The rotary converter you describe was in use in New Yorks Edison DC Distribution System from the early 1900s. It consisted of rotary converters from 1000 kW to 4500kW units. These units were fed from an 11 kV 25 Hz 3 phase system through a transformer that stepped it down to 600 volts (6 phase with a derived neutral). The output was 250 v dc with a neutral. The rotary converters without the derived neutral used a compensator (motor/gen) combination which could be used as (motor/motor)(motor/gen)or (gen/motor) when connected between the positive and negative buses to compensate for any umbalance between the positive and negative buses.
These converters could be started from the ac side using a wye-delta switch, if the unit came up in the correct polarity its speed and field could be adjusted to match the dc bus and then connected to it. Units could also be started from the dc side using a starting resistor box, when the resistance was all cut out, the unit was brought up to speed and synchronized to the ac side.
I believe these units were retired in the late 1970s or early 1980s and were replaced by submersible rectifiers to supply the remaining dc system in lower Manhattan.
