Characterization and Experimental Verification of the Axial Unbalanced Magnetic Force in Brushless DC Motors

The characteristics of the axial unbalanced magnetic force (UMF) in brushless dc (BLDC) motors were numerically and experimentally investigated. Three-dimensional finite element analysis was conducted to characterize the axial UMF. An experimental device was also developed to measure the axial UMF and verify the simulated results. The axial asymmetry of the permanent magnet (PM) with respect to the stator was found to be a major source of the axial UMF, whose driving frequencies are the least common multiples of the pole and slot. Additional uneven magnetization of the PM and rotor eccentricity introduce slot harmonics to the axial UMF, while additional stator eccentricity generates fundamental and pole harmonics.