Development of Asymmetrical Four-Phase Bearingless Motor with Unequal-Tooth-Pitch Stator

A bearingless motor integrates the function of a magnetic bearing, allowing for non-contact rotation; however, displacement sensors are required for active magnetic suspension control, but this increases the size of the motor. To reduce the size of the motor, this study proposes an unequal-tooth-pitch stator configuration for a 10-pole/12-slot bearingless motor, where displacement sensors and Hall effect sensors can be placed between the stator teeth. From the results of a finite element analysis (FEA), a stator shape that does not fluctuate the unbalanced magnetic pull caused by the changes in rotation angle was designed. Optimal asymmetrical four-phase combined winding arrangements were selected based on the magnetomotive force distribution. A current-suspension force model considering up to the fifth harmonic of the electric angle was obtained by the FEA and used for suspension force control. The test machine was successfully driven at 1500 rpm by magnetic levitation.