Disk drive in-hub radial-gap spindle motor with coils generating axial fields

TLDR: In this article, a brushless permanent magnet DC motor has a radial working gap but uses coils wound in the axial direction of the motor, and the magnetic flux from the permanent magnet is directed radially across the gap and into the stator teeth.

Abstract: An in-hub brushless permanent magnet DC motor has a radial working gap but uses coils wound in the axial direction of the motor. The coils generate fields that interact with the permanent magnet fields across the radial gap. The stator includes a plurality of angularly spaced stator teeth that have faces radially spaced across the gap from the permanent magnet. The stator teeth are connected to the base and extend axially into a cavity within the hub. The permanent magnet is a magnetically-segmented ring magnet located on an outer rim of the hub radially outwardly from the stator teeth. The magnetic flux from the permanent magnet is directed radially across the gap and into the stator teeth. Each stator tooth has an axially oriented post around which is wound a coil that generates an axial field. The axially-oriented part of each coil forms part of the magnetic circuit. The magnetic flux directed radially into each stator tooth is turned axially into the post and through the coil. The axially directed magnetic flux through the coil and post on each stator tooth is redirected from the axial direction to the circumferential direction by a ring magnetic flux guide. The ring flux guide interconnects the stator teeth and forms part of the magnetic circuit. Because the coils are wrapped around axial posts instead of the stator teeth a larger coil volume is obtained which results in higher motor efficiency and reduced heating of the bearing system. The angular spacing between adjacent teeth can be made very small or even eliminated since there is no need to locate a winding device between them as is required in conventional radial-gap motors. This results in a reduction in the motor cogging torque, which in turn allows a higher tooth/pole symmetry leading to better force balance and lower acoustic noise.