Electric locomotive
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An electric locomotive is a locomotive powered by electric motors and drawing current from an external source (through overhead lines or third rail), or from an on-board storage device such as a battery or a flywheel energy storage system. Locomotives with on-board prime movers such as diesel engines or gas turbines are not classed as electric locomotives when they use electric motors to turn the wheels, as the electric generator/motor system is considered to be a form of transmission.
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[edit] History
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 shown at the Royal Scottish Society of Arts Exhibition in 1841. The first electric train carrying passengers was presented at Berlin in 1879. However, the electric power available from galvanic cells was insufficient for electric traction to come into general use on railways.<ref>Renzo Pocaterra, Treni, De Agostini, 2003</ref>
The first mainline electrification was put into service on the Baltimore and Ohio Railroad in 1895, on the Baltimore Belt Line. 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 trackage 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 coupled onto the entire train, locomotive and all, and pulled it through the tunnels.<ref>B&O Power, Sagle, Lawrence, Alvin Stauffer</ref>
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, the first electric locomotives in the city 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.
In Europe, electrification projects initially focused on mountainous regions for several reasons: coal supplies were difficult and hydroelectric power was readily generated; and electric locomotives granted more axle traction on steeper lines. For example; today 100% of Swiss lines are electrified. Italian railways were the first to introduce electric traction on long lines over mountainous terrain (line of Valtellina, 1902), using three-phase power at 3600 V, with a maximum speed of 70 km/h. Similar lines followed, the most famous being that of St. Gotthard in Switzerland (1919), which used alternative current (AC) at 15,000 V. The use of high voltage AC power allowed the use of lighter lines as a higher voltage means lower current is required, hence smaller conductors can be used<ref>Alternating_current#Transmission, distribution, and domestic power supply</ref>.<ref>Renzo Pocaterra, Treni, De Agostini, 2003</ref>
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.
Subways used electric power from very early on, though they more typically used self-propelled cars than locomotives. Surface and elevated rapid transit systems generally used steam until forced to convert by ordinance.
In the United States, use of electric locomotives declined in the face of dieselization. Many of the advantages of electric locomotives over steam also applied to diesels, 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.
Many European main lines (eastern Europe included) were electrified by the 1960s. European electric locomotives technology had improved steadily from the 1920s onwards. As a comparison, 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. In 1955 French locomotive CC 7107 reached a speed of 300 km/h. In 1960 the SJ Class Dm 3 locomotives introduced on the Swedish Railways produced a record 7200 kW. Locomotives capable of commercial passenger service at 200 km/h appeared in Germany and France in the same period. Further improvements were brought about through the introduction of electronic control systems, which permitted the use of increasingly lighter and more powerful motors (standardising from 1990s onwards on asynchronous three-phase motors, fed through GTO-inverters).
In the 1980s, in other regions of the world, development of very high speed service also brought a revival of electrification. The Shinkansen of Japan and the TGV of France were the first systems for which devoted high-speed lines were built from scratch. Similar programs were undertaken in other countries, though in the United States the only new service was an extension of electrification over the Northeast Corridor from New Haven, Connecticut to Boston, Massachusetts.
[edit] Electric locomotive types
While some very small locomotives for use in mining are powered by batteries (thus avoiding sparking from power transmission), electric locomotives are normally supplied power from a trackside source. Diesel-electric locomotives are not classed as electric locomotives, but considered to have an onboard diesel prime mover with an electrical transmission.
There are three important characteristics of electric locomotives:
- the type of current used
- the method for collecting current
- the means used to power the wheels from the motors
[edit] Direct or alternating current
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 (several hundred volts); the equipment is therefore relatively massive because the currents involved must be large in order to transmit sufficient power. Power must be supplied at frequent intervals because the high currents required, result in large losses in the transmission system.
As alternating current motors were developed they became the predominant form, particularly on lengthy installations. 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 (eg 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).<ref>Alternating_current#Transmission.2C_distribution.2C_and_domestic_power_supply</ref> A similar high voltage, low current system could not be employed with direct current locomotives because there was no easy way to do the voltage/current transformation efficiently achieved by AC transformers.
