Minimization of cogging torque in a small axial-flux PMSG with a parallel-teeth stator

A stator with a steel core is well suited for small, low speed PM machines, where core losses are much lower than copper losses. However, a slotted stator introduces cogging torque. A stator with parallel-teeth inherently lowers cogging torque but there is need to further reduce it. This is particularly important in wind generators since it raises the cut-in wind speed, thereby lowering the energy captured yield for a given installed capacity. Pulsating torques also produces noise and mechanical vibrations which accelerates the wear of the machine and its support structure. This paper presents some effective methods of minimizing cogging torque, which have minimal effects on the performance of the machine. Three PM rotor configurations were considered, which include: a pole-arc ratio of 0.80, alternating pole-arcs of 0.61 and 0.80, and skewed PM poles with a skew one slot-pitch. The 3 rotor configurations were simulated numerically using 3D Finite Element Analysis (FEA) and tested experimentally with the same stator. The machine topology with a rotor pole-arc ratio of 0.80 was used as the reference design. From the experimental results, the alternating pole-arcs effectively reduced cogging-torque by 73% whereas the skewed poles reduced it by 48%. The 3D-FEA and experimental cogging torque results are also comparable. The paper also compares cogging torque reduction techniques which are well suited for a parallel-teeth stator.