Maglev (from magnetic levitation) is a system of train transportation that uses two sets of magnets: one set to repel and push the train up off the track, and another set to move the elevated train ahead, taking advantage of the lack of friction. Along certain "medium-range" routes (usually 320 to 640 km (200 to 400 mi)), maglev can compete favourably with high-speed rail and airplanes.
With maglev technology, the train travels along a guideway of magnets which control the train's stability and speed. While the propulsion and levitation require no moving parts, the bogies can move in relation to the main body of the vehicle and some technologies require support by retractable wheels at speeds under 150 kilometres per hour (93 mph). This compares with electric multiple units that may have several dozen parts per bogie. Maglev trains can therefore in some cases be quieter and smoother than conventional trains and have the potential for much higher speeds.[1]
Maglev vehicles have set several speed records, and maglev trains can accelerate and decelerate much faster than conventional trains; the only practical limitation is the safety and comfort of the passengers, although wind resistance at very high speeds can cause running costs that are four to five times that of conventional high-speed rail (such as the Tokaido Shinkansen).[2] The power needed for levitation is typically not a large percentage of the overall energy consumption of a high-speed maglev system.[3] Overcoming drag, which makes all land transport more energy intensive at higher speeds, takes the most energy. Vactrain technology has been proposed as a means to overcome this limitation. Maglev systems have been much more expensive to construct than conventional train systems, although the simpler construction of maglev vehicles makes them cheaper to manufacture and maintain.[citation needed]
The Shanghai maglev train, also known as the Shanghai Transrapid, has a top speed of 430 km/h (270 mph). The line is the fastest operational high-speed maglev train, designed to connect Shanghai Pudong International Airport and the outskirts of central Pudong, Shanghai. It covers a distance of 30.5 km (19 mi) in just over 8 minutes. For the first time, the launch generated wide public interest and media attention, propelling the popularity of the mode of transportation.[4] Despite over a century of research and development, maglev transport systems are now operational in just three countries (Japan, South Korea and China).[citation needed] The incremental benefits of maglev technology have often been considered hard to justify against cost and risk, especially where there is an existing or proposed conventional high-speed train line with spare passenger carrying capacity, as in high-speed rail in Europe, the High Speed 2 in the UK and Shinkansen in Japan.
Development[edit]
In the late 1940s, the British electrical engineer Eric Laithwaite, a professor at Imperial College London, developed the first full-size working model of the linear induction motor. He became professor of heavy electrical engineering at Imperial College in 1964, where he continued his successful development of the linear motor.[5] Since linear motors do not require physical contact between the vehicle and guideway, they became a common fixture on advanced transportation systems in the 1960s and '70s. Laithwaite joined one such project, the Tracked Hovercraft, although the project was cancelled in 1973.[6]
The linear motor was naturally suited to use with maglev systems as well. In the early 1970s, Laithwaite discovered a new arrangement of magnets, the magnetic river, that allowed a single linear motor to produce both lift and forward thrust, allowing a maglev system to be built with a single set of magnets. Working at the British Rail Research Division in Derby, along with teams at several civil engineering firms, the "transverse-flux" system was developed into a working system.
The first commercial maglev people mover was simply called "MAGLEV" and officially opened in 1984 near Birmingham, England. It operated on an elevated 600 m (2,000 ft) section of monorail track between Birmingham Airport and Birmingham International railway station, running at speeds up to 42 km/h (26 mph). The system was closed in 1995 due to reliability problems.[7]
History[edit]
First maglev patent[edit]
High-speed transportation patents were granted to various inventors throughout the world.[8] The first relevant patent, U.S. Patent 714,851 (2 December 1902), issued to Albert C. Albertson, used magnetic levitation to take part of the weight off of the wheels while using conventional propulsion.
