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Electric Locomotive

Electric locomotive

]] An electric locomotive is a locomotive powered by electric motors which draws current from an overhead wire, a third rail, or an on-board storage device such as a battery or a flywheel energy storage system. 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. 1841Modern electric locomotives range from small battery-powered machines for use in mines to large main-line locomotives of 6,000 horsepower (4.5 MW) or more. In reality most modern locomotives are electrically driven. Pure electric locomotives take their electrical supply from an external source while diesel-electric locomotives carry their own generating station. Main line electric locomotives first appeared at the beginning of the 20th century. The reason for their introduction was the problem of smoke, especially in tunnels caused by steam locomotives. In the UK this was the London underground system while in the USA, it was under river tunnels and the need to eliminate smoke in built up areas. Early electric locomotives all relied on external power sourcing. Once up and running they tend to be reliable and efficient, but the supply infrastructure is a large capital expense that does require ongoing maintenance. For this reason only heavily used lines could justify electrification. For suburban lines the reduction in pollution from steam locomotives was a benefit all were aware of.

Alternating current or direct current?

Early locomotives came in a variety of forms. Generally they were designed to run off the supplied current, so locomotives with a direct current (DC) supply had DC motors while alternating current (AC) supplied locomotives with AC motors. AC can be either single or three phase. While the former requires two wire supply, one overhead the other being the track, three phase requires three supply wires. Three phase locomotives therefore had two overhead supplies, the track being the third. DC supplies were either overhead or by means of a track level supply, commonly called the third rail. AC traction motors tended to be larger than DC motors. This often meant electric locomotives with steam engine type cranks. DC motors could be smaller and set up to drive the axles, usually through a gear, but in some early examples by being part of the axle. Even so, some notable DC electric locomotives had large DC motors driving large driving wheels. One possibility with electric locomotives is that the motor can be used as a generator during braking, feeding electricity back into the supply system; this is called regenerative braking. This is not a new idea, it was one reason for the adoption by some railways of 3 phase AC supplies. Especially in mountainous areas where the locomotive going down would generate much of the supply for a locomotive going up. The Swiss railway uses this system; three modern locomotives heading downwards generate enough power to power a single locomotive on its upward journey. Today all electric locomotives tend to have drive motors close to the axles, although some still have the motor in the body driving the wheels through internal drive shafts. Modern solid state electrical control systems means the motor does not need to match the supply. This means multi-voltage cross border locomotives are now quite common. Drive motors are generally DC, but there are 3 phase drive motors on some locomotives.

Locomotive

A locomotive (from lat. locus motivus) is a railway vehicle that provides the motive power for a train, and has no payload capacity of its own; its sole purpose is to move the train along the tracks. In contrast, many trains feature self-propelled payload-carrying vehicles; these are not normally considered locomotives, and may be referred to as multiple units or railcars; the use of these self-propelled vehicles is increasingly common for passenger trains, but very rare for freight (see however CargoSprinter). Vehicles which provide the motive power to haul an unpowered train, but are not generally considered locomotives because they have payload space or are rarely detached from their trains, are known as power cars. Traditionally, locomotives haul (pull) their trains. Increasingly common these days in local passenger service is push-pull operation, where a locomotive pulls the train in one direction and pushes it in the other, and is therefore optionally controlled from a control cab at the opposite end of the train. This is especially true of "High Speed Rail lines", such as the Japan’s Shinkansen and France’s TGV trains. TGV Grange class steam locomotive, at Bristol Temple Meads station, Bristol, England]]

Origins

The first successful locomotives were built by Cornish inventor Richard Trevithick. In 1804 his unnamed locomotive hauled a train along the tramway of the Penydarren ironworks, near Merthyr Tydfil in Wales. Although the locomotive hauled a train of 10 tons of iron and 70 passengers in five wagons over nine miles it was too heavy for the cast iron rails used at the time. The locomotive only ran three journeys before it was abandoned. In 1813, George Stephenson persuaded the manager of the Killingworth colliery where he worked to allow him to build a steam-powered machine. He built the Blucher, the first successful flanged-wheel adhesion locomotive. The flanges enabled the trains to run on top of the rails instead of in sunken tracks. This greatly simplified construction of switches (called "points" in UK) and rails, and opened the way to the modern railroad.

Benefits of locomotives

switches There are many reasons why the motive power for trains has been traditionally isolated in a locomotive, rather than in self-propelled vehicles. These include:
- Ease of maintenance - it is easier to maintain one locomotive than many self-propelled cars.
- Safety - it is often safer to locate the train's power systems away from passengers. This was particularly the case for steam locomotives, but still has some relevance for other power sources.
- Easy replacement of motive power - should the locomotive break down, it is easy to replace it with a new one. Failure of the motive power unit does not require taking the whole train out of service.
- Efficiency - idle trains do not waste expensive motive power resources. Separate locomotives mean that the costly motive power assets can be moved around as needed.
- Flexibility - large locomotives can be substituted for small locomotives where the gradients of the route become steeper and more power is needed.
- Obsolescence cycles - separating the motive power from the payload-hauling cars means that either can be replaced without affecting the other. At some times, locomotives have become obsolete when their cars are not, or vice versa.

Classification by motive power

Locomotives may generate mechanical work from fuel, or they may take power from an outside source. It is common to classify locomotives by their means of providing motive work - the common ones include:

Steam

power power The first railway locomotives (19th century) were powered by steam, first by burning wood, later coke and coal or petroleum. Because of the steam engine, some people took to calling the steam locomotives themselves "steam engines". The steam locomotive remained by far the most common type of locomotive until after World War II. The age of steam correlates highly to the coal era. The first steam locomotive was built by Richard Trevithick, and first ran on 21 February 1804, although it took some years before steam locomotive design became efficient and economically practical. Fairy Queen, built in 1855; plying between New Delhi and Alwar in India, is the longest-running steam locomotive in regular service in the world, but John Bull, built in 1831, is currently the oldest operable steam locomotive. John Bull is preserved in mostly static display at the Smithsonian Institution in Washington, DC. The all-time speed record for steam trains is held by an LNER Class A4 4-6-2 Pacific locomotive of the LNER in the United Kingdom, number 4468 Mallard, which pulling six carriages (plus a dynamometer car) reached 126 mph (203 km/h) on a slight downhill gradient down Stoke Bank on 3 July 1938. Aerodynamic passenger locomotives from other countries such as Germany and the United States attained speeds very close to this, and this is generally believed to be close to the practicable upper limit for the direct-coupled steam locomotive. Before the middle of the 20th century, electric and diesel-electric locomotives began replacing steam locomotives. Steam locomotives are less efficient than their more modern diesel and electric counterparts and require much greater manpower to operate and service. British Rail figures showed the cost of crewing and fuelling a steam locomotive was some two and a half times that of diesel power, and the daily mileage achievable was far lower. As labour costs rose, particularly after the second world war, non-steam technologies became much more cost-efficient. By the end of the 1960s-1970s, most western countries had completely replaced steam locomotives in commercial service. Freight locomotives generally were replaced later. Other designs, such as locomotives powered by gas turbines, have been experimented with, but have seen little use. By the end of the 20th century, almost the only steam power still in regular use in North America and Western European countries was on heritage railways specifically aimed at tourists and/or railroad enthusiasts, known as railfans or train spotters, although some narrow gauge lines in Germany which form part of the public transport system, running to all-year-round timetables retain steam for all or part of their motive power. Steam locomotives remained in commercial use in parts of Mexico into the late 1970s. Steam locomotives are in regular use in China, where coal is a much more abundant resource than petroleum for diesel fuel. India has switched in the 1990's from steam-powered trains to electric- and diesel-powered trains. In some mountainous and high altitude rail lines, steam engines remain in use because they are less affected by reduced air pressure than diesel engines. petroleum 73096, a 4-6-0 steam loco, at Virginia Water station, April 2004.]]

External links

- [http://www.steamlocomotive.com/ Database of surviving steam locomotives in North America]
- [http://steamrailroading.com/ Information on North American steam railroads in operation]

Diesel-mechanical

Diesel locomotives vary in the form of transmission used to convey the power from a diesel engine (or engines) to the wheels. The simplest form of transmission is by means of a gearbox, in the same way as on road vehicles. Diesel trains or locomotives that use this are called diesel-mechanical and began to appear (although limited in power) even before the first world war which saw a number of simplex diesel systems built for the war, a small number of which survive and are still operational today. It has, however, been found impractical to build a gearbox which can cope with a power output of more than 400 horsepower (300 kW) without breaking, despite a number of attempts to do so. Therefore this type of transmission is only suitable for low-powered shunting locomotives, or lightweight multiple units or railcars. For more powerful locomotives, other types of transmission have to be used.

