Cogging reduction in permanent magnet machines via skewed slot opening and its analytical modeling

Abstract

Air-gap magnetic energy variation with angular position produces cogging torque, which may results in mechanical vibration, acoustic noise, and torque ripple. Various cogging reduction methods of design modifications viz. skewed magnets, skewed slot, asymmetrical displacement of magnets/slots, etc. are reported in the literature. All such methods adversely affect machine performance in terms of air-gap magnetic field, back emf, and induced voltage. This paper introduces the cogging torque reduction by skewing of slot opening. In order to obtain machine performance, the no load magnetic field of the proposed machine is determined using combined methods of two-dimensional subdomain analytical analysis method and multislice method. The machine is considered as a stack of slices along axial direction. The adjacent slices differ in relative location of slot openings. The analytical field solution of each slice is obtained by use of subdomain method, and algebraic summation of slices is taken as field solution of actual machine. The analytical analysis developed is compared with finite element analysis (FEA). The close agreement of analytical results with FEA results confirms the validation of analytical solution. Furthermore, the machine parameters viz. cogging torque, back emf, and induced voltage are evaluated analytically, and results are compared with FEA solution. To demonstrate the effect of skewed slot opening on machine’s performance, a machine of same rating without skewing of slot opening is investigated, and their performances are compared.