Investigation of unbalanced magnetic force in permanent magnet brushless dc machines with diametrically asymmetric winding

The purpose of this paper is calculation of Unbalanced Magnetic Force (UMF) in permanent magnet brushless DC (PMBLDC) machines with diametrically asymmetric winding and investigation of UMF variations in the presence of phase advance angle. This paper presents an analytical model of UMF in surface mounted PMBLDC machines that have a fractional ratio of slot number to pole number. This model is according to a 2-D analytical field model. By an appropriate choice for slot number to pole number ratio, the magnitude of UMF is zero and this is achieved only when the stator slot and coils distribution is symmetrical about diameter of machine. The presented model is validated by 2-D finite element analysis and a good agreement is obtained between them. Also, UMF is calculated in the presence of different phase advance angles. UMF was calculated for 33-slot/34-pole and 36-slot/34- pole external rotor machines with analytical and finite element method. Machine with 33-slot/34-pole has significantly larger UMF than 36-slot/34-pole machine. Also UMF is calculated for 33-slot/34-pole machine in the presence of phase advanced angles and results show that the magnitude of UMF changes with the amount of phase advance angle periodically. The impact of phase advance angle method on the magnitude of UMF is investigated for the first time by finite element method. Due to increasing or decreasing of the magnitude of UMF in the presence of different phase advance angles, the magnitude of UMF is an important feature in the selection of appropriate phase advance angle.