An electric locomotive is a locomotive powered by electricity from an external source. Sources include overhead lines, third rail or an on-board electricity storage device such as a battery or a flywheel system.
Electrically-propelled locomotives with on-board fueled prime movers, such as diesel engines or gas turbines, are not classed as electric locomotives but as hybrid, as the electric generator/motor combination is only considered to be the power transmission system.
The initial advantage sought from electrification was its lack of pollution, at least from the locomotives themselves. The power plants, if they burn fossil fuels, can have the full range of emission controls applied to them; and electric locomotives can use electricity generated by any means, from hydroelectric power to windmills. Electric locomotives are also potentially extremely quiet, since there is no exhaust noise and little mechanical noise, although with high powered types the cooling fans are typically quite loud when in close proximity. The lack of reciprocating parts means that they are very easy on the track, reducing some maintenance-of-way costs.
Since power plant capacity is far beyond what any individual locomotive requires, enormous short-term power can be applied to the rails. Electric locomotives can accelerate very quickly, limited only by what the infrastructure can withstand; making them ideal for commuter service with its many stops. Electric locomotives like Italy's E412 although having a maximum speed in excess of 200 km/h can also perform with excellent efficiency in heavy freight service.
Electric locomotives allow the power plants to be run in their most efficient mode (a common problem with turbine-driven locomotives, which were inefficient at low speeds). Additional efficiencies can be gained from regenerative braking, which allows much of the kinetic energy of the train to be recovered and used to power other locomotives on the line.
They are potentially very reliable, as there is a minimum of moving parts.
New passenger services world-wide have tended to favor electric locomotives, because diesels are incapable of the higher speeds required; and electric traction remains the only choice for subway use.
The chief disadvantage of electrification is the high infrastructure cost. Moreover, diesels and even steam engines can operate under the wires, but electric locomotives cannot leave their territories on their own.
The first known electric locomotive was built by a Scotsman, Robert Davidson of Aberdeen in 1837 and was powered by galvanic cells ('batteries'). Davidson later built a larger locomotive named Galvani which was exhibited at the Royal Scottish Society of Arts Exhibition in 1841. The first electric passenger train was presented at Berlin in 1879. However, the limited electric power available from batteries prevented its general use on railways.
The first mainline electrification was on the Baltimore Belt Line of the Baltimore and Ohio Railroad (B&O) in 1895. This four-mile track connected the main portion of the B&O to the newly built line to New York, and it required a series of tunnels around the edges of Baltimore's downtown. Parallel tracks on the Pennsylvania Railroad had shown that coal smoke from steam locomotives would be a major operating issue, as well as a public nuisance. Three Bo+Bo units were initially used, at the south end of the electrified section; they coupled onto the entire train, locomotive and all, and pulled it through the tunnels.
Railroad entrances to New York City required similar tunnels, and the smoke problems were more acute there. A collision in the Park Avenue tunnel in 1902 led the New York State legislature to outlaw the use of smoke-generating locomotives south of the Harlem River after July 1, 1908. In response, electric locomotives began operation in 1904 on the New York Central Railroad. In the 1930s the Pennsylvania Railroad, which also had introduced electric locomotives because of the NYC regulation, electrified its entire territory east of Harrisburg, Pennsylvania.
Subways used electric power from very early on, though they more typically used self-propelled cars rather than locomotives. Surface and elevated rapid transit systems generally used steam until forced to convert by ordinance.
In Europe, electrification projects initially focused on mountainous regions for several reasons: coal supplies were difficult and hydroelectric power was readily available; and electric locomotives gave more traction on steeper lines. For example; today 100% of Swiss lines are electrified. Italian railways were the first to introduce electric traction on the long mountainous terrain of the Valtellina line in 1902, using three-phase power at 3,600 V, with a maximum speed of 70 km/h. Similar lines followed, the most famous being the St. Gotthard in Switzerland (1919), which used alternating current (AC) at 15,000 V. The use of high voltage AC power allowed the use of lighter lines as a higher voltage means a lower current is required, hence smaller conductors can be used.
In the United States, the Chicago, Milwaukee, St. Paul and Pacific Railroad (the Milwaukee Road), the last transcontinental line to be built, electrified its lines across the Rocky Mountains and to the Pacific Ocean. A few East Coast lines, notably the Virginian Railway and the Norfolk and Western Railway, found it expedient to electrify short sections of their mountain crossings. However, by this point, electrification was more associated with dense urban traffic, and the center of development shifted to Europe, where electrification was widespread.
