A study of cogging torque in permanent magnet synchronous machines with fractional slot windings

Results of an investigation of cogging torque in permanent magnet synchronous machines with fractional slot windings are given, and the nature of their origin is considered. This research is important because of the necessity to assess cogging torque at the stage of design of machines developed for areas of application in which there are rigid requirements for noise and vibration level. To simplify the solution, the design of an electrical machine with radial location of magnets is chosen as am object of research. A formula of cogging torque determined as the result of interaction between magnetic field harmonics and corresponding harmonics of the surface current density is developed. Conditions required for cogging torque are determined. Equations for calculating the number of air gap permeance harmonics and magnetomotive force harmonics contributing to the generation of this cogging torque are presented. This study shows that the cogging torque has two components. The first component is the result of interaction of permeance and magnetic field harmonics having the same number; the second component is caused by interaction between the permeance harmonics and mixed harmonics of the magnetic field induced by the stator slotting. An experimental investigation of cogging torque in a machine with 12 slots and 10 poles has been conducted. Fourier analysis of the experimental data verifies the analytical results of experimental data. © 2014, Allerton Press, Inc.