Railway electric traction
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Railway electric traction describes the various types of locomotive and multiple units that are used on electrification systems around the world
[edit] History
Railway electrification as a means of traction emerged at the end of the nineteenth century. It offered several benefits over the then predominate steam traction, particularly in respect of its quick acceleration (ideal for urban (metro) and suburban (commuter) services) and power (ideal for heavy freight trains through mountainous/hilly sections). A plethora of systems emerged in the first twenty years of the twentieth century....
[edit] DC Traction Units
DC traction units use direct current drawn from either a conductor rail or an overhead line.
[edit] AC Traction Units
Apart from a few cases, almost all AC Traction units draw alternating current from an overhead line.
[edit] Multi-System Units
Because of the variety of railway electrification systems, which can vary even within a country, trains often have to pass from one system to another. One way this is accomplished is by changing locomotives at the switching stations. These stations have overhead wires that can be switched from one voltage to another, and so the train arrives with one locomotive, and then departs with another. Often, however, this is inconvenient and time-consuming. The switching stations have very sophisticated components, and they are very expensive.
Another way is to use multisystem locomotives that can operate under several different voltages and current types. In Europe, it is common to use four-system locomotives (1.5 kV DC, 3 kV DC, 15 kV 16⅔ Hz AC, 25 kV, 50 Hz AC). These locomotives do not have to stop when passing from one electrification system to another, the changeover occurring where the train coasts for a short time.
Electro-diesel locomotives which can operate as an electric locomotive on electrified lines but have an onboard diesel engine for non-electrified sections or sidings have been used in several countries.
[edit] Czech Republic and Slovakia
In the Czech Republic and Slovakia, the railways have both 3000 V DC and 25 kV AC systems, but there are no switching stations - the two systems meet at breaks on overhead wires. Except for two breaks (Kutná Hora and Nedakonice), they are all away from stations.
Eurostar trains through the Channel Tunnel are multisystem: a significant part of the route near London is on southern England's 750 V DC third rail system, the route into Brussels is 3000 V DC overhead, while the rest of the route is 25 kV 50Hz overhead. The need for these trains to use third rail will end upon completion of the Channel Tunnel Rail Link in 2007. Southern England has some overhead/third rail dual-system locomotives and multiple units to allow through running between 750 V DC third rail south of London and the 25 kV AC overhead north and east of London.
[edit] United Kingdom
Early AC electrification systems in the UK used 6.25kV in urban areas, to prevent the need to raise many bridges to meet with the clearance standards of the time. This system was used on suburban routes in Glasgow and East London, the latter having been upgaded from 1500V DC. Further away from the cities, though, the voltage switched to 25kV. A system known as "Automatic Power Control" was developed to allow trains to automatically switch between the voltages whilst moving. All the driver had to do was shut off power and coast until clear of the neutral section; the system automatically opened the circuit breaker, detected a change in voltage and switched over the transformer to the correct input voltage setting, then closed the circuit breaker. This system proved unreliable, with the voltage changeover often failing to work. Since by this time enough knowledge of high voltage electrification was known, the rules were able to be changed to allow 25kV to be used in areas where 6.25kV had previously been used, and all new electrification was then at 25kV, with the 6.25kV systems graually disappearing, in particular spurred on by the desire to eliminate dual voltage equipment from new trains, the last line to retain 6.25kV being the semi-isolated London, Tilbury and Southend line, converted in 1983.
The British Rail Class ES1 locomotive operated on 600 V DC from a third rail or from an overhead line.
[edit] United States
In the United States, New Jersey Transit uses multisystem ALP-44 and ALP-46 locomotives for its Midtown Direct service into New York, and Amtrak uses multi-system AEM-7, HHP-8 and Acela locomotives on the Northeast Corridor between Washington DC and Boston. In both cases, through trains run on both newer, 25 kV 60 Hz built or refurbished by their respective agencies since the 1980s, and older, 12 kV 25 Hz inherited from the now-defunct Pennsylvania Railroad. The latter dates to the 1930s, when the Pennsylvania upgraded its electrified network from 650 V DC third rail.
[edit] India
In India 1500V DC and 25kV AC(50 Hz) is used for mainline trains.
The 1500V DC overhead system (negative earth, positive catenary) is used around Mumbai. The Mumbai region is the last bastion of 1500V DC electrified lines on Indian Railways. There are plans to change this to 25kV AC by 2010. The 25kV AC system with overhead supply from a catenary is used throughout the rest of the country. THe dual-voltage WCAM series locomotives haul intercity trains out of Mumbai DC suburban region. The new AC-DC EMU rakes used in Mumbai are also designed to operate with both DC and AC traction as the Mumbai area switches over to the 25kV AC system.
The Kolkata Metro uses 750V DC traction with a third-rail mechanism for delivering the electricity to the EMUs. The Kolkata trams use 550V DC with an overhead catenary system with underground return conductors. The catenary is at a negative potential.
The Delhi Metro uses 25kV AC overhead traction with a catenary system on the ground-level and elevated routes, and uses a rather unusual 'rigid catenary' or overhead power rail in the underground tunnel sections .
[edit] South Africa
South Africa has 15 km of dual system track, both 3 kV DC and 25 kV AC.
[edit] Battery electric rail vehicles
A few battery electric railcars and locomotives were used in the twentieth century, but generally the use of battery power was not practical except in underground mining systems, see Railcar and Locomotive sections.
[edit] See also
List of systems · Railway electric traction · Traction power network
Electric locomotive · EMUs · Electro-diesel locomotive
By Country : Great Britain · Japan · USA
