Electric locomotive

An electric locomotive is a locomotive powered by electric motors that are supplied with electricity generated by an external source. The locomotive draws current from an overhead wire (overhead lines), a third rail, or 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 considered electric locomotives even though they may use electric motors to turn the wheels; in this case the electric power system of the locomotive is considered to be a form of transmission.

History

The first known electric locomotive was built by a Scotsman, Robert Davidson of Aberdeen in 1837 and was powered by galvanic cells. Davidson later built a larger locomotive named Galvani which was shown at the Royal Scottish Society of Arts Exhibition in 1841.

This 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 of a unique configuration simply 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 the first electric locomotives began operation in 1904 on the New York Central. In the 1930s the Pennsylvania Railroad, which also had introduced eletrical working because of the NYC regulation, electrified its entire territory east of Harrisburg, Pennsylvania in two stages.

Other early electrifications focused on mountainous regions, particularly where coal supplies were difficult and hydroelectric power was readily generated. The Alps were the focus of early European electrification; 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.

Electric locomotives declined somewhat in the face of dieselization, which overcame some of the objections that led to some electrification projects. Development of very high speed service brought a revival of electrification. The Shinkansen of Japan and the TGV of France use electric locomotives, as diesel-electric locomotives cannot reasonably produce the power needed to accelerate these trains as required.

Electric locomotive types

While some very small locomotives for use in mining are powered by batteries (thus avoiding sparking from power transmission), eletric locomotives are normally supplied power from a trackside source. Diesel-electric locomotives, while they use an electrical transmission, are not considered to be true electric locomotives because they carry the prime mover on the locomotive itself.

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

Direct or alternating current

The most fundamental difference lies in the choice of direct (DC) or alternating current (AC); motors for one current type follow entirely different principles from the other. The earliest systems used direct current, for alternating current was not well-understood at first. 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 current does not travel as far.

As alternating current motors were developed, this mode became the predominant form, partocularly on lengthy installations. High voltages (tens of thousands of volts) are used to allow transmission of low currents (and therefore smaller wires etc.), and transformers in the locomotives reduce this to a voltage usable in the motors. AC traction allows the use of regenerative braking, in which the motors become generators and transform the motion of the train into power which is then fed back into the lines to power other locomotives. This system is particular advantageous in mountainous operations, as descending locomotives can produces a large portion of the power required for ascending trains.

AC traction sometimes uses three phase current rather than the single phase of household use. Rectifier locomotives, which used AC power transmission but DC motors, were not uncommon.

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. The FL-9 locomotives were equipped to run on power from two different electrical systems as well as run as conventional diesel-electrics.

Power transmission

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 which 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 "catenary" 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.

Driving the wheels

Modern electric locomotives, like their diesel-electric counterparts, almost universally use nose-suspended bogie mounted motors. One side of the motor is mounted the 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 earliests 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.

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.

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.

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.

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; Amtrak passengers of years gone by may recall the long station stop in New Haven, Connecticut as diesel and electric locomotives were swapped on their train. Diesels and even steam engines can operate under the wires, but electric locomotives cannot leave their territories on their own.

Overall, 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 has tended to favor electric locomotives, because diesels are incapable of the speeds required in new service; and they remain the only choice for subway use.

External links

Electric locomotives

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