Diesel-electric

railcar refueling at Dunsmuir, California]] The most common form of transmission is electric; a locomotive using electric transmission is known as a diesel-electric locomotive. With this system, the diesel engine drives a generator or alternator; the electrical power produced then drives the wheels using electric motors. In effect, such a locomotive is an electric locomotive which carries its own generating station along with it. Early diesel-electrics were switching engines used to move rail cars around in rail yards. The first went into service in 1918 with the Jay Street Connecting Railroad. Sixteen years later, the technology began to be applied to regular mainline service as streamlined passenger trains went into operation. Actually, a petroleum distillate-electric system powered the first such train, but diesel-electric systems soon proved to be more cost-effective because of higher efficiency and lower maintenance costs. The fuel for one early high-speed run from Chicago, Illinois to Denver, Colorado only cost US$14.64 (in 1934 dollars). In the 1970s, British Rail in the United Kingdom developed a high-speed diesel-electric train called the High Speed Train or HST. This train consists of two Class 43 locomotives (also known as power cars), one at each end, and a number of "Mark 3" carriages (usually 8). A complete HST set was originally designated as a Class 253 or 254 diesel multiple unit (DMU), but due to the frequent exchanges between sets the power cars were reclassified as locomotives and given class number 43. The unpowered carriages were simultaneously reclassified as individual coaches - the number of a DMU set should identify all its associated carriages as well. The prototype HST (designated Class 252) holds the world speed record for diesel traction, having reached a speed of 143 mph, although the operating speed of the production HST in service is 125 mph (200 km/h), hence the name "Inter-City 125". A variant of the Intercity 125, the XPT, is in service on New South Wales railways in Australia, but with a lower top speed and different carriages.

Diesel-hydraulic

Alternatively, diesel-hydraulic locomotives use hydraulic transmission to convey the power from the diesel engine to the wheels. On this type of locomotive, the power is transmitted to the wheels by means of a device called a torque converter. A torque converter consists of three main parts, two of which rotate, and one which is fixed. All three main parts are sealed in a housing filled with oil. The inner rotating part of a torque converter is called a centrifugal pump (or impeller), the outer part is called a turbine wheel (or driven wheel), and between them is a fixed guide wheel. All of these parts have specially shaped blades to control the flow of oil. The centrifugal pump is connected directly to the diesel engine, and the turbine wheel is connected to an axle, which drives the wheels. As the diesel engine rotates the centrifugal pump, oil is forced outwards at high pressure. The oil is forced through the blades of the fixed guide wheel and then through the blades of the turbine wheel, which causes it to rotate and thus turn the axle and the wheels. The oil is then pumped around the circuit again and again. The disposition of the guide vanes allows the torque converter to act as a "gearbox" with continuously variable ratio. If the output shaft is loaded so as to reduce its rotational speed, the torque applied to the shaft increases, so the power transmitted by the torque converter remains more or less constant. However, the range of variability is not sufficient to match engine speed to load speed over the entire speed range of a locomotive, so some additional method is required to give sufficient range. One method is to follow the torque converter with a mechanical gearbox which switches ratios automatically, similar to an automatic transmission on a car. Another method is to provide several torque converters each with a range of variability covering part of the total required; all the torque converters are mechanically connected all the time, and the appropriate one for the speed range required is selected by filling it with oil and draining the others. The filling and draining is carried out with the transmission under load, and results in very smooth range changes with no break in the transmitted power. Diesel-hydraulic multiple units, a less arduous duty, often use a simplification of this system, with a torque converter for the lower speed ranges and a fluid coupling for the high speed range. A fluid coupling is similar to a torque converter but the ratio of input to output speed is fixed; loading the output shaft results not in torque multiplication and constant power throughput but in reduction of the input speed with consequent lower power throughput. (In car terms, the fluid coupling provides top gear and the torque converter provides all the lower gears.) The result is that the power available at the rail is reduced when operating in the lower speed part of the fluid coupling range, but the less arduous duty of a passenger multiple unit compared to a locomotive makes this an acceptable tradeoff for reduced mechanical complexity. Diesel-hydraulic locomotives are slightly more efficient than diesel-electrics, but were found in many countries to be mechanically more complicated and more likely to break down. In Germany, however, diesel-hydraulic systems achieved extremely high reliability in operation. Persistent argument continues over the relative reliability of hydraulic engines, with continuing questions over whether data was manipulated politically to favour local suppliers over German ones. In the US and Canada, they are now greatly outnumbered by diesel-electric locomotives, while they remain dominant in some European countries. The most famous diesel-hydraulic locomotive is the German V200 which were built from 1953 in a total number of 136. The only diesel-electric locomotives of the Deutsche Bundesbahn were BR 288 (V 188), of which 12 were built in 1939 by the DRG. The high reliability of the German locomotives was paralleled by higher reliability of non-German locomotives built with German-made parts compared to that of the same designs built using parts made locally to German patterns under licence. Much of the unreliability experienced outside Germany was due to poor quality control in the local manufacture of engines and transmissions, and poor maintenance due to staff used to steam locomotives working on unfamiliar and much more complex designs in unsuitable conditions and failing to follow the unit-replacement maintenance methods which were part of the German success. It is notable that diesel-hydraulic multiple units, with the advantages of modern manufacturing techniques and improved maintenance procedures, are now extremely successful in widespread use, achieving excellent reliability.

Gas turbine-electric

DRG] Main article: Gas turbine-electric locomotive Locomotives powered by gas turbines were developed in many countries in the decades after World War II. These used jet-type engines (similar to the turboshaft engines in a turbine helicopter) driving an output shaft. The normal method of transmitting power to the wheels involved an electrical transmission similar to a diesel-electric locomotive - the turbines running at constant speed driving a generator, feeding to large electric motors driving the wheels. Gas turbine locomotives are very powerful, but also very noisy (they sounded similar to a jet aircraft at takeoff). Union Pacific operated the largest fleet of turbine locomotives and used them extensively, at one point claiming that the turbines hauled 10% of the railroad's freight. Their efficiency was quite low, but this was initially not a problem; Union Pacific's gas turbines were fueled with cheap 'Bunker C' (later No.6) heavy fuel oil. This cheap fuel source vanished when improved refinery techniques allowed it to be 'cracked' into lighter petroleum grades. After the oil crisis in the 1970s and the subsequent rise in fuel costs, gas turbine locomotives became uneconomic to operate, and many were taken out of service. This type of locomotive is now rare.

Electric

Main article: Electric locomotive Electric locomotive The electric locomotive is supplied externally with electric power, either through an overhead pickup or through a third-rail. While the cost of electrifying track is rather high, electric trains and locomotives are significantly cheaper to run than diesel ones, and are capable of superior acceleration as well as regenerative braking, making them ideal for passenger service in densely populated areas. Almost all high speed train systems (e.g. ICE, TGV, Shinkansen) use electric power, because the power needed for such performance is not easily carried on board. For example the most powerful electric locomotives that are used today on the channel tunnel freight services use 7 MW of power. The world speed record for a wheeled train was set in 1990 by a French TGV which reached a speed of 515.3 km/h (320 mph). While recently designed electrified railway 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). A small number of electric locomotives can also operate off battery power to enable short journeys or shunting to occur on non-electrified lines or yards. Pure battery locomotives also found usage in mines and other underground workings where diesel fumes or smoke are not safe and where external electricity supplies could not be used. Battery locomotives are also used on many underground railways for maintenance operations as they are required to operate in areas where the electricity supply has been temporarily disconnected. See also: Railway electrification system

Electro-diesel

Main article: Electro-diesel locomotive These are special locomotives that can either operate as an electric locomotive or a diesel locomotive. Dual-mode diesel-electric/third-rail locomotives are operated by the Long Island Rail Road and Metro-North Railroad between non-electrified territory and New York City because of a local law banning diesel-powered locomotives in Manhattan tunnels. For the same reason Amtrak operates a fleet of dual-mode locomotives in the New York area. British Rail operated dual diesel-electric/electric locomotives designed to run primarily as electric locomotives. This allowed railway yards to remain un-electrified as the third-rail power system is extremely hazardous in a yard area.

