A rotary converter plant is a facility at which one form of electricity is converted into another form of electricity by using a combination of an electric motor and an electric generator. The installed combinations of motors and generators at a plant determine the possible type(s) of conversion. Such facilities also allow the setting of voltages and frequencies, if appropriate equipment is installed.
Rotary converter plants existed before the invention of mercury arc rectifiers in the 1920s at each facility where DC gained from an AC power grid was fed into an overhead line or a third rail of a railway. In spite of modern semi-conductor technology, they are still common for feeding railway systems with AC of another frequency than that of the main electricity grid. Rotary converter plants were also used for coupling power grids of different frequencies and operation modes. For the latter, the former Neuhof Substation was a good example. Former machinery transmitters like the Alexanderson alternator were, strictly speaking, rotary converter plants.