For using the ST-3/5 as motor, power transistors have advanced to the point that it is now more practical to do commutation via electronics, thus avoiding a major mechanical modification. I think rotational position feedback will be required. (added to the shaft or pulley; maybe a just a couple magnets for two hall sensors) The field coils can be alternated as well as the stator, if need be.
I'm busy on an low EMI, 120V input inverter project now. I've got a modified square wave version with a single controlled rise/fall H-bridge and a toroidal step up transformer, and a stepped sine presently using 2 H-bridges and two toroidal transformers. I'm only doing 3 steps in each direction plus zero, at present. The goal is to make motors happy; some get hot and growl on MSW. This is a low frequency, transformer isolated design based on the venerable Trace SW design. Running 500 watts of heat lamps, the output AC can only be heard on the lowest end of the AM radio within a few inches of the power cord. The DC side is totally silent. My THD with my on-hand transformers is 18%; the wave is too broad at the top to get to 120VRMS. A typical MSW output is 24%. I should be able to get down to generator class which is 9-15% THD, or better.