I'm aware of some elvators that still use a rotary converter setup exactly like a single phase to three phase or changing frequency. You can't easily get three phase out of single phase, but if you have three phase AC, then getting DC from it is easy, isn't it?
This rotary converter I have seen is a Westinghouse 25kW "MG set" that is made of a 40hp 460v 3ph motor and a 240v DC 105A generator in one case. Why do they continue to use these as an elevator power supply rather than converting to diode rectifiers?
AC motor -> DC generator -> DC motor seems inefficient. The MG set itself is probably maintenance intensive too. I can't think of a reason why they'd continue using it.. Any idea?