The 1960s saw the electrification of many European main lines (Eastern Europe included) European electric locomotives technology had improved steadily from the 1920s onwards. By comparison, the Milwaukee Road class EP-2 (1918) weighed 240 t, with a power of 3,330 kW and a maximum speed of 112 km/h; in 1935, German E 18 had a power of 2,800 kW, but weighed only 108 tons and had a maximum speed of 150 km/h. On March 29 1955 French locomotive CC 7107 reached a speed of 331 km/h. In 1960 the SJ Class Dm 3 locomotives introduced on the Swedish Railways produced a record 7,200 kW. Locomotives capable of commercial passenger service at 200 km/h appeared in Germany and France in the same period. Further improvements resulted from the introduction of electronic control systems, which permitted the use of increasingly lighter and more powerful motors (standardising from the 1990s onwards on asynchronous three-phase motors, fed through GTO-inverters).
In the United States, the use of electric locomotives declined in the face of dieselization. Diesels also shared some of the electric locomotive’s advantages of over steam, and the cost of building and maintaining the power supply infrastructure, which had always worked to discourage new installations, brought on the elimination of most mainline electrification outside the Northeast. Except for a few captive systems (e.g. the Black Mesa and Lake Powell), by 2000 electrification was confined to the Northeast Corridor and some commuter service; even there, freight service was handled by diesels.
In the 1980s, development of very high speed service brought a revival of electrification. The Japanese Shinkansen and the French TGV were the first systems for which devoted high-speed lines were built from scratch. Similar programs were undertaken in Italia, Germany and Spain; in the United States the only new service was an extension of electrification over the Northeast Corridor from New Haven, Connecticut to Boston, Massachusetts.
Electric locomotive typesEditar
An electric locomotive can be supplied with power from
- Rechargeable energy storage systems, as battery or ultracapacitor-powered mining locomotives.
- A stationary source, such as a third rail or overhead wire.
The distinguishing design features of electric locomotives are:
- The type of electrical power used, either alternating current or direct current.
- The method for store (batteries, ultracapacitors) or collecting (transmision) electrical power.
- The means used to mechanically couple the traction motors to the driving wheels (drivers).
Direct or alternating currentEditar
The most fundamental difference lies in the choice of direct (DC) or alternating current (AC). The earliest systems used direct current as, initially, alternating current was not well understood. Direct current locomotives typically run at relatively low voltage (750 to 3,000 volts); the equipment is therefore relatively massive because the currents involved are large in order to transmit sufficient power. Power must be supplied at frequent intervals as the high currents result in large transmission system losses.
As alternating current motors were developed, they became the predominant form, particularly on longer routes. High voltages (tens of thousands of volts) are used because this allows the use of low currents; transmission losses are proportional to the square of the current (e.g. twice the current means four times the loss). Thus, high power can be conducted over long distances on lighter and cheaper wires. Transformers in the locomotives transform this power to a low voltage, high current for the motors (the magnet fields, hence power, from the motor is proportional to electric current). A similar high voltage, low current system could not be employed with direct current locomotives because there is no easy way for DC to do the voltage/current transformation so efficiently achieved by AC transformers.
AC traction sometimes uses three phase current rather than the single phase of household use. Speed control of three-phase AC motors remained problematic until the introduction of power electronic control circuits in the 1960s. Italy was the only country to try to solve the problem by using three-phase motors fed by three-phase lines: this system, however, caused other maintenance and technology problems, and was abandoned in the 1970s.
The previous direct commutators had problems at both start and low velocities. Rectifier locomotives, which used AC power transmission and DC motors, were common. Today's advanced electric locomotives have invariably Three-phase AC induction motors. These Polyphase machines are powered from GTO inverters. The cost of electronic devices in a modern locomotive can be up to 50% of the total cost of the vehicle.
Electric traction allows the use of regenerative braking, in which the motors are used as brakes and become generators that transform the motion of the train into electrical power that is then fed back into the lines. This system is particularly advantageous in mountainous operations, as descending locomotives can produces a large portion of the power required for ascending trains.
Most systems have a characteristic voltage, and in the case of AC power a system frequency. Many locomotives over the years were equipped to handle multiple voltages and frequencies as systems came to overlap or were upgraded. American FL9 locomotives were equipped to handle power from two different electrical systems and could also operate as a conventional diesel-electric.
