Cogging torque in slotless permanent magnet machines

Slotless permanent magnet machines theoretically do not produce any cogging torque, provided the stator inner surface reluctance remains unchanged. However, a cogging torque may occur due to a change in the electromagnetic design following material cost reduction in the manufacturing process. When the electromagnetic design is changed to reduce the waste in the manufacturing process (stamping of the laminations) by dividing the stator toroidal ferromagnetic core into two equal parts, gluing them together introduces a very small non magnetic gap between them, which affects the stored magnetic energy along the circumference and consequently gives rise to occurrence of a cogging torque. Using the finite element method its value is analyzed analytically and verified by a laboratory measurement. Results of our work, presented in this paper, will help scientists and engineers to understand and avoid the causes for the occurrence of the cogging torque in designing slotless permanent magnet machines, where the design itself is subjected to the manufacturing process.