Magnetic levitation

third-rail The newest technology in trains is magnetic levitation (maglev). These electrically powered trains have a special open motor which floats the train above the rail without the need for wheels. This greatly reduces friction. Very few systems are in service and the cost is very high. The experimental Japanese magnetic levitation train has reached 552 km/h (343 mph). The transrapid maglev train connects Shanghai's airport with the city. The first commercial maglev trains ran in the 1980s in Birmingham, United Kingdom, providing a low-speed shuttle service between the airport and its railway station. Despite the huge interest and excitement in the technology it was abandoned and replaced by a cable-hauled guideway a few years later.

Classification by use

The three main categories of locomotives are often subdivided in their usage in rail transport operations. There are passenger locomotives, freight locomotives and switcher (or shunting) locomotives. These categories mainly depend on manoeuvrability, traction power and speed. Some locomotives are designed to work in mountain railways.

References

[http://www.gutenberg.org/etext/11164 An engineer's guide from 1891] [http://www.keveney.com/Locomotive.html Animated engines, Steam Locomotive] 1 Locomotive Category:Rail transport ja:機関車 ko:기관차

Electric motor

An electric motor converts electrical energy into mechanical motion. The reverse task, that of converting mechanical motion into electrical energy, is accomplished by a generator or dynamo. In many cases the two devices differ only in their application and minor construction details, and some applications use a single device to fill both roles. For example, traction motors used on locomotives often perform both tasks if the locomotive is equipped with dynamic brakes.

Operation

Most electric motors work by electromagnetism, but motors based on other electromechanical phenomena, such as electrostatic forces and the piezoelectric effect, also exist. The fundamental principle upon which electromagnetic motors are based is that there is a mechanical force on any wire when it is conducting electricity while contained within a magnetic field. The force is described by the Lorentz force law and is perpendicular to both the wire and the magnetic field. In a rotary motor, there is a rotating element, the rotor. The rotor rotates because the wires and magnetic field are arranged so that a torque is developed about the rotor's axis. Most magnetic motors are rotary, but linear types also exist. In a rotary motor, the rotating part (usually on the inside) is called the rotor, and the stationary part is called the stator. The motor contains electromagnets that are wound on a frame. Though this frame is often called the armature, that term is often erroneously applied. Correctly, the armature is that part of the motor across which the input voltage is supplied or that part of the generator across which the output voltage is generated. Depending upon the design of the machine, either the rotor or the stator can serve as the armature.

DC motors

One of the first electromagnetic rotary motors was invented by Michael Faraday in 1821 and consisted of a free-hanging wire dipping into a pool of mercury. A permanent magnet was placed in the middle of the pool. When a current was passed through the wire, the wire rotated around the magnet, showing that the current gave rise to a circular magnetic field around the wire. This motor is often demonstrated in school physics classes, but brine is sometimes used in place of the toxic mercury. This is the simplest form of a class of electric motors called homopolar motors. The modern DC motor was invented by accident in 1873, when Zénobe Gramme connected a spinning dynamo to a second similar unit, driving it as a motor. The classic DC motor has a rotating armature in the form of an electromagnet with two poles. A rotary switch called a commutator reverses the direction of the electric current twice every cycle, to flow through the armature so that the poles of the electromagnet push and pull against the permanent magnets on the outside of the motor. As the poles of the armature electromagnet pass the poles of the permanent magnets, the commutator reverses the polarity of the armature electromagnet. During that instant of switching polarity, inertia keeps the classical motor going in the proper direction. (See the diagrams below.) inertia inertia inertia
DC motor speed generally depends on a combination of the voltage and current flowing in the motor coils and the motor load or braking torque. The speed of the motor is proportional to the voltage, and the torque is proportional to the current. The speed is typically controlled by altering the voltage or current flow by using taps in the motor windings or by having a variable voltage supply. As this type of motor can develop quite high torque at low speed it is often used in traction applications such as locomotives. However, there are a number of limitations in the classic design, many due to the need for brushes to rub against the commutator. The rubbing creates friction, and the higher the speed, the harder the brushes have to press to maintain good contact. Not only does this friction make the motor noisy, but it also creates an upper limit on the speed and causes the brushes eventually to wear out and to require replacement. The imperfect electric contact also causes electrical noise in the attached circuit. These problems vanish when you turn the motor inside out, putting the permanent magnets on the inside and the coils on the outside thus designing out the need for brushes in a brushless design. However such designs need electronic circuits to control the switching of the electromagnets (the function that is performed in conventional motors by the commutator).

Wound field DC motor

The permanent magnets on the outside (stator) of a DC motor may be replaced by electromagnets. By varying the field current it is possible to alter the speed/torque ratio of the motor. Typically the field winding will be placed in series (series wound) with the armature winding to get a high torque low speed motor, in parallel (shunt wound) with the armature to get a high speed low torque motor, or to have a winding partly in parallel, and partly in series (compound wound) for a balance. Further reductions in field current are possible to gain even higher speed but correspondingly lower torque. This technique is ideal for electric traction (see Traction motor) and many similar applications where its use can eliminate the requirement for a mechanically variable transmission.

Universal motors

A variant of the wound field DC motor is the universal motor. The name derives from the fact that it may use AC or DC supply current, although in practice they are nearly always used with AC supplies. The principle is that in a wound field DC motor the current in both the field and the armature (and hence the resultant magnetic fields) will alternate (reverse polarity) at the same time, and hence the mechanical force generated is always the same. In practice the motor must be specially designed to cope with the AC current (impedance/reluctance must be taken into account), and the resultant motor is generally less efficient than an equivalent pure DC motor. The advantage of the universal motor is that AC supplies may be used on motors which have the typical characteristics of DC motors, specifically high starting torque and very compact design if high running speeds are used. The negative aspect is the maintenance and reliability problems caused by the commutator, and as a result such motors will rarely be found in industry but are the most common type of AC supplied motor in devices such as food mixers and power tools which are only used intermittently. Continuous speed control of a universal motor running on AC is very easily accomplished using a thyristor circuit while stepped speed control can be accomplished using multiple taps on the field coil. Household blenders that advertise many speeds frequently combine a field coil with several taps and a diode that can be inserted in series with the motor (causing the motor to run on half-wave DC with half the RMS voltage of the AC power line). Unlike the other common forms of AC motors (induction motors and synchronous motors), universal motors can easily exceed one revolution per cycle of the mains current (that is, exceed 3000 rpm on a 50 Hz system or 3600 rpm on a 60 Hz system). This makes them especially useful for certain appliances such as blenders, vacuum cleaners, and hair dryers where high-speed operation is desired. With the very low cost of semiconductor rectifiers, some applications that would have previously used a universal motor now use a pure DC motor, usually with a permanent magnet field. This is especially true if the semiconductor circuit is also used for variable-speed control.

AC motors

A typical AC motor consists of two parts: # An outside stationary stator having coils supplied with AC current to produce a rotating magnetic field, and; # An inside rotor attached to the output shaft that is given a torque by the rotating field. There are two fundamental types of AC motor depending on the type of rotor used:
- The synchronous motor, which rotates exactly at the supply frequency or a submultiple of the supply frequency, and;
- The induction motor, which turns slightly slower, and typically (though not necessarily always) takes the form of the squirrel cage motor. The rotating magnetic field principle, though commonly credited to Nikola Tesla in 1882 or thereabouts, was productively employed by mainstream scientists such as Michael Faraday and James Clerk Maxwell in the 1820s. Tesla, however, exploited the principle to design a unique two-phase induction motor in 1883. Michael von Dolivo-Dobrowlsky invented the first modern three-phase "cage-rotor" in 1890. Introduction of the motor from 1888 onwards initiated what is known as the Second Industrial Revolution, making possible the efficient generation and long distance distribution of electrical energy using the alternating current transmission system, also of Tesla's invention (1888)[http://www.tfcbooks.com/tesla/system.htm]. The first successful commercial three phase generation and long distance transmission system was designed by Almerian Decker at Mill Creek No. 1 [http://www.electrichistory.com] in Redlands California.[http://www.redlandsweb.com] Although the statement that "AC motors generally come in two types: single phase and three phase" is often made, this distinction is of insufficient importance to assign the term "types." What is actually meant is that, for more important purposes of commercial power distribution, AC motors are commonly employed in a "three-phase" system whereby three discrete waveforms--each logically displaced 120 degrees from its neighbor--are transmitted in unison. It is common for an individual subscriber to have only one of these phases actually present on the premises, allowing only single phase motors to be used.

