Decoupling Manufacturing Sources of Cogging Torque in Fractional Pitch PMSM

Fractional pitch is commonly used to significantly reduce cogging torque in PMSM, however, maximum benefit is dependent on accurate stator and rotor manufacturing. This paper presents a method of decoupling the stator and rotor contributions to total cogging torque. Rotor causes are further decoupled into magnet placement and strength variation. Decoupling is possible due to stator and rotor affected harmonics being independent of one another. Magnet strength and position decoupling is based on the analysis of the cogging torque waveform generated by the rotor interaction with a single slot stator and utilizes the zero torque produced when a single magnet is directly over a slot. Superposition and least squares minimization is then used to determine strength variation and simulate cogging torque with and without placement and strength variation. Analysis of ten production stators and rotors is presented and discussed, with the overall findings confirming that for the motors tested, the largest contributors to manufacturing induced cogging torque were the stator, magnet placement inaccuracy and magnet strength variation. Eliminating stator variation would improve cogging torque by 45%, perfect magnet placement would result in a 29% reduction in cogging torque and eliminating magnet strength variations would achieve a 7% reduction.