensorless control technology. Aiming at a three-phase bearingless motor, the equivalent two-phase mutual inductance model between two sets of stator windings is deduced. Then, based on the mutual inductance measurement method of a simple two-phase bearingless motor, a novel measurement algorithm of the equivalent two-phase mutual inductance is proposed, and the proposed measurement algorithm is applicable for a three-phase bearingless motor. Finally, based on a three-phase bearingless induction prototype motor, the experimental measurement of the equivalent two-phase mutual inductance is carried out. From the experimental results, it is clear that within the limited radial eccentricity of the rotor, the measured value of the equivalent two-phase mutual inductance is approximately proportional to the radial displacement of the rotor, and the correction coefficient k_c of the equivalent two-phase mutual inductance model is derived; the consistency between measured and calculated values has verified the validities of the equivalent two-phase mutual inductance model and the proposed measurement algorithm from one instance. Thus, the theory foundation has been laid for research on the displacement sensorless vector control technology of a three-phase bearingless motor.