Three-phase AC induction motors

For higher-power applications where a polyphase electrical supply is available, the three-phase (or polyphase) AC induction motor is used. The phase differences between the three phases of the polyphase electrical supply create a rotating electromagnetic field in the motor. Through electromagnetic induction, the rotating magnetic field induces a current in the conductors in the rotor, which in turn sets up a counterbalancing magnetic field that causes the rotor to turn in the direction the field is rotating. The rotor must always rotate slower than the rotating magnetic field produced by the polyphase electrical supply; otherwise, no counterbalancing field will be produced in the rotor. Induction motors are the workhorses of industry and motors up to about 500 kW in output are produced in highly standardized frame sizes, making them nearly completely interchangeable between manufacturers (although European and North American standard dimensions are of course different). There are two types of rotors used in induction motors. Most use the squirrel cage rotor. An alternate design, called the wound rotor, is used when variable speed is required. In this case, the rotor has the same number of poles as the stator and the windings are made of wire, connected to slip rings on the shaft. Carbon brushes connect the slip rings to an external controller such as a variable resistor that allows changing the motor's slip rate. In certain high-power variable speed wound-rotor drives, the slip-frequency energy is captured, rectified and returned to the power supply through an inverter. Compared to squirrel cage rotors, wound rotor motors are expensive and require maintenance of the slip rings and brushes, but they were the standard form for variable speed control before the advent of compact power electronic devices. Transistorized inverters with variable frequency drive can now be used for speed control and wound rotor motors are becoming less common. (Transistorized inverter drives also allow the more-efficient three-phase motors to be used when only single-phase mains current is available.) Several methods of starting a polyphase motor are used. Where the large inrush current and high starting torque can be permitted, the motor can be started across the line, by applying full line voltage to the terminals. Where it is necessary to limit the starting inrush current (where the motor is large compared with the short-circuit capacity of the supply), reduced voltage starting using either series inductors, an autotransformer, thyristors, or other devices are used. A technique sometimes used is star-delta starting, where the motor coils are initially connected in wye for acceleration of the load, then switched to delta when the load is up to speed. Transistorized drives can directly vary the applied voltage as required by the starting characteristics of the motor and load. This type of motor is becoming more common in traction applications such as locomotives, where it is known as the asynchronous traction motor. The speed of the AC motor is determined primarily by the frequency of the AC supply and the number of poles in the stator winding, according to the relation: :

N_ = 120F/p

where :N_s = Synchronous speed, in revolutions per minute :F = AC power frequency :p = Number of poles, usually an even number but always a multiple of the number of phases Actual RPM for an induction motor will be less than this calculated synchronous speed by an amount known as slip that increases with the torque produced. With no load the speed will be very close to synchronous. When loaded, standard motors have between 2-3% slip, special motors may have up to 7% slip, and a class of motors known as torque motors are rated to operate at 100% slip (0 RPM/full stall). The slip of the AC motor is calculated by: :

S = (N_ - N)/N_

where :N_r = Rotational speed, in revolutions per minute. :S = Slip, in percent. As an example, a typical four-pole motor running on 60 Hz might have a nameplate rating of 1725 RPM at full load, while its calculated speed is 1800. The speed in this type of motor has traditionally been altered by having additional sets of coils or poles in the motor that can be switched on and off to change the speed of magnetic field rotation. However, developments in power electronics mean that the frequency of the power supply can also now be varied to provide a smoother control of the motor speed.

Three-phase AC synchronous motors

If connections to the rotor coils of a three-phase motor are taken out on slip-rings and fed a separate field current to create a continuous magnetic field (or if the rotor consists of a permanent magnet), the result is called a synchronous motor because the rotor will rotate in synchronism with the rotating magnetic field produced by the polyphase electrical supply. A synchronous motor can also be used as an alternator. Nowadays, synchronous motors are frequently driven by transistorized variable frequency drives. This greatly eases the problem of starting the massive rotor of a large synchronous motor. They may also be started as induction motors using a squirrel-cage winding that shares the common rotor: once the motor reaches synchronous speed, no current is induced in the squirrel-cage winding so it has little effect on the synchronous operation of the motor. Synchronous motors are occasionally used as traction motors; the TGV may be the best-known example of such use.

Single-phase AC induction motors

A polyphase induction motor will continue to rotate even if one phase is disconnected, at reduced torque. However, a polyphase motor at standstill will not generate any net starting torque if connected only to a single-phase supply. The key to the design of single-phase motors, then, is to provide a rotating magnetic field to produce starting torque. A common single-phase motor is the shaded pole motor, which is used in devices requiring lower torque, such as electric fans or other small household appliances. In this motor, small single-turn copper "shading coils" create the moving magnetic field. Part of each pole is encircled by a copper coil or strap; the induced current in the strap opposes the change of flux through the coil (Lenz's Law), so that the maximum field intensity moves across the pole face on each cycle. Another common single-phase AC motor is the split-phase induction motor, commonly used in major appliances such as washing machines and clothes dryers. Compared to the shaded pole motor, these motors can generally provide much greater starting torque by using a special startup winding in conjunction with a centrifugal switch. In the split phase motor, the startup winding is designed with a higher resistance than the running winding. This creates an LR circuit which slightly shifts the phase of the current in the startup winding. When the motor is starting, the startup winding is connected to the power source via a set of spring-loaded contacts pressed upon by the not-yet-rotating centrifugal switch. The starting winding is wound with fewer turns of smaller wire than the main winding, so it has a higher resistance. The extra resistance creates a small phase shift, not more than about 30 degrees, between the flux due to the main winding and the flux of the starting winding. The starting direction of rotation may be reversed simply by exchanging the connections of the startup winding relative to the running winding. The phase of the magnetic field in this startup winding is shifted from the phase of the mains power, allowing the creation of a moving magnetic field which starts the motor. Once the motor reaches near design operating speed, the centrifugal switch activates, opening the contacts and disconnecting the startup winding from the power source. The motor then operates solely on the running winding. The starting winding must be disconnected since it would increase the losses in the motor. In a capacitor start motor, a starting capacitor is inserted in series with the startup winding, creating an LC circuit which is capable of a much greater phase shift (and so, a much greater starting torque). The capacitor naturally adds expense to such motors. Another variation is the Permanent Split-Capacitor (PSC) motor. This motor operates similarly to the capacitor-start motor described above, but there is no centrifugal starting switch and the second winding is permanently connected to the power source. PSC motors are frequently used in air handlers, fans, and blowers and other cases where a variable speed is desired. By changing taps on the running winding but keeping the load constant, the motor can be made to run at different speeds.

Single-phase AC synchronous motors

Small single-phase AC motors can also be designed with magnetized rotors (or several variations on that idea). The rotors in these motors do not require any induced current so they do not slip backward against the mains frequency. Instead, they rotate synchronously with the mains frequency. Because of their highly accurate speed, such motors are usually used to power mechanical clocks, audio turntables, and tape drives; formerly they were also much used in accurate timing instruments such as strip-chart recorders or telescope drive mechanisms. The shaded-pole synchronous motor is one version. Because inertia makes it difficult to instantly accelerate the rotor from stopped to synchronous speed, these motors normally require some sort of special feature to get started. Various designs use a small induction motor (which may share the same field coils and rotor as the synchronous motor) or a very light rotor with a one-way mechanism (to ensure that the rotor starts in the "forward" direction).

Stepper motors

Closely related in design to three-phase AC synchronous motors are stepper motors, where an internal rotor containing permanent magnets or a large iron core with salient poles is controlled by a set of external magnets that are switched electronically. A stepper motor may also be thought of as a cross between a DC electric motor and a solenoid. As each coil is energized in turn, the rotor aligns itself with the magnetic field produced by the energized field winding. Unlike a synchronous motor, in its application, the motor may not rotate continuously; instead, it "steps" from one position to the next as field windings are energized and deenergized in sequence. Depending on the sequence, the rotor may turn forwards or backwards. Simple stepper motor drivers entirely energize or entirely deenergize the field windings, leading the rotor to "cog" to a limited number of positions; more sophisticated drivers can proportionally control the power to the field windings allowing the rotors to position "between" the "cog" points and thereby rotate extremely smoothly. Computer controlled stepper motors are one of the most versatile forms of positioning systems, particularly when part of a digital servo-controlled system.

