The system provides regenerative braking with the power fed back to the system, so is particularly suitable for mountain railways. 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, and a relatively low voltage compared with later AC systems.
Disadvantages
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.
Historical systems
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 rack railways, where the overhead wiring is less complicated and restrictions on the speeds available less important. Modern motors and their control systems avoid the fixed speeds of traditional systems, as they are built with solid-state converters. The four current such railways are
This category does not cover railways with a single-phase 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 at the Ganz Works, and used in Hungary and Italy, used rotary phase 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 United States.
Locomotives
Usually, the locomotives had one, two, or four motors on the body chassis, 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 working was used to allow two or four different speeds, and resistances were required for starting. In Italy freight locomotives used plain cascade with two speeds, ; while express locomotives used cascade combined with pole-changing, giving four speeds, 37, 50, 75 and 100 km/h. With the use of 3,000 or 3,600 volts at 16⅔ Hz, the supply could be fed directly to the motor without an onboard transformer. Generally, the motor fed a single axle, with other wheels linked by connecting rods, as the induction motor is sensitive to speed variations and with non-linked motors on several axles the motors on worn wheels would do little or even no work as they would rotate faster. This motor characteristic led to a mishap in the Cascade Tunnel to a GN east-bound freight train with four electric locomotives, two on the head and two pushing. The two pushers suddenly lost power and the train gradually slowed to a stop, but the lead unit engineer was unaware that his train had stopped, and held the controller on the power position until the usual time to transit the tunnel had elapsed. Not seeing daylight, he finally shut down the locomotive, and found that the wheels of his stationary locomotive had ground through two-thirds of the rail web.
Overhead wiring
Generally two separate overhead wires are used, with the rail for the third phase, though occasionally three overhead wires are used. At junctions, crossovers and crossings the two lines must be kept apart, with a continuous supply to the locomotive, which must have two live conductors wherever it stops. Hence two collectors per overhead phase are used, but the possibility of bridging a dead section and causing a short circuit from the front collector of one phase to the back collector of the other phase must be avoided. The resistance of the rails used for the third phase or return is higher for AC than for DC due to "skin effect", but lower for the low frequency used than for industrial frequency. Losses are also increased, though not in the same proportion, as the impedance is largely reactive. train in Brazil The locomotive needs to pick up power from two overhead conductors. Early locomotives on the Italian State Railways used a wide bow collector which covered both wires but later locomotives used a wide pantograph with two collector bars, side by side. A three-phase system is also prone to larger lengthwise gaps between sections, owing to the complexity of two-wire overhead, and so a long pickup base is needed. In Italy this was achieved with the long bow collectors reaching right to the ends of the locomotive, or with a pair of pantographs, also mounted as far apart as possible. In the United States, a pair of trolley poles were used. They worked well with a maximum speed of. The dual conductor pantograph system is used on four mountain railways that continue to use three-phase power.