Early United States patents for a linear motor propelled train were awarded to German inventor Alfred Zehden. The inventor was awarded U.S. Patent 782,312 (14 February 1905) and U.S. Patent RE12700 (21 August 1907). [note 1] In 1907, another early electromagnetic transportation system was developed by F. S. Smith.[9] In 1908, Cleveland mayor Tom L. Johnson filed a patent for a wheel-less "high-speed railway" levitated by an induced magnetic field.[10] Jokingly known as "Greased Lightning," the suspended car operated on a 90-foot test track in Johnson's basement "absolutely noiseless[ly] and without the least vibration."[11] A series of German patents for magnetic levitation trains propelled by linear motors were awarded to Hermann Kemper between 1937 and 1941.[note 2] An early maglev train was described in U.S. Patent 3,158,765, "Magnetic system of transportation", by G. R. Polgreen (25 August 1959). The first use of "maglev" in a United States patent was in "Magnetic levitation guidance system"[12] by Canadian Patents and Development Limited.
Technology
In the public imagination, "maglev" often evokes the concept of an elevated monorail track with a linear motor. Maglev systems may be monorail or dual rail—the SCMaglev MLX01 for instance uses a trench-like track—and not all monorail trains are maglevs. Some railway transport systems incorporate linear motors but use electromagnetism only for propulsion, without levitating the vehicle. Such trains have wheels and are not maglevs.[note 3] Maglev tracks, monorail or not, can also be constructed at grade or underground in tunnels. Conversely, non-maglev tracks, monorail or not, can be elevated or underground too. Some maglev trains do incorporate wheels and function like linear motor-propelled wheeled vehicles at slower speeds but levitate at higher speeds. This is typically the case with electrodynamic suspension maglev trains. Aerodynamic factors may also play a role in the levitation of such trains.
The two main types of maglev technology are:
- Electromagnetic suspension (EMS), electronically controlled electromagnets in the train attract it to a magnetically conductive (usually steel) track.
- Electrodynamic suspension (EDS) uses superconducting electromagnets or strong permanent magnets that create a magnetic field, which induces currents in nearby metallic conductors when there is relative movement, which pushes and pulls the train towards the designed levitation position on the guide way.
Electromagnetic suspension (EMS)[edit]
In electromagnetic suspension (EMS) systems, the train levitates above a steel rail while electromagnets, attached to the train, are oriented toward the rail from below. The system is typically arranged on a series of C-shaped arms, with the upper portion of the arm attached to the vehicle, and the lower inside edge containing the magnets. The rail is situated inside the C, between the upper and lower edges.
Magnetic attraction varies inversely with the square of distance, so minor changes in distance between the magnets and the rail produce greatly varying forces. These changes in force are dynamically unstable—a slight divergence from the optimum position tends to grow, requiring sophisticated feedback systems to maintain a constant distance from the track, (approximately 15 mm [0.59 in]).[61][62]
The major advantage to suspended maglev systems is that they work at all speeds, unlike electrodynamic systems, which only work at a minimum speed of about 30 km/h (19 mph). This eliminates the need for a separate low-speed suspension system, and can simplify track layout. On the downside, the dynamic instability demands fine track tolerances, which can offset this advantage. Eric Laithwaite was concerned that to meet required tolerances, the gap between magnets and rail would have to be increased to the point where the magnets would be unreasonably large.[63] In practice, this problem was addressed through improved feedback systems, which support the required tolerances.
Electrodynamic suspension (EDS)[edit]
In electrodynamic suspension (EDS), both the guideway and the train exert a magnetic field, and the train is levitated by the repulsive and attractive force between these magnetic fields.[64] In some configurations, the train can be levitated only by repulsive force. In the early stages of maglev development at the Miyazaki test track, a purely repulsive system was used instead of the later repulsive and attractive EDS system.[65] The magnetic field is produced either by superconducting magnets (as in JR–Maglev) or by an array of permanent magnets (as in Inductrack). The repulsive and attractive force in the track is created by an induced magnetic field in wires or other conducting strips in the track.
A major advantage of EDS maglev systems is that they are dynamically stable—changes in distance between the track and the magnets creates strong forces to return the system to its original position.[63] In addition, the attractive force varies in the opposite manner, providing the same adjustment effects. No active feedback control is needed.