Brushless DC motors

Midway between ordinary DC motors and stepper motors lies the realm of the brushless DC motor. Built in a fashion very similar to stepper motors, these often use a permanent magnet external rotor, three phases of driving coils, one or more Hall effect devices to sense the position of the rotor, and the associated drive electronics. The coils are activated, one phase after the other, by the drive electronics as cued by the signals from the Hall effect sensors. In effect, they act as three-phase synchronous motors containing their own variable frequency drive electronics. Brushless DC motors are commonly used to drive fans, the spindles within CD, CD-ROM (etc.) drives, and mechanisms within office products such as laser printers and photocopiers. They also find significant use in high-performance electric model aircraft. They have several advantages over conventional motors:
- Compared to AC fans using shaded-pole motors, they are very efficient, running much cooler than the equivalent AC motors. This cool operation leads to much-improved life of the fan's bearings.
- Without a commutator to wear out, the life of a DC brushless motor can be significantly longer compared to a DC motor using brushes and a commutator
- The same Hall effect devices that provide the commutation can also provide a convenient tachometer signal for closed-loop control (servo-controlled) applications. In fans, the tachometer signal can be used to derive a "fan okay" signal.
- The motor can be easily synchronized to an internal or external clock, leading to precise speed control. Modern DC brushless motors range in power from a fraction of a watt to many kilowatts.

Coreless DC motors

A coreless DC motor is a specialized form of an ordinary DC motor. Optimized for rapid acceleration, these motors have a rotor that is constructed without any iron core. The rotor can take the form of a winding-filled cylinder inside the stator magnets, a basket surrounding the stator magnets, or a flat pancake (possibly formed on a printed wiring board) running between upper and lower stator magnets. The windings are typically stabilized by being impregnated with epoxy resins. Because the rotor is much lighter in weight (mass) than a conventional rotor formed from copper windings on steel laminations, the rotor can accelerate much more rapidly, often achieving a mechanical time constant under 1 ms. This is especially true if the windings use aluminum rather than the heavier copper. But because there is no metal mass in the rotor to act as a heat sink, even small coreless motors must often be cooled by forced air. These motors were commonly used to drive the capstan(s) of magnetic tape drives and are still widely used in high-performance servo-controlled systems.

Linear motors

A linear motor is essentially an electric motor that has been "unrolled" so that instead of producing a torque (rotation), it produces a linear force along its length by setting up a traveling electromagnetic field. Linear motors are most commonly induction motors or stepper motors. You can find a linear motor in a maglev (Transrapid) train, where the train "flies" over the ground.

Textbooks

- Shanefield D. J., Industrial Electronics for Engineers, Chemists, and Technicians, William Andrew Publishing, Norwich, NY, 2001. A self-teaching textbook that briefly covers electric motors, transformers, speed controllers, wiring codes and grounding, transistors, digital, etc. Easy to read and understand, up to an elementary level on each subject, not a suitable reference book for technologists already working in any of those fields.
- Woodsom and Melcher, unnamed book, for graduates
- Fitzgerald/Kingsley/Kusko (Fitzgerald/Kingsley/Umans in later years), Electric Machinery, classic text for junior and senior electrical engineering students. Originally published in 1952, 6th edition published in 2002. Authors still listed as Fitzgerald/Kingsley/Umans although Fitzgerald and Kingsley are now deceased.
- Slemon and Straughen, unnamed book, less advanced
- Van E. Mablekos, title unknown, very easy reading
- Bedford and Hoft, unnamed book on power electronics, outdated Principles of Inverter Circuits (1964); John Wiley & Sons (Inverter circuits are used for adjustable frequency motor speed control)
- Dewan and Straughen, another unnamed book on power electronics
- B. R. Pelly, "Thyristor Phase-Controlled Converters and Cycloconverters: Operation, Control, and Performance" (New York: John Wiley, 1971).

References

Donald G. Fink and H. Wayne Beaty, Standard Handbook for Electrical Engineers, Eleventh Edition, McGraw-Hill, New York, 1978, ISBN 007020974X. Category:Energy conversion Category:Electromagnetic components
- Category:Nikola Tesla Category:BEV Components ja:電動機

Current (electricity)

In electricity, current refers to electric current, which is the flow of electric charge. Lightning is an example of an electric current, as is the solar wind, the source of the polar aurora. Probably the most familiar form of electric current is the flow of conduction electrons in a metallic wire. This is how utility companies deliver electricity. In electronics, electric current is most often the flow of electrons through conductors and devices such as resistors, but it is also the flow of ions inside a battery or the flow of holes within a semiconductor.

Relation between current and charge

The symbol typically used for the amount of current (the amount of charge Q flowing per unit of time t) is I, from the German word Intensität, which means 'intensity'. :

I =

Formally this is written as :

i(t) =

or inversely as

q(t) = \int_^ i(x)\, dx

Conventional current

Conventional current was defined early in the history of electrical science as a flow of positive charge. In solid metals, like wires, the positive charges are immobile, and only the negatively charged electrons flow in the direction opposite conventional current, but this is not the case in most non-metallic conductors. In other materials, charged particles flow in both directions at the same time. Electric currents in electrolytes are flows of electrically charged atoms (ions), which exist in both positive and negative varieties. For example, an electrochemical cell may be constructed with salt water (a solution of sodium chloride) on one side of a membrane and pure water on the other. The membrane lets the positive sodium ions pass, but not the negative chlorine ions, so a net current results. Electric currents in plasma are flows of electrons as well as positive and negative ions. In ice and in certain solid electrolytes, flowing protons constitute the electric current. To simplify this situation, the original definition of conventional current still stands. There are also instances where the electrons are the charge that is physically moving, but where it makes more sense to think of the current as the movement of positive "holes" (the spots that should have an electron to make the conductor neutral). This is the case in a p-type semiconductor. The SI unit of electrical current is the ampere. Electric current is therefore sometimes informally referred to as amperage or ampage, by analogy with the term voltage. Though this is a valid term, some engineers frown on it.

The speed of an electric current

The charged particles whose movement causes an electric current do not always move in straight lines. In metals, for example, they follow an erratic path, bouncing from atom to atom, but generally drifting in the direction of the electric field. The speed at which they drift can be calculated from the equation: :

I=nAvQ \!\

where :I is the current :n is number of charged particles per unit volume :A is the cross-sectional area of the conductor :v is the drift velocity, and :Q is the charge on each particle. For example, in a copper wire of cross-section 0.5 mm², carrying a current of 5 A, the drift velocity of the electrons is of the order of a millimetre per second. To take a different example, in the near-vacuum inside a cathode ray tube, the electrons travel in near-straight lines ("ballistically") at about a tenth of the speed of light. However, we know that an electric signal travels much faster than this; usually close to the speed of light. These results show that the speed of the charged particles is not necessarily related to the speed of the electric signal. To understand how signals travel faster than the particles that carry them, it is necessary to understand the properties of electromagnetic waves (see article).

Current density

Current density is the current per unit (cross-sectional) area. Mathematically, current is defined as the net flux through an area. Thus: :

I = j \cdot A

where, in the MKS or SI system of measurement, :I is the current, measured in amperes :j is the "current density" measured in amperes per square metre :A is the area through which the current is flowing, measured in square metres The current density is defined as: :

j=\int_i n_i \cdot x_i \cdot \mathbf

where :n is the particle density (number of particles per unit volume) :x is the mass, charge, or any other characteristic whose flow one would like to measure. :u is the average velocity of the particles in each volume Current density is an important consideration in the design of electrical and electronic systems. Most electrical conductors have a finite, positive resistance, making them dissipate power in the form of heat. The current density must be kept sufficiently low to prevent the conductor from melting or burning up, or the insulating material failing. In superconductors, excessive current density may generate a strong enough magnetic field to cause spontaneous loss of the superconductive property.

