> I can easily imagine designing a VFD that works just fine over a respectable range of DC input voltages by adjusting its duty cycle accordingly. [...] I can easily imagine designing a VFD that works just fine over a respectable range of DC input voltages by adjusting its duty cycle accordingly.

I imagine it's not the waveform or current that matters so much, as the voltage. These motors would be powering massive blades encountering incredible resistance, so you need megavolts to move them, with an input voltage all the way down to near zero.

> H bridges at tens of MHz

Imagine the MOSFETs on this thing! Do they have something that scales up to MV? That sounds like an engineering challenge in itself.

Full disclaimer: electronics is not my wheelhouse, though I have played around with motor controllers.