Three dimensional analysis of magnetic force and torque in a BLDC motel due to the axial variation of yoke and pole

The characteristics of magnetic force and torque due to yoke and pole patterns in the axial direction were analyzed by a 3 dimensional finite element method, Maxwell stress tensor and Fourier spectral analysis for a brushless DC (BLDC) motor. This research shows that the magnetic field in a small air gap produces not only the torque as a resultant of the tangential force, but also an axial resultant for an axially asymmetric motor, which has a yoke-ceiling or skew magnets. This axial resultant has driving frequencies corresponding to the integer multiple of the product of the number of poles and phases. The axial resultant may increase the possibility of mechanical resonance of the motor as well as the surrounding structure.