Electromagnetism

Every electric current produces a magnetic field. The magnetic field can be visualized as a pattern of circular field lines surrounding the wire. Electric current can be directly measured with a galvanometer, but this method involves breaking the circuit, which is sometimes inconvenient. Current can also be measured without breaking the circuit by detecting the magnetic field it creates. Devices used for this include Hall effect sensors, current clamps and Rogowski coils.

Ohm's law

Ohm's law predicts the current in an (ideal) resistor (or other ohmic device) to be the quotient of applied voltage over electrical resistance: :

I = \frac

where :I is the current, measured in amperes :V is the potential difference measured in volts :R is the resistance measured in ohms

Electrical safety

The danger of an electric shock depends on the current (in milliamperes), duration and the current's path in the body:
- 1 mA causes a tingle
- 5 mA causes a slight shock
- 50 to 150 mA may result in death, e.g. through rhabdomyolysis (muscle breakdown) and resultant acute renal failure
- 1-4 A causes ventricular fibrillation
- 10 A causes cardiac arrest (only at this current will a typical home fuse break the circuit) Currents through the heart and the nervous system are the most dangerous. As most dangerous sources are voltage sources, the current present depends on the resistance of the body between the points of contact and any current limiting built into the source. The comparison between the dangers of alternating current and direct current has been a subject of debate ever since the War of Currents in the 1880s. DC tends to cause continuous muscular contractions that make the victim hold on to a live conductor, thereby increasing the risk of deep tissue burns. On the other hand, mains-frequency AC tends to interfere more with the heart's electrical pacemaker, leading to an increased risk of fibrillation. AC at higher frequencies holds a different mixture of hazards, such as RF burns and the possibility of tissue damage with no immediate sensation of pain.

External links

- [http://www.unitconversion.org/unit_converter/current.html Online Current Converter] - convert between various units of current, such as ampere, biot, abampere, statampere, and so on
- [http://www.unitconversion.org/unit_converter/current-v.html Interactive Current Conversion Table] - convert selected unit to all other units of current
- [http://amasci.com/amateur/elecdir.html Which direction does electricity really flow?] Category:Electromagnetism Category:Magnetism ko:전류 ja:電流 th:กระแสไฟฟ้า

Third rail

stop in the Washington, D.C. area, electrified to 750 volts. The third rail is at the top of the image, covered by the white canopy above it. The two lower rails are the ordinary running rails; current from the third rail eventually returns to the power station through these.]] Washington, D.C. :"Third rail" is sometimes used as a metaphor in politics: see third rail (metaphor). A third rail can also be part of a dual gauge setup. A third rail is a method of providing electricity to power a railroad, typically a mass transit system. Well-known examples of rail transit systems utilizing a third rail include the New York City subway system, the Los Angeles and Washington, DC Metro systems, the San Francisco BART system, the Chicago 'L', most of the Metro-North Railroad and Long Island Rail Road in New York, the Toronto subway, and the MBTA in Boston. In the UK, third rails are used on the London Underground system (which uses a fourth rail as well), the suburban railway network in and around south London, long-distance services across the south of England and the Glasgow Subway system. Subway systems (U-Bahnen) in Germany and the suburban trains (S-Bahnen) in Hamburg and Berlin use a third rail. The metro systems of Amsterdam, Netherlands, Moscow and St. Petersburg, Russia also use third rails to power their trains, as do parts of the Paris metro system. This third rail system of electrification is unrelated to the third rail used in dual-gauge railways.

History

Third-rail electric systems are, apart from on-board batteries, the oldest means of supplying electric power to trains. An experimental electric train using this method of power supply was developed by the German firm of Siemens & Halske and shown at the Berlin Exhibition of 1879. Third-rail systems began to be used in public transit in the 1880s for tram (or streetcar) systems and standard-gauge railways. A third rail supplied power to the world's first electric underground railway, the City & South London Railway, which opened in 1890. A widespread belief that African-American inventor Granville Woods invented the third rail is based on his , granted in 1901. However, by that time there had been numerous other patents for electrified third-rail systems, including Thomas Edison's of 1882.

Technical aspects

1882 and WAGN routes primarily to the north and west of London.]] WAGN The third rail is located either in between the two running rails or by the side of them. The electricity is transmitted to the train by means of a sliding "shoe" which is held in contact with the rail. On many systems an insulating cover is provided above the third rail to protect employees working near the track; sometimes the shoe is designed to contact the side or bottom of the third rail, allowing the protective cover to be mounted directly to its top surface. The third rail is an alternative to electrified overhead lines that transmit power to trains by means of pantograph arms attached to the trains. On some metro/light-rail lines, part of the line has a third rail and another part overhead wires, and vehicles allow both, e.g. in Rotterdam, Metro-North's New Haven Division, Boston's Blue Line or Milan subway (line M1). Whereas overhead-wire systems can operate at 25 kV or more, using alternating current (AC), the smaller clearance around a live rail imposes a maximum of about 1200 V (suburban trains in Hamburg), and direct current (DC) is used. As with overhead wires, the return current on a third-rail system usually flows through one or both running rails, and leakage to ground is not considered serious. Where trains run on rubber tires, as on part of the Paris Métro, a separate live rail must be provided for the return current; this third and fourth rail design has other advantages and a few steel-wheel systems also use it, the largest being the London Underground. In line M1 of the Milan underground, the third rail is used as the return electrical line (with potential near the ground) and the live electrical connection is made with a sliding block on the side of the car contacting an electrical bar located next to the railway (between the railway and the opposite direction railway) approximately 1 m (3') above the rail level. In this manner there are four rails. In the northern part of the line the more common overhead lines system is used. One method for reducing current losses is to attach strips of aluminum (which is a better conductor of electricity than steel) to the steel third rail. Because aluminum has a different coefficient of thermal expansion from steel, the strips must be applied on both sides and riveted at frequent intervals. (The third rail in the photo above employs this system. Click on the photo to see it more clearly.)

Disadvantages of third rail

Third-rail systems have a number of significant problems and disadvantages, including:
- Safety: Having an unguarded electrified rail is a major safety hazard, and many people have been killed by touching the rail or by stepping on it while attempting to cross the tracks. There are urban legends that people have died as a result of urinating on the third rail, the urine stream completing an electrical circuit that results in the victim being electrocuted. However, this was disproven from an episode of MythBusters. [http://kwc.org/blog/archives/2004/2004-01-21.mythbusters_third_rail.html] A new tramway system in Bordeaux surmounts the safety problem by using a third rail divided into insulated segments. Each segment is charged only while completely covered by a tram, so there is no risk of a person or animal coming into contact with a charged rail.
- Limited capacity: A relatively low voltage is necessary in a third-rail system, otherwise electricity would arc from the rail. This low voltage means that electrical feeder sub-stations have to be set up at frequent intervals along the line in order to feed electricity into the system. This increases the cost of operating the railway. The low voltage also means that the system is prone to overload; this makes third-rail systems unsuitable for trains demanding high amounts of power such as freight trains or high-speed trains. These inherent limitations of third-rail systems have largely restricted their use to relatively low-speed, lightweight, trains of the type used in mass transit systems, although 750 V DC third rail is used on many hundreds of main line railway route miles across south and southeast England. Capacity is also limited by speed restrictions - 160 km/h (100 mph) is considered to be the maximum speed at which a contact shoe can reliably collect power from a third-rail system.
- Infrastructure restrictions: Junctions and other pointwork make it necessary to leave gaps in the live rail at times, as do level crossings. This is not usually a problem as most third-rail rolling stock has multiple current collection shoes along the length of the train, but under certain circumstances it is possible for a train to become "gapped" - stalled with none of its shoes in contact with the live rail. When this happens it is usually necessary for the train to be shunted back onto a live section either by a rescue locomotive or another service train, although in some circumstances it is possible to use jumper cables to temporarily hook the train's current collectors to the nearest section of live rail. Especially given that gapping tends to happen at complex, important junctions, it can be a major source of disruption.
- Inefficient contact: Fallen leaves, snow and other debris on the conductor rail can adversely affect the efficiency of the contact between the conductor rail and the pickup shoes, leaving trains stalled because of the lack of power.

