Investigation of Effects of Asymmetries on the Performance of Permanent Magnet Synchronous Machines

Asymmetries in permanent magnet synchronous machines (PMSM) can result from manufacturing tolerances or optimization techniques that deliberately introduce them on rotor, stator, or both. This paper analyzes the impact of such unbalance on the air gap pressure, that is radial and tangential to the direction of motion, by means of Maxwell Stress Tensor; also, their consequences on torque pulsations and unbalanced magnetic pulling. Neglecting the manufacturing variations in the analysis of high power density PMSMs underestimates the resultant torque ripple. Additionally, new vibration modes occur due to unbalanced radial pressure. On the other hand, a geometry optimized to minimize torque ripple through controlled asymmetry is investigated, the approach compensates harmonics in torque effectively, but such compensation does not have the same effect on radial forces. Finite element analysis and experimentation are used to support the analysis and it is found that an asymmetric PMSM with low torque ripple will not necessarily produce lower vibrations and noise.