Influence of Manufacturing Tolerance in Ring Magnet on Cogging Torque of Surface Permanent Magnet Synchronous Machine

Small-size surface permanent magnet (SPM) synchronous machines with ring magnet are widely used due to their compact size, low cost, and ease of assembly. However, the manufacturing tolerance of ring magnet can significantly increase cogging torque in SPM machines, making it challenging for machine designers to reduce cogging torque during mass production. This paper presents an in-depth analysis of the mechanism of cogging torque influenced by manufacturing tolerance based on the co-energy method. Theoretical derivation and finite element (FE) analysis have shown that the cogging torque of a
Q
-slot/2
p
-pole SPM machine with a ring magnet is mainly caused by the
nQ
th (
n
= 1, 2, 3…) harmonics of the squared magnetic field of the ring magnet. Therefore, a simple quality index is proposed to suppress the cogging torque before assembling the ring magnet into an SPM machine, which is helpful for designers of magnetizing fixtures and SPM machines. All the analysis has been verified through 2D FE simulation and experiments on 18 prototypes, consistent with the theoretical analysis.