This statement is corroborated on
Beyond this, increasing generator size makes it more and more difficult to guarantee that direct-drive turbines can maintain the size of the air gap, i.e., the radial distance between the generator rotor and stator. Technical solutions require either great care to be taken during component production or a stronger magnetic field, which, in turn, increases the weight and needs more rare-earth elements for the permanent magnets.
This confirms the questioning of A large diameter ring style generator, and also, more generally, some of my topics relying on a large ring generator, such as Beaujean’s AWES as a start for a collaborative project, Rotor surrounding a round kite, a variant from Flettner balloon and VAWT side by side , and perhaps elsewhere.