However, at slow speeds, the current induced in these coils and the resultant magnetic flux is not large enough to levitate the train. For this reason, the train must have wheels or some other form of landing gear to support the train until it reaches take-off speed. Since a train may stop at any location, due to equipment problems for instance, the entire track must be able to support both low- and high-speed operation.
Another downside is that the EDS system naturally creates a field in the track in front and to the rear of the lift magnets, which acts against the magnets and creates magnetic drag. This is generally only a concern at low speeds, and is one of the reasons why JR abandoned a purely repulsive system and adopted the sidewall levitation system.[65] At higher speeds other modes of drag dominate.[63]
The drag force can be used to the electrodynamic system's advantage, however, as it creates a varying force in the rails that can be used as a reactionary system to drive the train, without the need for a separate reaction plate, as in most linear motor systems. Laithwaite led development of such "traverse-flux" systems at his Imperial College laboratory.[63] Alternatively, propulsion coils on the guideway are used to exert a force on the magnets in the train and make the train move forward. The propulsion coils that exert a force on the train are effectively a linear motor: an alternating current through the coils generates a continuously varying magnetic field that moves forward along the track. The frequency of the alternating current is synchronized to match the speed of the train. The offset between the field exerted by magnets on the train and the applied field creates a force moving the train forward.
Tracks[edit]
The term "maglev" refers not only to the vehicles, but to the railway system as well, specifically designed for magnetic levitation and propulsion. All operational implementations of maglev technology make minimal use of wheeled train technology and are not compatible with conventional rail tracks. Because they cannot share existing infrastructure, maglev systems must be designed as standalone systems. The SPM maglev system is inter-operable with steel rail tracks and would permit maglev vehicles and conventional trains to operate on the same tracks.[63] MANin Germany also designed a maglev system that worked with conventional rails, but it was never fully developed.[citation needed]
Evaluation[edit]
Each implementation of the magnetic levitation principle for train-type travel involves advantages and disadvantages.
| Technology | Pros | Cons |
|---|---|---|
| EMS[66][67](Electromagnetic suspension) | Magnetic fields inside and outside the vehicle are less than EDS; proven, commercially available technology; high speeds (500 km/h or 310 mph); no wheels or secondary propulsion system needed. | The separation between the vehicle and the guideway must be constantly monitored and corrected due to the unstable nature of electromagnetic attraction; the system's inherent instability and the required constant corrections by outside systems may induce vibration. |
| EDS[68][69] (Electrodynamic suspension) | Onboard magnets and large margin between rail and train enable highest-recorded speeds (603 km/h or 375 mph) and heavy load capacity; demonstrated successful operations using high-temperature superconductors in its onboard magnets, cooled with inexpensive liquid nitrogen[citation needed]. | Strong magnetic fields on the train would make the train unsafe for passengers with pacemakers or magnetic data storage media such as hard drives and credit cards, necessitating the use of magnetic shielding; limitations on guideway inductivity limit maximum speed;[citation needed] vehicle must be wheeled for travel at low speeds. |
| Inductrack System[70][71](Permanent Magnet Passive Suspension) | Failsafe Suspension—no power required to activate magnets; Magnetic field is localized below the car; can generate enough force at low speeds (around 5 km/h or 3.1 mph) for levitation; given power failure cars stop safely; Halbach arrays of permanent magnets may prove more cost-effective than electromagnets. | Requires either wheels or track segments that move for when the vehicle is stopped. Under development as of 2008; no commercial version or full-scale prototype. |
Neither Inductrack nor the Superconducting EDS are able to levitate vehicles at a standstill, although Inductrack provides levitation at much lower speed; wheels are required for these systems. EMS systems are wheel-free.
The German Transrapid, Japanese HSST (Linimo), and Korean Rotem EMS maglevs levitate at a standstill, with electricity extracted from guideway using power rails for the latter two, and wirelessly for Transrapid. If guideway power is lost on the move, the Transrapid is still able to generate levitation down to 10 km/h (6.2 mph) speed,[citation needed] using the power from onboard batteries. This is not the case with the HSST and Rotem systems.
