Applications
A typical use for an AC/DC converter was for railway electrification, where utility power was supplied as alternating current but the trains were designed to work on direct current. Before the invention of mercury arc rectifiers and high-power semiconductor rectifiers, this conversion could only be accomplished using motor-generators or rotary converters.
Most machinery and appliances were operated by DC power at the turn of the century which was provided by rotary converter substations for residential, commercial and industrial consumption. Rotary converters provided high current DC power for industrial electrochemical processes such as electroplating. Steel mills needed large amounts of on site DC power for their main roll drive motors.
Read more about this topic: Rotary Converter