Motor Torque and Magnetic Levitation Force Generation in Bearingless Brushless Multipole Motors

In this paper, the principles of motor torque and suspension force generation in bearingless brushless motors with high pole numbers are explained, graphically illustrated, and analyzed in detail. The necessary harmonic components of the flux density distribution in the air gap are calculated and it is visualized how these can be generated by superposition of harmonics depending on a specific rotor pole/stator slot ratio. Especially, for bearingless motors with a fractional pole/slot ratio, it is shown how superior and precise suspension performance and high rotational speeds can be achieved. A basic analytical model is introduced and the considerations are exemplified for a 26-pole/24-slot bearingless brushless single-layer motor with concentrated windings compared to alternative feasible configurations. Adequate performance criteria for the motor as well as the bearing comparison are defined.