Semi-analytical method for air-gap main magnetic field computation of direct drive permanent magnet torque motors

This paper presents a semi-analytical method to computer the air gap magnetic field of direct drive permanent magnet (PM) torque motors for being evaluated slotting effects accurately. The field region is divided into the smooth air gap region and the extended slot sub-regions. The magnetic vector potentials in the air gap region are first solved by the analytical method, and those in the extended slot sub-regions solved secondly by the numerical method. Boundary values in the extended parts of slot sub-regions are obtained by the analytical solutions while permanent magnets are dealt with the equivalent surface current sheets. After combined the results of each slot sub-region the flux density distributions in air gap with slot opening are obtained. The magnetic field distribution and back-EMF computed with the proposed method are verified with those obtained from finite element method (FEM) and measurement.