Bearingless Single-Phase Motor with Fractional Pitch Windings

The bearingless single-phase motor with four concentrated full pitch windings performs levitation force and torque generation with only one winding system. The axial position and the tilt angle are stabilized passively by magnetic reluctance forces achieved by the permanent magnet disk rotor. Flux distribution in the air gap due to the permanent magnet shape is sinusoidal. Owing to the non-sinusoidal stator current distribution, the levitation force vector describes an elliptical force locus curve when the rotor is turned and current density distribution is kept constant. The slim shape of the elliptical locus curve causes problems concerning the stability of the position controller at certain rotor angles, especially, where the force vector crosses the flat parts of the locus curve. To improve the behavior a method is shown which influences the shape of the force locus curve by means of a modified magnetic circuit design of the stator. A fractional pitch winding system with four concentrated coils is used. The space between the poles is filled with an intermediate pole without coil. The result of the modification is a wider shape of the elliptical force locus curve which leads to a better behavior of the position controller in the sections where the force vector crosses the critical flat parts of the locus curve. The disadvantage of a fractional pole pitch is reduction in torque which must be considered for the motor design.