I don't know anything about 2-speed motors and don't' own one, but this thread piqued my interest. I have a question.
Baldor has one and two winding 2-speed motors. A 1HP one winding loses 75% of it's HP at low speed (1/2 speed) and a 1HP two winding loses 56%. With a VFD you lose 50% of the HP at half speed. So by opting to control the speed a with a VFD you would actually gain HP at 1/2 speed? Or at least that would be the case with the motors link to below?
Baldor 3-Phase TEFC Two Winding Variable Torque Two Speed Motors
HP, Torque, and RPM all live in a fixed Mathematical relationship to one another. Look it up. Calculation tools re is online. Also simple.
Never mind how you GET there on any one or chosen combination of those parameters, be that alternate winding set, VFD, DC motor, or servo.
For the lower-RPM to NOT lose HP, the Torque would have to go UP.
Add more "poles" and one can goal for that. Compare 2, 4, 6, 8 pole ONE speed AC motors.
Fewer poles - worse - pulsed less frequently as the means of reducing RPM - it only gets weaker, and also rougher, "sooner".
If there was a
cheap and easy "all electronic" way around that
(a VFD is not..), neither of "cone head" step-pulley belted ratios, nor geared-head ratios, would need to exist.
Whether stepless variable speed, fixed-count multi-speed, or solitary FIXED speed, the world is full of devices that STILL need at least one mechanical ratio to keep motor size, weight, cost, and performance where it needs to be for pragmatic use.
IOW?
Don't expect a lot of "magic" to have been overlooked, lo a hundred years and more, still seeking.
"VFD" isn't even CLOSE to "new" in concept nor delivery.
The only thing "new" is that they are now small, light, and solid-state.