Advantages of third rail

Third-rail systems are less expensive to install than overhead wire systems, less prone to weather damage (other than flooding and icing, which cause major problems), and better able to be fitted into small tunnels. One further argument cited in favour of third-rail systems is visual intrusion, since they do not need an overhead wire system which some people perceive as unsightly; Singapore, for example, has banned their use outside tunnels. While sometimes used in new transit system construction, third rails are now considerably less popular than overhead systems. In the U.S., they are still the usual means of powering heavy rapid transit lines that are completely grade-separated. Monorail typically use a variation of the third rail (with a fourth rail as well) for current transmission in the form of cables or other electrical conductors placed on the sides of the guideway and contacted by a sprung shoe. Many older railways still use third rails and DC power, even where overhead lines would otherwise be practicable, due to the high cost of retrofitting.

Compromise systems

There are and have been several systems in which third rail has been used for part of the system, and overhead lines for the remainder. These exist sometimes because of the connection of separately-owned railways using the different systems, or because of local ordinances. In New York City, electric trains that must use third rail leaving Grand Central Terminal on the former New York Central Railroad (now Metro-North Railroad) switch to or from overhead lines when they need to operate out onto the former New York, New Haven and Hartford Railroad (now Amtrak) line to Connecticut. The switch is made "on the fly" controlled from the engineer's position. In Manhattan, New York City, and in Washington, D.C., local ordinances required electrified street railways to draw current from a buried third rail accessed by means of a collector that passed through a slot between the running rails. When streetcars on such systems entered territory where overhead lines were allowed, they stopped over a pit where a man detached the collector (plow) and the motorman placed a trolley pole on the overhead.

External links

- [http://patimg1.uspto.gov/.piw?Docid=00263132&PageNum=2&idkey=NONE Thomas Edison's third rail patent (1882)] Category:Rail infrastructure ja:第三軌条方式

Battery (electricity)

:For other uses, see battery (disambiguation). battery (disambiguation) In science and technology, a battery is a device that stores energy and makes it available in an electrical form. Batteries consist of electrochemical devices such as one or more galvanic cells (or, more recently, fuel cells). The first possible evidence of batteries in history are the Baghdad Batteries from sometime between 250 BCE and 640 CE. The modern development of batteries started with the Voltaic pile developed by the Italian physicist Alessandro Volta in 1800. The worldwide battery industry generates 48 billion dollars in sales annually [http://www.dfj.com/cgi-bin/artman/publish/article_141.shtml (2005 estimate)].

Cell vs. battery

Strictly, an electrical "battery" is an interconnected array of one or more similar "cells". That distinction, however, is considered pedantic in most contexts (other than the expression dry cell), and in current English usage it is more common to call a single cell used on its own a battery than a cell. For example, a hand lamp (flashlight) (torch) is said to take one or more "batteries" even though they may be D cells. A car battery is a true "battery" because it uses multiple cells. Multiple batteries or cells may also be refered to as a battery pack as a set of multi-cell 12 V batteries in an electric vehicle.

Electrical component

electric vehicle The cells in a battery can be connected in parallel or in series, or both. A parallel combination of cells has the same voltage as a single cell, but can supply a higher current (the sum of the currents from all the cells). A series combination has the same current rating as a single cell but its voltage is the sum of the voltages of all the cells. Most practical electrochemical batteries, such as 9 volt flashlight (torch) batteries and 12 V automobile (car) batteries, have a series structure. Parallel arrangements suffer from the problem that, if one cell discharges faster than its neighbour, current will flow from the full cell to the empty cell, wasting power and possibly causing overheating. Even worse, if one cell becomes short-circuited due to an internal fault, its neighbour will be forced to discharge its maximum current into the faulty cell, leading to overheating and possibly explosion. Cells in parallel are therefore usually fitted with an electronic circuit to protect them against these problems. In both series and parallel types, the energy stored in the battery is equal to the sum of the energies stored in all the cells. A battery can be simply modelled as a perfect voltage source (i.e. one with zero internal resistance) in series with a resistor. The voltage source depends mainly on the chemistry of the battery, not on whether it is empty or full. When a battery runs down, its internal resistance increases. When the battery is connected to a load (e.g. a light bulb), which has its own resistance, the resulting voltage across the load depends on the ratio of the battery's internal resistance to the resistance of the load. When the battery is fresh, its internal resistance is low, so the voltage across the load is almost equal to that of the battery's internal voltage source. As the battery runs down and its internal resistance increases, the proportion of its internal voltage that gets through the internal resistance to appear at the load gets smaller, so the battery's ability to deliver power to the load decreases.

Battery concepts

Battery capacity

The capacity of a battery to store charge is often expressed in ampere hours (1 A·h = 3600 coulombs). If a battery can provide one ampere (1 A) of current (flow) for one hour, it has a real-world capacity of 1 A·h. If it can provide 1 A for 100 hours, its capacity is 100 A·h. Because of the chemical reactions within the cells, the capacity of a battery depends on the discharge conditions such as the magnitude of the current, the duration of the current, the allowable terminal voltage of the battery, temperature, and other factors. Battery manufacturers use a standard method to determine how to rate their batteries. The battery is discharged at a constant rate of current over a fixed period of time, such as 10 hours or 20 hours, down to a set terminal voltage per cell. So a 100 ampere-hour battery is rated to provide 5 A for 20 hours at room temperature. The efficiency of a battery is different at different discharge rates. When discharging at low rate, the battery's energy is delivered more efficiently than at higher discharge rates. This is Peukert's Law.

Battery lifetime

Disposable alkaline batteries are designed to be used only once. Even if never taken out of the original package, disposable (or "Primary") batteries can lose two to twenty-five percent of their original charge every year, depending heavily on temperature. This is known as the "self discharge" rate and is due to chemical reactions that occur within the cell even if no load is applied to it. Storing batteries at lower temperatures reduces the rate of these side reactions and extends the storage life of the battery. Rechargeable batteries self discharge more rapidly than disposable alkaline batteries. In fact, they can self-discharge up to three percent a day (again, depending on temperature). Due to their poor shelf life, they shouldn't be left in a drawer and then relied upon to power a flashlight or a small radio in an emergency. For this reason, it’s a good idea to keep a few alkaline batteries on hand. With the exception of lead-acid batteries, most NI-MH batteries can be recharged 500-1000 times while NI-CD batteries can only be recharged about 400 times. Special "reserve" batteries intended for long storage in emergency equipment or munitions keeps the electrolyte of the battery separate from the plates until the battery is activated, allowing the cells to be filled with the electrolyte. Storage lives for such batteries can be years or decades, however, their construction is more expensive than more common forms.

Terms used for automobile battery power ratings

:Cranking amps(CA) is the current a battery can provide at 32º F (0 Celsius). Defined as the numbers of amperes an automotive battery at 32 degrees F (0 degrees C) can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12 volt battery). :Cold Cranking Amps (CCA) is the current a battery can provide at (0º F). It is defined as the amperes an automotive battery at 0 degrees F (-17.8 degrees C) can deliver for 30 seconds and maintain at least 1.2 volts per cell (7.2 volts for a 12-volt battery). :Reserve Capacity Minutes(RCM) is a battery's ability to sustain a minimum stated electrical load; it is defined as the time (in minutes) that a lead-acid battery at 80º F will continuously deliver 25 amps before its voltage drops below 10.5 volts.

Battery explosion

Under extreme conditions, certain types of batteries can explode. A battery explosion is usually caused by the misuse or malfunction of a battery (such as the recharging of a non-rechargeable battery or shorting a car battery). With car batteries, explosions are most likely to occur when a short circuit generates currents of very high magnitude. A short circuit malfunction in a battery placed in parallel with other batteries ("jumped") can cause its neighbour to discharge its maximum current into the faulty cell, leading to overheating and possible explosion. In addition, car batteries liberate hydrogen when they are overcharged (because of electrolysis of the water in the electrolyte). Normally the amount of overcharging is very small and so is the amount of explosive gas developed, and the gas dissipates quickly. However, when "jumping" a car battery, the high current can cause the rapid release of large volumes of hydrogen, which could be ignited by a spark nearby (for example, when removing the jumper cables). When a non-rechargeable battery is recharged at a high rate, an explosive gas mixture of hydrogen and oxygen may be produced faster than it can escape from within the walls of the battery, leading to pressure build-up and a possible explosion. In extreme cases, the battery acid may spray violently from the casing of the battery and cause injury. Additionally, disposing of a battery in fire may cause an explosion as steam builds up within the sealed case of the battery. Overcharging, which is charging a battery beyond its electrical capacity, can also lead to a battery explosion, leakage, or irreversible damage to the battery. It may also cause damage to the charger or device in which the overcharged battery is later used.

