Improved Torque Capacity for Flux Modulated Machines by Injecting DC Currents into the Armature Windings

Flux modulated machine (FMM) comprises three parts: a conventional stator with armature winding, a rotor with permanent magnet, and iron flux modulation pole. Thanks to the inherent magnetic gearing effect, FMMs have been gaining more attention for their features of high torque density. So far, the windings of existing FMMs are fed with pure sinusoidal current. In this paper, to further improve the torque density using the magnetic gear principle, dc-biased sinusoidal current is innovatively injected into the armature coils which are arranged in a specific way, and the electromagnetic performance is analyzed by the finite element analysis. The results show that the electromagnetic torque contains not only the preexisting PM torque, but also the additional torque produced by the dc current interacted with the ac armature current. What is more, it is found that with the proposed dc-biased current, the torque density is improved under the constant copper loss compared with the existing pure sinusoidal current. Besides, the flux adjustment ability is greatly enhanced since the exciting field can be easily adjusted by the injected dc current.