Three-phase AC railway electrification

Three-phase AC railway electrification was used in Italy, Switzerland and the United States in the early twentieth century. Italy was the major user, from 1901 until 1976, although lines through two tunnels also used the system; the Simplon Tunnel between Switzerland and Italy from 1906 to 1930 (but not connected to the Italian system), and the Cascade Tunnel of the Great Northern Railway in the United States from 1909 to 1939. The first standard gauge line was in Switzerland, from Burgdorf to Thun (40 km or 25 mi), since 1899.

The system provides regenerative braking with the power fed back to the system, so is particularly suitable for mountain railways (provided the grid or another locomotive on the line can accept the power). The locomotives use three-phase induction motors. Lacking brushes and commutators, they require less maintenance. The early Italian and Swiss systems used a low frequency (16⅔ Hz), and a relatively low voltage (3,000 or 3,600 volts) compared with later AC systems.

The overhead wiring, generally having two separate overhead lines and the rail for the third phase, was more complicated, and the low-frequency used required a separate generation or conversion and distribution system. Train speed was restricted to one to four speeds, with two or four speeds obtained by pole-changing or cascade operation or both.

The following is a list of the railways that have used this method of electrification in the past:

The system is only used today for four rack (mountain) railways, where the overhead wiring is less complicated and restrictions on the speeds available less important. New motive power avoids speed restrictions as it is built with solid-state converters. The four systems are as follows:

They are nowadays industrial rather than low frequency (50 Hz, or 60 Hz (Brazil)), using between 725 and 3,000 volts.

This category does not cover railways with a single-phase (or DC) supply which is converted to three-phase on the locomotive or power car, e.g. most railway equipment from the 1990s and earlier using solid-state converters. The Kando system of the 1930s developed by Kálmán Kandó and used in Hungary and Italy used rotating converters on the locomotive to convert the single-phase supply to three phases, as did the Phase-splitting system on the Norfolk and Western Railroad in the USA.

Usually the locomotives had one, two or four motors on the body chassis (not on the bogies), and did not require gearing. The induction motors are designed to run at a particular synchronous speed, and when they run above the synchronous speed downhill, power is fed back to the system. Pole changing and cascade (concatenation) working was used to allow two or four different speeds, and resistances (often liquid rheostats) were required for starting. In Italy freight locomotives used plain cascade with two speeds, 25 and 50 km/h (16 and 31 mph); while express locomotives used cascade combined with pole-changing giving four speeds, 37, 50, 75 and 100 km/h (23, 31, 46 and 62 mph). With the use of 3,000 or 3,600 volts at 16⅔ (16.7) Hz, the supply could be fed directly to the motor without an onboard transformer.

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