Winding Arrangement to Increase Suspension Force in Bearingless Motors with Brushless DC Structure

This paper presents a new winding arrangement to increase the suspension force in a bearingless motor with a brushless DC structure. The bearingless drive technique is applied to a conventional brushless DC motor and the motor structure, principle of suspension force generation and rotor suspension control strategy are presented. The finite element method (FEM) is used to analyze the machine and a control strategy is developed which will stably support the rotor. In a bearingless motor with a brushless DC structure the suspension force is smaller than in the equivalent permanent magnet (PM) synchronous motor with fully-pitched windings despite the machines being of similar size. This is because the suspension force is generated by the unbalanced air-gap flux density; and the air-gap areas in which the flux density is unbalanced are narrower in the brushless DC structure. In this paper, as an example, a 24-slot 16-pole bearingless brushless DC motor is highlighted and the new winding arrangement is proposed which increases the suspension force. The suspension forces are computed by FEM. An increase in the suspension force is confirmed when compared with the conventional winding arrangement. From the computed results, the proposed winding arrangement is effective in increasing the suspension force under equal motor dimensions.