Cogging torque and UMF (unbalanced magnetic force) are major excitation sources of acoustic noise and vibration originated from HDD spindle motors. They are generally outer rotor type motors with fluid dynamic bearings (FDBs). The FDBs support and constrain the rotating disk-spindle system in five degrees of freedom except axial rotating direction. Unbalanced mass of the disk-spindle system generates whirling motion and changes the characteristics of UMF. Several researchers have investigated the harmonic contents of cogging torque and UMF by numerical and analytical methods [1]–[3]. Lee and Jang [4] experimentally and numerically investigated the characteristics of the UMF of a HDD spindle motor due to manufacturing errors such as the uneven magnetization of permanent magnet (PM) and the eccentricity of rotor and stator. However, they discussed only the cogging torque and UMF, and did not investigate the effect of the cogging torque and UMF on acoustic noise and vibration of a HDD spindle system.

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