The installed combinations of motors and generators at a plant determine the possible type(s) of conversion.
Rotary converter plants were commonplace in railway electrification before the invention of mercury arc rectifiers in the 1920s.
Rotary converter plants were also used for coupling power grids of different frequencies and operation modes.
Former machinery transmitters like the Alexanderson alternator were, strictly speaking, rotary converter plants.
In spite of modern power semiconductor technology, rotary converters are still common for feeding railway systems with AC of a different frequency from that of the main electricity grid.