Cogging Torque Reduction in U-Core TFPM Generator Using Different Halbach-Array Structures

Transverse Flux PM Machines (TFPM) have higher torque density than the other structures of electrical machines. Due to the higher number of poles in these machines, they can be used in wind turbine and direct drive applications. One of the most important characteristic of wind turbine's generators is the requirements for the low cogging torque. But, the TFPM machines due to their open-slot structure of the stator core, inherently have high level of cogging toque which limits the application of these machines as wind generator. In order to reduce the content of cogging torque in these structures, a TFPM machine with Halbach array magnets is developed in this study. Using Halbach array in TFPM structure can avoid the drop in average torque of machine, improve the harmonic content of the flux-linkage and EMF, reducing the torque ripple and cogging torque. The proposed structure can implement in all TFPM machines which have surface mounted PMs in their rotors. The proposed structures with various number of magnet pieces per pole in the array and pole pitches have analyzed by 3D-FEM and compared with each other by considering the torque ripple, harmonic content of back-EMF, and electromagnetic torque. The results showed considerable decrease in cogging torque with an increase in average torque of machine.