Common battery types

Rechargeable and disposable batteries

electrolysis From a user's viewpoint, at least, batteries can be generally divided into two main types—rechargeable and non-rechargeable (disposable). Each is in wide usage. Disposable batteries, also called primary cells, are intended to be used once, until the chemical changes that induce the electrical current supply are complete, at which point the battery is discarded. These are most commonly used in smaller, portable devices with either low current drain, only used intermittently, or used well away from an alternative power source. See also: waste. By contrast, rechargeable batteries or secondary cells can be re-charged after they have been drained. This is done by applying externally supplied electrical current which causes the chemical changes that occur in use to be reversed. Devices to supply the appropriate current are called chargers or rechargers. The oldest form of rechargeable battery still in modern usage is the wet cell lead-acid battery. This battery is notable in that it contains a liquid in an unsealed container, requiring that the battery be kept upright and the area be well-ventilated to deal with the explosive hydrogen gas which is vented by these batteries during overcharging. The lead-acid battery is also very heavy for the amount of electrical energy it can supply. Despite this, its low manufacturing cost and its high surge current levels make its use common where the weight and ease of handling are not concerns. A common form of lead-acid battery is the modern car battery. This can deliver about 10,000 watts of power for a short period, and has a peak current output that varies from 450 to 1100 amperes. The battery's electrolyte is sulfuric acid, which can cause serious injury if splashed on the skin or eyes. A more expensive type of lead-acid battery called a gel battery (or "gel cell") contains a semi-solid electrolyte to prevent spillage. More portable rechargeable batteries include several "dry cell" types, which are sealed units and are therefore useful in appliances like mobile phones and laptops. Cells of this type (in order of increasing power density and cost) include nickel-cadmium (nicad or NiCd), nickel metal hydride (NiMH), and lithium-ion (Li-Ion) cells.

Disposable

- Zinc-carbon battery
- Alkaline battery
- Silver-oxide battery
- Lithium battery
- Mercury battery
- Zinc-air battery

Rechargeable

- Lead-acid battery
- Absorbed_Glass_Mat
- Gel battery
- Li-ion battery
- Li-Polymer battery
- NaS battery
- NiMH battery
- NiCd battery
- Sodium-metal chloride battery
- NiZn battery

Homemade cells

Almost any liquid or moist object that has enough ions to be electrically conductive can serve as the electrolyte for a cell. As a novelty or science demonstration, it is possible to insert two electrodes into a lemon, potato, glass of soft drink, etc. and generate small amounts of electricity. As of 2005, "two-potato clocks" are widely available in hobby and toy stores; they consist of a pair of cells, each consisting of a potato (lemon, etc.) with two electrodes inserted into it, wired in series to form a battery with enough voltage to power a digital clock. Homemade cells of this kind are of no real practical use, because they produce far less current—and cost far more per unit of energy generated—than commercial cells, due to the need for frequent replacement of the fruit or vegetable.

Traction batteries

Traction batteries (secondary batteries or accumulators) are designed to provide power to move a vehicle, such as an electric car or tow motor. To prevent spilling, the electrolyte in traction batteries is gelled. The electrolyte may also be embedded in a glass wool which is wound so that the cells have a round cross-sectional area (AGM-type). The following types are also in use[http://www.madkatz.com/ev/battery.html]:
- Zebra NiNaCl (or NaNiCl) battery operating at 270 °C requiring cooling in case of temperature excursions
- NiZn battery (higher cell voltage 1.6 V and thus 25% increased specific energy, very short lifespan) Lithium-ion batteries are now pushing out NiMh-technology in the sector while for low investment costs the lead-acid technology remains in the leading role[http://www.e-mobile.ch/pdf/2005/Subat_WP5-006.pdf].

Flow Batteries

Flow batteries are a special class of battery where additional quantities of electrolyte are stored outside the main power cell of the battery, and circulated through it by pumps or by movement. Flow batteries can have extremely large capacities and are used in marine applications and are gaining populatity in grid energy storage applications.

Common battery sizes

Disposable cells and some rechargeable cells come in a number of standard sizes, so the same battery type can be used in a wide variety of appliances. Some of the major types used in portable appliances include the A-series (A, AA, AAA, AAAA), B, C, D, F, G, J, and N, 3R12, 4R25 and variants, PP3 and PP9, and the lantern 996 and PC926. These and less common types are included in the list of battery sizes appearing in the following section (the list can be opened as a separate page as well). A good cross-reference of different manufacturer's battery and cell designations can be found here [http://www.gpina.com/consumer/primary/button.htm] and here [http://batterywholesale.com/lithium_cross.html].

History

There is some evidence—in the form of the Baghdad Batteries from sometime between 250 BCE and 640 CE (while Baghdad was under Parthian and Sassanian dynasties of ancient Persia) of galvanic cells having been used in ancient times. Such ancient knowledge in the history of electricity bears no known continuous relationship to the development of modern batteries. The hypothesis that these devices had an electrical function, while plausible, remains unproven, as with devices discovered in Egyptian digs that are alleged to be batteries as well. In 1748, Benjamin Franklin coined the term battery to describe the simple capacitor he experimented with, which was an array of charged glass plates. He adapted the word from its earlier sense meaning a beating, which is what an electric shock from the apparatus felt like. In those days, the entertaining effect of an electric shock was one of the few uses of the technology. Other experimenters made batteries from a number of Leyden jars connected in parallel. The definition was later widened to include an array of electrochemical cells or capacitors. The Voltaic pile was a chemical battery developed by the Italian physicist Alessandro Volta in 1800. Volta researched the effects which different metals produced when exposed to salt water. In 1801, Volta demonstrated the Voltaic cell to Napoleon Bonaparte (who later ennobled him for his discoveries). The discoverer of biological electricity Luigi Galvani, researched the same effect with two pieces of the same metal exposed to salt water. The scientific community at this time called this battery a pile, accumulator, because it held charge, or artificial electrical organ. Some early battery researchers called the device a gravity cell because gravity kept the two sulfates separated. The name crowfoot cell was also commonly used because of the shape of the zinc electrode used in the batteries. In 1800, William Nicholson and Anthony Carlisle used a battery to decompose water into hydrogen and oxygen. Sir Humphry Davy researched this chemical effect at the same time. Davy researched the decomposition of substances (called electrolysis). In 1813, he constructed a 2,000-plate paired battery in the basement of Britain's Royal Society, covering 889 ft² (83 m²). Through this experiment, Davy deduced that electrolysis was the action in the voltaic pile that produced electricity. In 1820, the British researcher John Frederic Daniell improved the voltaic cell. The Daniell cell consisted of copper and zinc plates and copper and zinc sulfates. It was used to operate telegraphs and doorbells. Between 1832 and 1834, Michael Faraday conducted experiments with a ferrite ring, a galvanometer, and a connected battery. When the battery was connected or disconnected, the galvanometer deflected. Faraday also developed the principle of ionic mobility in chemical reactions of batteries. In 1839, William Robert Grove developed the first fuel cell, which produced electrical energy by combining hydrogen and oxygen. Grove developed another form the electric cell using zinc and platinum electrodes. These electrodes were exposed to two acids separated by a diaphragm. In the 1860s, Georges Leclanché of France developed a carbon-zinc battery. It was a wet cell, with electrodes plunged into a body of electrolyte fluid. It was rugged, manufactured easily, and had a decent shelf life. An improved version called a dry cell was later made by sealing the cell and changing the fluid electrolyte to a wet paste. The Leclanché cell is a type of primary (non-rechargeable) battery. In the 1860s, Raymond Gaston Planté invented the lead-acid battery. He immersed two thin solid lead plates separated by rubber sheets in a dilute sulfuric acid solution to make a secondary (rechargeable) battery. The original invention had a short shelf life, though. Around 1881, Émile Alphonse Faure, with his colleagues, developed batteries using a mixture of lead oxides for the positive plate electrolyte. These had faster reactions and higher efficiency. In 1878, the air cell battery was developed. In 1897, Nikola Tesla researched a lightweight carbide cell and a oxygen-hydrogen storage cell. In 1898 Nathan Stubblefield received approval for a battery patent (US600457): this electrolytic coil patent is referred to as an "earth battery<