Spindle motor having reduced torque ripple

Claim 1

A spindle motor for a disc drive, said disc drive including a stationary frame; an elongate spindle shaft fixed with respect to the frame, extending in a longitudinal direction and centered on a longitudinal spindle axis; a hub; at least one disc fixed with respect to the hub; and a means for mounting the hub to rotate relative to the spindle shaft and about the spindle axis; said spindle motor including: an annular longitudinally disposed stator integral with and concentric with the spindle shaft, and a rotor assembly integral with the hub, said rotor assembly including a longitudinally disposed annular magnet concentric with the spindle shaft, substantially centered with respect to the stator in the longitudinal direction, spaced apart from the stator to define an annular air gap, and having a longitudinal length greater than a longitudinal length of the stator whereby the length of the stator determines the length of the air gap; and wherein said magnet is longitudinally and radially symmetrical and includes a plurality of arcuate and longitudinally disposed poles substantially identical to one another in size and shape, adjacent ones of the poles being separated from one another by longitudinal and radial flux gaps running the length of the magnet, each of the poles having a longitudinally extending medial region and first and second longitudinally extending end regions on opposite sides of the medial region, each of the medial region and end regions spanning the entire length of the air gap, each of the poles further being selectively magnetized to provide a first magnetic flux level from the medial region across the air gap, and a second magnetic flux level, greater than the first level, from each of the end regions across the air gap.