While recently designed systems invariably operate on alternating current, many existing direct current systems are still in use — e.g. in Sudáfrica, Spain, and the United Kingdom (750 V and 1,500 V); Netherlands and Ireland (1,500 V); Belgium, Italia, Poland, Russia (3,000 V), and the cities of Mumbai and Chicago, Illinois (which will be switched to AC by 2025).
See also Railway electrification system
Electrical circuits require two connections (or for three phase AC, three connections). From the very beginning the trackwork itself was used for one side of the circuit. Unlike model railroads, however, the trackwork normally supplies only one side; the other side(s) of the circuit are provided separately.
The original Baltimore and Ohio Railroad electrification used a sliding shoe in an overhead channel, a system quickly found to be unsatisfactory. It was replaced with a third rail system, in which a pickup (the "shoe") rode underneath or on top of a smaller rail parallel to the main track, somewhat above ground level. There were multiple pickups on both sides of the locomotive in order to accommodate the breaks in the third rail required by trackwork. This system is preferred in subways because of the close clearances it affords.
- Trolley pole: a long flexible pole, which engages the line with a wheel or shoe.
- Bow collector: a frame that holds a long collecting rod against the wire.
- Pantograph: a hinged frame that holds the collecting shoes against the wire in a fixed geometry.
Of the three, the pantograph method is best suited for high-speed operation. Some locomotives are equipped to use both overhead and third rail collection.
Driving the wheelsEditar
During the initial development of railroad electrical propulsion, a number of drive systems were devised to couple the output of the traction motors to the wheels. One of the earliest methods was the jackshaft drive[cita requerida]. In this arrangement, the traction motor is mounted within the body of the locomotive and drives the jackshaft through a set of gears. This system was employed because the first traction motors were too large and heavy to mount directly on the axles. Due to the number of mechanical parts involved, frequent maintenance was necessary. The jackshaft drive was abandoned for all but the smallest units when smaller and lighter motors were developed,
Several other systems were devised as the electric locomotive matured. One drive type, referred to as the quill drive, mounted the traction motor above or to the side of the axle and coupled to the axle through a reduction gear and a semi-flexible shaft (the quill). The Pennsylvania Railroad GG1 locomotive used a quill drive. Again, as traction motors continued to shrink in size and weight, quill drives gradually fell out of favor.
Another drive example was the "bi-polar" system, in which the motor armature was the axle itself, the frame and field assembly of the motor being attached to the truck (bogie) in a fixed position. The motor had two field poles, which allowed a limited amount of vertical movement of the armature. This system was of limited value since the power output of each motor was limited. The EP-2 bi-polar electrics used by the Milwaukee Road compensated for this problem by using a large number of powered axles.
Modern electric locomotives, like their Diesel-electric counterparts, almost universally use axle-hung traction motors, with one motor for each powered axle. In this arrangement, one side of the motor housing is supported by plain bearings riding on a ground and polished journal that is integral to the axle. The other side of the housing has a tongue-shaped protuberance that engages a matching slot in the truck (bogie) bolster, its purpose being to act as a torque reaction device, as well as a support. Power transfer from motor to axle is effected by spur gearing, in which a pinion on the motor shaft engages a bull gear on the axle. Both gears are enclosed in a liquid-tight housing containing lubricating oil. The gear ratio employed is dictated by the type of service in which the locomotive is used. Numerically high ratios are commonly found on freight units, whereas numerically low ratios are typical of passenger engines.
The Whyte notation system for classifying steam locomotives is not adequate for describing the varieties of electric locomotive arrangements, though the Pennsylvania Railroad applied classes to its electric locomotives as if they were steam or concatenations of such. For example, the PRR GG1 class indicates that it is arranged like two 4-6-0 class G locomotives that are coupled back-to-back.
In any case, the UIC classification system was typically used for electric locomotives, as it could handle the complex arrangements of powered and unpowered axles, and could distinguish between coupled and uncoupled drive systems.
Success and failure of electric tractionEditar
In the United States it was estimated that it cost as much to electrify a railroad as it cost to build it in the first place. Overhead lines and third rails require greater clearances, and the right-of-way must be better separated to protect the public from electrocution, as well as from trains which approach much more quietly than diesels or steam.