Propulsion[edit]
EMS systems such as HSST/Linimo can provide both levitation and propulsion using an onboard linear motor. But EDS systems and some EMS systems such as Transrapid levitate but do not propel. Such systems need some other technology for propulsion. A linear motor (propulsion coils) mounted in the track is one solution. Over long distances coil costs could be prohibitive.
Stability[edit]
Earnshaw's theorem shows that no combination of static magnets can be in a stable equilibrium.[72] Therefore a dynamic (time varying) magnetic field is required to achieve stabilization. EMS systems rely on active electronic stabilization that constantly measures the bearing distance and adjusts the electromagnet current accordingly. EDS systems rely on changing magnetic fields to create currents, which can give passive stability.
Because maglev vehicles essentially fly, stabilisation of pitch, roll and yaw is required. In addition to rotation, surge (forward and backward motions), sway (sideways motion) or heave (up and down motions) can be problematic.
Superconducting magnets on a train above a track made out of a permanent magnet lock the train into its lateral position. It can move linearly along the track, but not off the track. This is due to the Meissner effect and flux pinning.
Guidance system[edit]
Some systems use Null Current systems (also sometimes called Null Flux systems).[64][73] These use a coil that is wound so that it enters two opposing, alternating fields, so that the average flux in the loop is zero. When the vehicle is in the straight ahead position, no current flows, but any moves off-line create flux that generates a field that naturally pushes/pulls it back into line.
Proposed technology enhancements[edit]
Evacuated tubes[edit]
Some systems (notably the Swissmetro system) propose the use of vactrains—maglev train technology used in evacuated (airless) tubes, which removes air drag. This has the potential to increase speed and efficiency greatly, as most of the energy for conventional maglev trains is lost to aerodynamic drag.[74]
One potential risk for passengers of trains operating in evacuated tubes is that they could be exposed to the risk of cabin depressurization unless tunnel safety monitoring systems can repressurize the tube in the event of a train malfunction or accident though since trains are likely to operate at or near the Earth's surface, emergency restoration of ambient pressure should be straightforward. The RAND Corporation has depicted a vacuum tube train that could, in theory, cross the Atlantic or the USA in around 21 minutes.[75]
Rail-Maglev Hybrid[edit]
The Polish startup Nevomo (previously Hyper Poland) is developing a system for modifying existing railway tracks into a maglev system, on which conventional wheel-rail trains, as well maglev vehicles can travel.[76] Vehicles on this so-called ‘magrail’ system will be able to reach speeds of up to 300 km/h at significantly lower infrastructure costs than stand-alone maglev lines. Similar to proposed Vactrain systems, magrail is designed to allow a later-stage upgrade with a vacuum cover which will enable vehicles to reach speeds of up to 600 km/h due to reduced air pressure, making the system similar to a hyperloop, but without the necessity for dedicated infrastructure corridors.[77]
Energy use[edit]
Energy for maglev trains is used to accelerate the train. Energy may be regained when the train slows down via regenerative braking. It also levitates and stabilises the train's movement. Most of the energy is needed to overcome air drag. Some energy is used for air conditioning, heating, lighting and other miscellany.
At low speeds the percentage of power used for levitation can be significant, consuming up to 15% more power than a subway or light rail service.[78] For short distances the energy used for acceleration might be considerable.
The force used to overcome air drag increases with the square of the velocity and hence dominates at high speed. The energy needed per unit distance increases by the square of the velocity and the time decreases linearly. However power increases by the cube of the velocity. For example, 2.37 times as much power is needed to travel at 400 km/h (250 mph) than 300 km/h (190 mph), while drag increases by 1.77 times the original force.[79]
Aircraft take advantage of lower air pressure and lower temperatures by cruising at altitude to reduce energy consumption but unlike trains need to carry fuel on board. This has led to the suggestion of conveying maglev vehicles through partially evacuated tubes.
