<p>The airgap flux density harmonics of permanent magnet synchronized machines (PMSMs) have a significant effect on the cogging torque and torque ripple. The existing reduction methods for cogging torque and torque ripple only reduce the harmonic content, while the coupling relationship between torque harmonics, flux density harmonics, magnetomotive force (MMF) harmonics and permeance harmonics has not been analyzed. Based on this, this paper analyzes the effect of flux density harmonics on cogging torque and torque ripple of fractional slot centralized winding PMSMs by comparing multi-pole few-slot and few-pole multi-slot machines. Firstly, the distribution of the airgap flux density harmonics is analyzed based on the permeance model and the MMF distribution model, and the computational model from flux density to torque is established by using the Maxwell tensor method. Based on this, the relationship between flux density harmonics, cogging torque and torque ripple is analyzed. The mutual canceling effect of the torque components generated by the flux density harmonics of particular orders is proposed, and the general laws and principles of the particular orders under different conditions are investigated. The proposed model and the mutually canceling effect provide a theoretical basis for the future optimization of the shape of the stator core and the permanent magnets to reduce the torque harmonics.</p>