I have allready converted some aircraft instruments with stepper motors, vehicle instrument steppers, I posted that on another link, this time I'd like to drive synchro's direct.
Yes I'd considered square wave, and what cheap inverters call modded sine wave, I assumed that would cause issues with accuracy and jitters, but maybe not at 400hz, that would have to be tried, having said that I'd need 4 high voltage bridge drivers and an inverter, 4 audio amps and mains transformers would give me 4 clean sine waves, that is however a lot of stuff for a little synchro.
Ron yes 200ma at 12vwas as far as I wanted to go with the little brush wires to the rotor, if I went to 120v then I'm sure I'd just see a little blue flash, the inductance and max flux density the core will stand denotes how long a voltage can be present, 50 hz would probably fry them too.
Something I'd considered too was putting a neo magnet in place of the rotor coil, turning magnets is a messy job though.