Records[edit]
The highest-recorded maglev speed is 603 km/h (375 mph), achieved in Japan by JR Central's L0 superconducting maglev on 21 April 2015,[94] 28 km/h (17 mph) faster than the conventional TGVwheel-rail speed record. However, the operational and performance differences between these two very different technologies is far greater. The TGV record was achieved accelerating down a 72.4 km (45 mi) slight decline, requiring 13 minutes. It then took another 77.25 km (48 mi) for the TGV to stop, requiring a total distance of 149.65 km (93 mi) for the test.[95] The MLX01 record, however, was achieved on the 18.4 km (11.4 mi) Yamanashi test track – 1/8 the distance.[96] No maglev or wheel-rail commercial operation has actually been attempted at speeds over 500 km/h (310 mph).
History of maglev speed records[edit]
 | This section needs additional citations for verification. (January 2018) |
| Year | Country | Train | Speed | Notes |
|---|---|---|---|---|
| 1971 | West Germany | Prinzipfahrzeug | 90 km/h (56 mph) | |
| 1971 | West Germany | TR-02 (TSST) | 164 km/h (102 mph) | |
| 1972 | Japan | ML100 | 60 km/h (37 mph) | manned |
| 1973 | West Germany | TR04 | 250 km/h (160 mph) | manned |
| 1974 | West Germany | EET-01 | 230 km/h (140 mph) | unmanned |
| 1975 | West Germany | Komet | 401 km/h (249 mph) | by steam rocket propulsion, unmanned |
| 1978 | Japan | HSST-01 | 308 km/h (191 mph) | by supporting rockets propulsion, made in Nissan, unmanned |
| 1978 | Japan | HSST-02 | 110 km/h (68 mph) | manned |
| 1979-12-12 | Japan | ML-500R | 504 km/h (313 mph) | (unmanned) It succeeds in operation over 500 km/h for the first time in the world. |
| 1979-12-21 | Japan | ML-500R | 517 km/h (321 mph) | (unmanned) |
| 1987 | West Germany | TR-06 | 406 km/h (252 mph) | (manned) |
| 1987 | Japan | MLU001 | 401 km/h (249 mph) | (manned) |
| 1988 | West Germany | TR-06 | 413 km/h (257 mph) | (manned) |
| 1989 | West Germany | TR-07 | 436 km/h (271 mph) | (manned) |
| 1993 | Germany | TR-07 | 450 km/h (280 mph) | (manned) |
| 1994 | Japan | MLU002N | 431 km/h (268 mph) | (unmanned) |
| 1997 | Japan | MLX01 | 531 km/h (330 mph) | (manned) |
| 1997 | Japan | MLX01 | 550 km/h (340 mph) | (unmanned) |
| 1999 | Japan | MLX01 | 552 km/h (343 mph) | (manned/five-car formation). Guinness authorization. |
| 2003 | Japan | MLX01 | 581 km/h (361 mph) | (manned/three formation). Guinness authorization.[97] |
| 2015 | Japan | L0 | 590 km/h (370 mph) | (manned/seven-car formation)[98] |
| 2015 | Japan | L0 | 603 km/h (375 mph) | (manned/seven-car formation)[94] |
https://en.wikipedia.org/wiki/Maglev#First_maglev_patent
A linear induction motor (LIM) is an alternating current (AC), asynchronous linear motor that works by the same general principles as other induction motorsbut is typically designed to directly produce motion in a straight line. Characteristically, linear induction motors have a finite primary or secondary length, which generates end-effects, whereas a conventional induction motor is arranged in an endless loop.[1]
Despite their name, not all linear induction motors produce linear motion; some linear induction motors are employed for generating rotations of large diameters where the use of a continuous primary would be very expensive.
As with rotary motors, linear motors frequently run on a three-phase power supply and can support very high speeds. However, there are end-effects that reduce the motor's force, and it is often not possible to fit a gearbox to trade off force and speed. Linear induction motors are thus frequently less energy efficient than normal rotary motors for any given required force output.
LIMs, unlike their rotary counterparts, can give a levitation effect. They are therefore often used where contactless force is required, where low maintenance is desirable, or where the duty cycle is low. Their practical uses include magnetic levitation, linear propulsion, and linear actuators. They have also been used for pumping liquid metals.[2]
https://en.wikipedia.org/wiki/Linear_induction_motor
A linear motor is an electric motor that has had its stator and rotor "unrolled" thus instead of producing a torque (rotation) it produces a linear force along its length. However, linear motors are not necessarily straight. Characteristically, a linear motor's active section has ends, whereas more conventional motors are arranged as a continuous loop.
