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We present an analytical model of unbalanced magnetic force (UMF) in fractional-slot surface-mounted permanent magnet (PM) machines having diametrically asymmetrical winding distribution but no static/dynamic rotor eccentricities. It is based on a 2-D analytical field model and accounts for the influence of both the radial and tangential force waves under any load condition. It is capable of providing insight into the generation, harmonic contents, and characteristics of the UMF and accurately predicting the magnitudes, rotation directions, phase angles of its harmonics and various components, such as the UMFs due to armature reaction only, mutual interaction between both PM and armature reaction fields, and radial and tangential force waves. The cancellation and additive effects between the UMF components resulting from the radial and tangential force waves are revealed for the first time by the analytical model. We show that such effects strongly depend on the UMF harmonic order, slot/pole number combinations, and the internal/external rotor machine topologies. We have validated the analytical model by finite-element analyses and partially by experimental results.
Date of Publication: July 2010