A Perspective on the Normative Performance Description of Electrically Excited Flux-Modulating Machines in Wind Generator Applications

Flux-modulating machines are emerging as noteworthy machines, particularly in wind turbines. Due to their brushless medium-speed operations, they can be deployed for applications where reliability is a critical factor. Two popular machines in this category are brushless doubly fed machines (BDFMs) and wound-field flux-switching machines (WFFSMs). Although these machines work according to similar flux cross-coupling principles, they have almost contrasting descriptions. While the low power density (power per volume) of BDFMs is well documented in the literature, WFFSMs are commonly touted for their high power densities. In this paper, the performances of BDFMs and WFFSMs are compared to harmonize the perspective on their performances in wind generation applications. It is revealed that the compared BDFM and WFFSM topologies have identical performances with some notable nuances. They generate similar power for a given volume, with the WFFSM having greater efficiency, but they have lower power factors. Compared with conventional topologies like doubly fed induction generators, both flux-modulating topologies have considerably lower power densities. Nevertheless, it has been demonstrated that some design insights for one topology (WFFSM or BDFM) are transferable to the other, such as high-performing pole-pair combinations and magnetic loadings. It should also be noted that design developments such as dual stator configurations are beneficial for both topologies, as they enable significant increases in the power generated for a given volume.