A 10 HP brushless DC motor using an inner permanent-magnet rotor was studied for noise and vibration reduction. The analytic methods used might be of interest to others with similar goals. The flux distribution in the gap is described by the product of three components, the maximum flux density, a normalized magnetomotive force function depending only on the rotor magnets, and a normalized permeance function depending only on the stator tooth geometry. The magnetomotive-force function and permeance functions are expanded in cosine series by Fourier analysis. These were used to solve for cogging torques and unbalanced side forces in the motor. Specific components are identified and modified to reduce noise and vibration, especially at problem frequencies.
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