Source and Reduction of Uneven Magnetization of the Permanent Magnet of a HDD Spindle Motor

Uneven magnetization of a permanent magnet (PM) generates cogging torque with slot harmonics and an unbalanced magnetic force, which are the major sources of vibration and noise in a hard disk drive (HDD) spindle motor. We investigated several potential sources of uneven magnetization of a ring-shaped PM magnetized in a capacitor-discharge impulse magnetizer by transient finite element analysis. We verified our simulation model by comparing the simulated surface magnetic flux density along the ring-shaped PM with the measured flux density. Our research findings demonstrate that positioning error of the coil of the magnetizing yoke is the dominant source of uneven magnetization. High magnetic field intensity in the magnetization moment due to high magnetizing voltage or the presence of a back yoke decreases the uneven magnetization of the PM by compensating for the coil-positioning error.