AC traction sometimes uses three phase current rather than the single phase of household use. Transmission of three-phase current to AC motors remained problematic until the 1960s, with appearance of semiconductor electronic circuits for the commutation of current (the previous direct commutators had problems both at start and low velocities). Rectifier locomotives, which used AC power transmission but DC motors, were not uncommon. Today's advanced electric locomotives have invariably Three-phase AC induction motors, fed by apposite electronic circuits. Polyphase machines are powered through feeding circuits called GTO inverters. The cost of electronic devices in a modern locomotive can amount 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 particular 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 run on power from two different electrical systems as well as run as conventional diesel-electrics.
While recently designed systems invariably operate on alternating current, many existing direct current systems are still in use — e.g. in South Africa, Spain, and the United Kingdom (750 V and 1500 V); Netherlands (1500 V); Belgium, Italy, Poland (3000 V), and the cities of Mumbai and Chicago, Illinois (which will be switched to AC by 2025).
[edit] Power transmission
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 with model railroads, however, the trackwork normally supplies only one side; the other side(s) of the circuit are provided separately.
The original B&O electrification used a sliding shoe in an overhead channel, a system that was 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 the predominant choice for DC systems, and is preferred in subways because of the close clearances it affords.
AC systems tend to prefer overhead lines, often called "catenaries" after the support system used to hold the wire parallel to the ground. Three collection methods are used:
- Trolley pole: a long flexible pole which engages the line with a wheel or shoe
- Bow collector: a frame which holds a long collecting rod against the wire
- Pantograph: a hinged frame which holds the collecting shoes against the wire in a fixed geometry
Some locomotives are equipped to use both overhead and third rail collection.
[edit] Driving the wheels
</div> Modern electric locomotives, like their diesel-electric counterparts, almost universally use nose-suspended bogie mounted motors. One side of the motor is mounted the bogie frame; the other rests upon the axle. Transmission between the motor and the axle is through gears.Earlier locomotives used a variety of mechanisms to transmit motor power to the wheels. The earliest systems employed on railroads put the motor concentric with the driven axle, using a quill drive. A more extreme example was the "bi-polar" system, in which motor shaft was the axle itself. This assembly was able to move vertically because the motor only had two stator poles. This system was generally abandoned 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.
A system which lasted considerably longer was the use of jackshafts. In this case the motors themselves were mounted above the frame in the carbody, and connecting rods were used to drive the wheels. This system dropped from favor as motors became smaller and lighter.
[edit] Wheel arrangements
The Whyte notation system for classifying steam locomotives is not adequate to 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 famous PRR GG1 class indicates that it is arranged like two 4-6-0 class G locomotives 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 power and unpowered axles, and could distinguish between coupled and uncoupled drive systems.
[edit] Advantages
The initial advantage sought from electrification was its lack of pollution, at least from the locomotives themselves. Even then 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 (with modern motors) no clanking rods or other mechanical noise. The lack of reciprocating parts means that they are very easy on the track, reducing some maintenance-of-way costs.
Since the power plants have capacity far beyond what any individual locomotive requires, enormous short term power can be applied to the rails. Electric locomotives can accelerate extremely quickly, limited only by what the infrastructure can withstand; this makes them ideal for commuter service with its many stops.
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 with 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 complication and particularly of moving parts.
[edit] Disadvantages
The chief disadvantage of electrification is the high infrastructure cost. 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.<ref>"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.</ref> Diesels and even steam engines can operate under the wires, but electric locomotives cannot leave their territories on their own.
In North America, the flexibility of diesel locomotives and the relative inexpense of their infrastructure has led them to prevail except where legal or other operational constraints dictate the use of electricity. That said, new passenger service in all parts of the world has tended to favor electric locomotives, because diesels are incapable of the speeds required by the new services; and they remain the only choice for subway use.
[edit] See also
- Air brake (rail)
- Locomotive
- Railway brakes
- Railway electrification system
- Overhead lines
- Boxcab
- Robert Davidson - electric locomotive pioneer
- Emily Davenport - electric locomotive pioneer
[edit] References
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[edit] External links
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