Permanent magnet brushless dc motor having reduced cogging

Abstract From the Canadian patent

An improved salient pole configuration for a dynamoelectric machine is disclosed, particularly for a permanent magnet brushless DC motor wherein cogging torque is a problem. One or more apertures or "reluctance holes" are disposed within the head portion of the salient poles in the stator such that the reluctance holes are substantially adjacent but not adjoining the pole face, thereby forming a bridge between the aperture and the pole face. The reluctance hole bridge provides a substantially constant air gap flux density to reduce cogging, while at the same time provides a low reluctance shunt path for the armature reaction flux around the permanent magnet to improve the demagnetization …