2009年1月20日星期二

Railway electric traction

Railway electric traction describes the various types of locomotive and multiple units that are used on electrification systems around the world


History
Railway electrification as a means of traction emerged at the end of the nineteenth century, although experiments in electric rail have been traced back to the mid-nineteenth century . Thomas Davenport, in Brandon, Vermont, erected a circular model railroad on which ran battery-powered locomotives (or locomotives running on battery-powered rails) in 1834. Robert Davidson, of Aberdeen, Scotland, created an electric locomotive in 1839 and ran it on the Edinburgh-Glasgow railway at 4 miles per hour.The earliest electric locomotives tended to be battery-powered. In 1880, Thomas Edison built a small electrical railway, using a dynamo as the motor and the rails as the current-carrying medium. The electric current flowed through the metal rim of otherwise wooden wheels, being picked up via contact brushes.

Electrical traction offered several benefits over the then predominant 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.


DC Traction Units
DC traction units use direct current drawn from either a conductor rail or an overhead line.


AC Traction Units
Apart from a few cases, almost all AC Traction units draw alternating current from an overhead line.


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.


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 High Speed 1 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.


United Kingdom
Main article: Railway electrification in Great Britain
Electrification in the UK began in a piecemeal fashion. The earliest main line (as opposed to metro and tramway) systems were divided between low voltage 3rd rail (commonly about 600V DC) and overhead systems (a variety of voltages, both DC and AC were used). The 3rd rail systems of this period eventually gave rise to the 750V DC system in the southern part of the UK, and a separate area with the same system around Merseyside.

Cheap loans to stimulate economic development in the 1930s gave rise to the several schemes of 1500V DC electrification, mostly completed post war, notably between Liverpool Street and Shenfield, and the Woodhead Line. Starting with the West Coast Mainline electrification in the 1960s, the 25 kV AC overhead system was adopted for all subsequent mainline electrification in the UK (except for extensions to other existing systems, mostly on the souther 3rd rail network).

In some areas with restricted clearences, particularly in urban areas in east London (converted from 1500 V DC, and on suburban routes around Glasgow), 6.25 kV was used. 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 somewhat unreliable, and with experience, it was found that less clearance was needed for 25 kV than had initially been allowed for. This allowed the 6.25 kV sections to be converted to 25 kV, with the last section, at the London end of the London Tilbury and Southend line, being converted in 1983.


United States
Main article: Railroad electrification in the 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.




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 .

PDA Travel Charger


Phone Car Charger


mp3 solar charger


12V Battery Chargers


Agu Fuse Holder


anl fuse holder


Universal Travel Charger


dell battery charger


dc car charger


ag13 button battery


dell cpi battery


AA Size Battery


li polymer batteries


C Size Battery


circuit breaker abb


dc circuit breakers


electric razor battery


emergency lights battery


cr 123 battery


flashlight no batteries


coin cell batteries

没有评论:

发表评论