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This paper investigates the characteristics of the unbalanced magnetic force (UMF) of a brushless dc (BLDC) motor due to uneven magnetization of a permanent magnet (PM), rotor eccentricity, stator eccentricity, and interactions between these factors. An experimental device is developed to measure the UMF of an electric motor directly. Finite-element analysis is performed to compare experimental and simulated results. The characteristics of the measured UMF match those of the simulated one. The UMF always exists in the symmetric design of pole, slot and winding of BLDC motors if there are manufacturing or operating errors such as uneven magnetization of the PM, or rotor and stator eccentricities. This research shows that the driving frequencies of the UMF due to uneven magnetization of the PM are the first harmonic and the harmonics of slot number plusmn 1. It also shows that rotor and stator eccentricities mainly generate the first harmonic and the harmonics of the pole number, respectively.