TABLE OF CONTENTS Despite the general acceptance and growth of AC power systems, the need continued for DC in electrochemistry, variable speed motors, and traction systems. Although George Westinghouse demonstrated AC traction motors, they were operated at 25 Hz rather than the prevailing 60 Hz used for utility systems. Metropolitan transit systems, however, required DC for the traction motors. The New York subway system generated and distributed 25-Hz power but ran the cars on DC through rotary converters. With the predominately 60 Hz power systems, the problem became one of converting 60 Hz AC to DC. The terms rectifier and converter are often used interchangeably to denote a system of any sort that converts AC to DC. Present-day usage seems to favor rectifier to denote an uncontrolled conversion system and converter to denote a controlled system.
Rotary converters, double wound, rotating synchronous machines, had been used on 25-Hz power to generate DC since the early days. The need for DC by some industries had influenced the choice of 25 Hz for the initial generation at Niagara Falls because, in the early days of magnetic materials, it was difficult to make the rotary converters operate on 60 Hz. In later years, such operation became feasible, and rotary converter substations were scattered around the outlying regions of urban transit systems to provide DC for the trolly wires. Generally a large-diameter, narrow machine, the rotary converter was a fixture in DC power conversion for more than half a century.
