Cogging torque of a brushless DC (BLDC) motor is a source of the vibration and noise of the hard disk drive (HDD) spindle motor, and the excitation frequencies of the cogging torques are the harmonics of the least common multiple of the poles and slots in ideal BLDC motors. However, manufacturing errors generate the additional harmonics to the cogging torque. Especially, the uneven magnetization of the permanent magnet (PM) in BLDC motors generates additional slot harmonics to the cogging torque. In small BLDC motors such as 2.5″ HDD spindle motors, the effect of additional slot harmonics is as serious as that of the fundamental harmonics of the cogging torque. A coil-positioning error of the magnetizer is a major source among the several potential sources of uneven magnetization of a ring-shaped PM in a HDD spindle motor [1]. It is difficult to estimate and to reduce the coil-positioning error when manufacturing a magnetizer. In addition, each magnetizer has different range of the coil-positioning error.

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