Three-phase motor in railway traction

Abstract

In the 1890s, polyphase systems were developed for distribution of power over relatively long distances, but three-phase motors found only limited use in railway traction because the fixed frequency of supply restricted operating speeds to a few, inflexible rates of rotation. The first use of the three-phase traction motor was on systems which supplied three-phase power direct to the locomotive, with two contact wires above each track. This system was used in Italy, but, though tried in Austria, Switzerland and the USA, it was not widely used elsewhere, and it was replaced either by single-phase AC systems using AC commutator motors, or by high voltage DC railways. In the United States, Alexanderson introduced the split-phase technique, using a locomotive mounted single-phase motor coupled to a three-phase generator, which enabled a single-phase supply in the contact wire to be used in conjunction with three-phase traction motors. This never became a standard method, and it was rivalled by the motor-generator system which used a locomotive mounted single-phase motor, driving a convertor to supply the traction motors with DC. The methods used before 1960 for supplying three-phase traction motors needed complex fixed works, or else they required locomotive mounted equipment which was heavy and large. The advent of electronics changed traction motor design in the 1960s, particularly through the introduction of the compact current-inverter. Between 1962 and 1965, Brush pioneered current-inverter three-phase traction using a diesel-electric locomotive as a test bed. The Brush trials were followed by work in Germany, and a Co—Co diesel—electric locomotive successfully introduced three-phase traction motor technology into modern practice in 1971. The DB Class 120 electric locomotive established current-inverter technology, and the asynchronous motor, in modern traction between 1980 and 1982. By the mid-1980s these were accepted as standard practice.