Manufacturing-Induced Cogging Torque in Segmented Stator Permanent-Magnet Machines With Respect to Steel Punching

There are applications like electric-power-assisted steering system in which strict requirements on the cogging torque of the machine should be fulfilled. Numerous techniques and methods are introduced to minimize the inherent cogging torque of the machine, but manufacturing tolerances and imperfections still can cause extra cogging torque. In this article, the impact of punching on the manufacturing-induced cogging torque of segmented permanent-magnet synchronous machines is investigated. The punching model is implemented by modifying the BH curve of the steel as a function of distance from the cut edges. The parameters of the new BH curve are identified by means of standard Epstein frame measurements using steel strips with different widths. Then, the punching model is incorporated into the finite element (FE) model of a 10-pole 12-tooth segmented stator machine designed for power steering application. It is observed that deviation among the magnetic properties of the teeth caused by punching can introduce extra low-order harmonics in the cogging torque of the machine. The amplitude of the new orders can be comparable or even much bigger than the inherent orders, where not only the magnitude of deviation but also the arrangement of teeth with given punching parameters play a role.