This subject has been discussed before, but a few points are worth mention perhaps;
The simplest mechanical means of obtaining a true polyphase ac supply from single phase ac is with a 'Motor-Generator/Alternator', comprising 2 pretty standard machines coupled by a drive mechanism.
I built one for my workshop-
https://www.electrical-contractor.net/ubb/Forum5/HTML/000937.html A more efficient arrangement is to have
two stator frames arranged in line on a single shaft, known as a 'Motor-Converter'. A motor-armature directly drives a generating-armature wound on the same shaft. These were extensively used in ac to dc conversion for traction purposes and were generally reversible for dc to ac operation or starting. Of course, single to polyphase ac with such a machine is easily arranged.
Another type is the true 'Rotary Converter', in which both windings for motoring and generation are on the
same armature and stator. Usually these converted dc to ac for traction, but single to polyphase ac machines can be built. You just need to get your head round the fact that dc or single phase ac currents operate simultaneously with polyphase currents in the same windings!
As to modern practice, these machines are hopelessly inefficient, noisy, short lived and expensive when set against solid-state switching devices. Commercial transmission of HVDC is a practical reality and conversion to true polyphase ac is regularly done for both domestic, commercial and traction end users. The small 'inverters' on the market for home workshop/light industrial use don't make true 3-phase, nor do the ingenious big 3-phase 'master motors' which others have described on ECN, leading to poor performance, 'only-one-tool-at-a-time' and lousy efficiency. I think a motor-generator set up as I described gives the best 3-phase for home workshop use. When you get out to the 5kva level, it probably works out not a lot more expensive, certainly more robust and low tech. The technology exists for a manufacturer [or home builder for that matter!] to make a device which will create true polyphase ac from single phase ac using solid state devices, but at what cost for components I wonder?
Alan
