Rotary motion was used for conversion long before semiconductor devices were invented: https://en.wikipedia.org/wiki/Rotary_converter https://en.wikipedia.org/w

Rotary motion was used for conversion long before semiconductor devices were invented: https://en.wikipedia.org/wiki/Rotary_converter https://en.wikipedia.org/wiki/Rotary_converter I don't know what the limits on efficiency are, and the Wikipedia article mainly addresses AC-to-DC conversion rather than DC-to-AC, but I assume simply from the fact that they aren't used these days that they aren't an improvement on solid-state inverters.