Design and Analysis of a Permanent Magnet Frameless Motor

Compared to the conventional frame motor, the frameless motor (FlM) has lower torque ripple and higher power density, which becomes the ideal selection in the robotic field. In this article, a novel permanent magnet (PM) FlM is proposed, which has a modular three-teeth stator structure and toroidal windings. The air gap flux density and cogging torque are analyzed by analytical method (AM), and back electromotive force (EMF) and torque of the PMFlM are calculated by magnetic circuit method, which can be used to achieve the best parameter values of PMFlM. The torque density of PMFlM is improved by increasing the axial length of the rotor and PM poles. A PM synchronous motor (SM) with toroidal winding and air gap in the stator pole (topology II) and traditional PMSM (topology III) are designed. A prototype is manufactured, and the electromagnetic characteristics of PMFlM are analyzed by both the experimental test and finite element method (FEM). While the three topologies have the same volume, the amplitude of the back EMF and the torque density of PMFlM are improved by 55.7% and 35.8% compared to those of topology II, respectively. The torque density of PMFlM is larger by 3.5% than that of topology III.