Design aspects and performance of a slotless PM motor for hard disk drives

Slotted motors are presently used in all hard disk drives (HDDs). However, the advantages of a slotless spindle providing low static cogging torque and negligible torque ripples would be required to meet the demands of the HDD industry, as indicated by the trends in increased capacity and smaller formats. The location of the windings in the airgap of a slotless machine resulting in an extended airgap produces lower inductance values, which will provide the drive with better dynamics. These features are discussed in this article. Aspects of the design and construction of a slotless permanent magnet (PM) motor are described and, where appropriate, comparisons are made with a slotted motor. Both motors are intended for use in a 3.5-inch, half-height HDD. The design of the slotless motor meets all the specifications for application in the HDD. The test and performance of the motor show that the slotless motor is superior in performance in comparison with the slotted motor.