Early electric systems used low-voltage DC. Electric motors were fed directly from the traction supply and were controlled using a combination of resistors and relays that connected the motors in parallel or series. The most common DC voltages are 600 V and 750 V for trams and metros and 1,500 V, 650/750 V third rail for the former Southern Region of the UK and 3 kV overhead. The lower voltages are often used with third or fourth rail systems, whereas voltages above 1 kV are normally limited to overhead wiring for safety reasons.
Suburban trains (SBahn) lines in Hamburg, Germany, operate using a third rail with 1,200 V, the French SNCF Culoz-Modane line in the Alps used 1,500 V and a third rail until 1976, when a catenary was installed and the third rail was removed. In the UK, south of London, 750 V third rail is used while, for inner London, 650 V is used to allow inter-running with London Underground which uses a 650 V fourth rail system but with the 4th (centre) rail connected to the running rails in inter-running areas.
During the mid-20th century, rotary converters or mercury arc rectifiers were used to convert utility (mains) AC power to the required DC voltage at feeder stations. Today, this is usually done by semiconductor rectifiers after stepping down the voltage from the utility supply. The DC system is quite simple but it requires thick cables and short distances between feeder stations because of the high currents required. There are also significant resistive losses. In the United Kingdom, the maximum current that can be drawn by a train is 6,800 A at 750 V. The feeder stations require constant monitoring and, on many systems, only one train or locomotive is allowed per section.
The distance between two feeder stations at 750 V on third-rail systems is about 2.5 km (1.6 mi). The distance between two feeder stations at 3 kV is about 25 km (16 mi). If auxiliary machinery, such as fans and compressors, is powered by motors fed directly from the traction supply, they may be larger because of the extra insulation required for the relatively high operating voltage. Alternatively, they can be powered from a motorgenerator set, which offers an alternative way of powering incandescent lights which otherwise would have to be connected as series strings (bulbs designed to operate at traction voltages being particularly inefficient).
Now solid-state converters (SIVs) and fluorescent lights can be used. The Tyne and Wear Metro is the only United Kingdom system that uses 1,500 V DC. 1,500 V DC is used in the Netherlands, Japan, Hong Kong (parts), Ireland, Australia (parts), India (around the Mumbai area alone, to be converted to 25 kV AC like the rest of the country), France, New Zealand (Wellington) and the United States (Chicago area on the Metra Electric district and the South Shore Line interurban line). In Slovakia, there are two narrow-gauge lines in the High Tatras (one a cog railway). In Portugal, it is used in the Cascais Line and, in Denmark, on the suburban S-train system.
Nottingham Express Transit in United Kingdom uses a 750 V DC overhead, in common with most modern tram systems. In the United Kingdom, 1,500 V DC was used in 1954 for the Woodhead trans-Pennine route (now closed); the system used regenerative braking, allowing for transfer of energy between climbing and descending trains on the steep approaches to the tunnel. The system was also used for suburban electrification in East London and Manchester, now converted to 25 kV AC. 3 kV DC is used in Belgium, Italy, Spain, Poland, the northern Czech Republic, Slovakia, Slovenia, western Croatia, South Africa and former Soviet Union countries (also using 25 kV 50 Hz AC).
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