For most large systems the cost of electrifying the whole system is impractical, and generally only some divisions are electrified. In the United States only certain dense urban areas and some mountainous areas were electrified, and the latter have all been discontinued. The junction between electrified and unelectrified territory is the locale of engine changes; for example, Amtrak trains had extended stops in New Haven, Connecticut as diesel and electric locomotives were swapped, a delay which contributed to the electrification of the remaining segment of the Northeast Corridor in 2000.
In North America, the flexibility of diesel locomotives and the relative low cost of their infrastructure has led them to prevail except where legal or other operational constraints dictate the use of electricity. An example of this being the superiority of electric locomotives providing Head end power and acceleration when used by AMTRAK and Commuter Railroads in The Northeast.
Electrification is widespread in Europe. Due to higher density schedules the operating costs of the locomotives are more dominant with respect to the infrastructure costs than in the US, and electric locomotives have much lower operating costs than diesels. In addition, governments were motivated to electrify their railway networks due to coal shortages during the First and Second World War.
It should also be noted that diesel locomotives have little power compared to electric locomotives, given the same weight and dimensions. For instance, the 2,200 kW of a modern British Rail Class 66 were already met in 1927 by the electric SBB-CFF-FFS Ae 4/7 (2,300 kW), which is even a bit lighter. However, it should be noted that for low speeds tractive effort is more important than power, which is a reason why diesel engines are competitive for slow freight traffic (as it is common in the US), but not for passenger or mixed passenger/freight traffic like on many European railway lines, especially not lines with steep grades like the Gotthardbahn or the Brenner railway, where heavy freight trains must be run at comparatively high speeds (80 km/h or more).
These factors led to high degrees of electrification in most European countries. In some countries like Switzerland, even electric shunters are common and many private sidings can be served by electric locomotives.
The recent political developments in many European countries to enhance public transit have led to another boost for electric traction. High-speed trains like the TGV or ICE can only be run economically using electric traction, and the operation of branch lines is usually less in deficit when using electric traction, due to cheaper and faster rolling stock and more passengers due to more frequent service and more comfort. In addition, gaps of unelectrified track are closed to avoid replacing electric locomotives by diesels for these sections. Note that the necessary modernisation and electrification of these lines is in most cases only possible due to state subsidies.
Both Victorian Railways and New South Wales Government Railways, which pioneered electric traction in Australia in the early 20th century and continue to operate 1500 V DC Electric Multiple Unit services, have withdrawn their fleets of main line electric locomotives.
In both states, the use of electric locomotives on principal interurban routes proved to be a qualified success. In Victoria, because only one major line (the Gippsland line) had been electrified, the economic advantages of electric traction were not fully realised due to the need to change locomotives for trains that extended beyond the range of the electrified network. VR's entire electric locomotive fleet was withdrawn from service by 1987, and the Gippsland line electrification was dismantled by 2004. Similarly, the new fleet of 86 class locomotives introduced to NSW in 1983 had a relatively short life as the costs of changing locomotives at the extremities of the electrified network, together with the higher charges levied for electricity use, saw diesel-electric locomotives make inroads into the electrified network and the electric locomotive fleet was progressively withdrawn.
Queensland Rail, conversely, implemented electrification relatively recently and utilises the more recent 25 kV AC technology with around 1,000 km of the QR narrow gauge network now electrified. It operates a fleet of electric locomotives to transport coal for export, the most recent of which are those of the 3,000 kW (4,020 HP) 3300/3400 Class.
- Renzo Pocaterra, Treni, De Agostini, 2003
- B&O Power, Sagle, Lawrence, Alvin Stauffer
- Alternating current#Transmission, distribution, and domestic power supply
- Renzo Pocaterra, Treni, De Agostini, 2003
- Alternating current#Transmission.2C distribution.2C and domestic power supply
- "New York to Boston, under wire - Amtrak begins all-electric Northeast Corridor service between Boston and Washington, D.C", Railway Age, March 2000, accessed from FindArticles.com on 28 Sep. 2006.
- «L class electric locomotives». victorianrailways.net. Consultado el 2007-04-026.
- «VR History». victorianrailways.net. Consultado el 26 de abril de 2007.
- «SETS Fleet - Electric Locomotive 8606». Sydney Electric Train Society. Consultado el 26 de abril de 2007.
- «QR: 3300/3400 class». railpage.com.au. Consultado el 26 de abril de 2007.
- Wikimedia Commons alberga una categoría multimedia sobre Zam/Proyectos/Locomoción eléctrica.
- Electric traction
- Electric engines
[[Category:Electric rail transport [[Category:Locomotoras [[Category:Hybrid locomotives