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Excitation frequencies of cogging torque in a brushless dc motor are composed of the fundamental and its harmonics of the least common multiple of the poles and slots in ideal brushless dc motors, and the slot harmonics of the multiple of slot number due to the manufacturing errors. One of the main sources of the slot harmonics is the uneven magnetization of the permanent magnet. We investigated numerically and experimentally the uneven magnetization of permanent magnet due to the design of the magnetizing fixture and its effect on the harmonics of cogging torque. We proposed a new design of the magnetizing fixture to uniformly magnetize the permanent magnet in such a way to reduce both the fundamental harmonic and the slot harmonics of cogging torque. The slot harmonics and the fundamental harmonic of cogging torque of a brushless dc motor can be suppressed by including the back-yoke and the slots of the teeth in the magnetizing fixture, respectively.
Date of Publication: Oct. 2011