Flux-modulating machines are emerging as noteworthy machines, particularly in wind turbines. They typically have two windings on their stators, which are not directly coupled, but use a flux-modulating rotor to cross-couple the two stator windings. These brushless machines are typically medium-speed machines, and they boast reliability suitable for remote areas with low accessibility.

Two popular machines in this category are the brushless doubly fed machines (BDFMs) and wound field flux switching machines (WFFSMs). Although these machines work according to similar operating principles, they have almost contrasting descriptions. BDFMs, viewed as alternatives to doubly fed induction generators (DFIGs), are noted for their comparably lower power density and efficiency. Conversely, WFFSMs are typically touted for their high power densities, hence the suggestions of replacing PM machines.

In this paper, the performances of BDFMs and WFFSMs are compared. Select topologies that operate at the same speed are evaluated: the 4/6 BDFM and the 24/10 WFFSM. Instead of applying a direct comparison between these two topologies, optimized 250 kW designs are compared to their conventional parallel topology. BDFMs are compared with DFIGs, while WFFSMs are compared with wound field synchronous machines. The optimization processes are conducted using the non-dominated sorting genetic algorithm coupled with response surface approximations from FEA evaluated designs.

It is shown that a 4/6, 250 kW BDFM typically has about 5 % less efficiency compared to a 20-pole DFIG, while being almost two times the volume. The efficiency in WFFSMs is better due to less copper loss; however, their power factors are low. This is due to the cross-coupling nature of operations in BDFMs and WFFSMs, which leads to significantly lower flux utilization compared to their directly coupled counterparts. However, their reliability advantages cannot be dismissed.