Influence of alternate slot openings on torque-speed characteristics and cogging torque of fractional slot IPM brushless AC machines

This paper presents a comparative study of torque-speed characteristics and cogging torque in three interior permanent magnet (IPM) brushless AC machines having different slot openings, viz. open slot, closed slot and hybrid slot (sandwiched open and closed slot), and unskewed and step skewed rotors. The influence of such alternate slot openings on the torque-speed characteristics, magnetic cross-coupling level and cogging torque is examined. Furthermore, the influence of the manufacturing limits and tolerances on the cogging torque of machines having these alternate slot openings is also investigated. In addition, a 2D finite element (FE) technique to analyse the machine with hybrid slot is proposed. Three prototypes, i.e. the machine with closed slot and step skewed rotor, the machine with hybrid slot and without skew and the machine with hybrid slot and step skewed rotor, are built and tested to verify the FE analyses.