As the data storage technology advances, there is an increasing demand to develop hard disk drive (HHD) spindles supported by fluid film bearings (FFB) in order to achieve extremely high recording density. The effect of the unbalanced electromagnetic pull is a particular design issue in electromagnetic aspects for such spindles, the common topology of which is permanent magnet brushless (BLDC) motor. Since its direction and magnitude are changing with the rotor position, such an unbalanced pull causes vibration and acoustic noise. For FFB spindles, the unbalanced pull affects the rotor dynamics and also the performance of the journal bearing. In order to investigate these effects in BLDC motors, an appropriate modeling of the air gap magnetic field is needed. As pointed out by many researchers, an one-dimensional modeling of the magnetic field in BLDC machines does not generally suffice for providing accurate and reliable information about the machine performance. Zhu [1] developed an analytical technique for the magnetic field problem in PM BLDC machines. This approach uses Boules [2] solution in polar coordinates to obtain the distribution of radial flux density in airgap and then modify it with relative air gap permeace. The results appeared to be close to the finite element predictions. However, since this approach does not apply to the tangential component of the flux density, further assumptions are necessary if using this technique for force or torque calculations[3]. An analytical model for predicting the magnetic field and electromagnetic pull in brushless dc motors is presented in this paper. The unbalanced pull caused by the eccentricity of the stator and the is also discussed. The computed results obtained from this model compared with the predictions obtained from finite element analysis.
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