A typical mode of operation is as a Lorentz-type actuator, in which the applied force is linearly proportional to the current and the magnetic field .
Linear motors are by far most commonly found in high accuracy engineering[1] applications. It is a thriving field of applied research with dedicated scientific conferences[2] and engineering text books.[3]
Many designs have been put forward for linear motors, falling into two major categories, low-acceleration and high-acceleration linear motors. Low-acceleration linear motors are suitable for maglev trains and other ground-based transportation applications. High-acceleration linear motors are normally rather short, and are designed to accelerate an object to a very high speed, for example see the coilgun.
High-acceleration linear motors are typically used in studies of hypervelocity collisions, as weapons, or as mass drivers for spacecraft propulsion.[citation needed] They are usually of the AC linear induction motor (LIM) design with an active three-phase winding on one side of the air-gap and a passive conductor plate on the other side. However, the direct current homopolar linear motor railgun is another high acceleration linear motor design. The low-acceleration, high speed and high power motors are usually of the linear synchronous motor (LSM) design, with an active winding on one side of the air-gap and an array of alternate-pole magnets on the other side. These magnets can be permanent magnets or electromagnets. The motor for the Shanghai maglev train, for instance, is an LSM.
Synchronous[edit]
In this design the rate of movement of the magnetic field is controlled, usually electronically, to track the motion of the rotor. For cost reasons synchronous linear motors rarely use commutators, so the rotor often contains permanent magnets, or soft iron. Examples include coilguns and the motors used on some maglev systems, as well as many other linear motors. In high precision industrial automation linear motors are typically configured with a magnet stator and a moving coil. Hall effect sensor is attached to the rotor to track the magnetic flux of the stator. The electrical current is typically provided from a stationary servo drive to the moving coil by a moving cable inside a cable carrier.
Induction[edit]
In this design, the force is produced by a moving linear magnetic field acting on conductors in the field. Any conductor, be it a loop, a coil or simply a piece of plate metal, that is placed in this field will have eddy currents induced in it thus creating an opposing magnetic field, in accordance with Lenz's law.[6] The two opposing fields will repel each other, thus creating motion as the magnetic field sweeps through the metal.
Homopolar[edit]
In this design a large current is passed through a metal sabot across sliding contacts that are fed from two rails. The magnetic field this generates causes the metal to be projected along the rails.
Piezo electric[edit]
Piezoelectric drive is often used to drive small linear motors.
Usage[edit]
Linear motors are commonly used for actuating high performance industrial automation equipment. Their advantage, unlike any other commonly used actuator, such as ball screw, timing belt, or rack and pinion, is that they provide any combination of high precision, high velocity, high force and long travel.
Linear motors are widely used. One of the major uses of linear motors is for propelling the shuttle in looms.
Linear motors have been used for sliding doors and various similar actuators. Also, they have been used for baggage handing and even large-scale bulk materials transport.
Linear motors are sometimes used to create rotary motion, for example, they have been used at observatories to deal with the large radius of curvature.
Linear motors may also be used as an alternative to conventional chain-run lift hills for roller coasters. The coaster Maverick at Cedar Point uses one such linear motor in place of a chain lift.
A linear motor has been used for accelerating cars for crash tests.[11]
Industrial automation[edit]
The combination of high precision, high velocity, high force, and long travel makes brushless linear motors attractive for driving industrial automations equipment. They serve industries and applications such as semiconductor steppers, electronics surface-mount technology, automotive cartesian coordinate robots, aerospace chemical milling, optics electron microscope, healthcare laboratory automation, food and beverage pick and place.
https://en.wikipedia.org/wiki/Linear_motor
https://en.wikipedia.org/wiki/Regenerative_brake
https://en.wikipedia.org/wiki/Vactrain
https://en.wikipedia.org/wiki/Linear_motor
https://en.wikipedia.org/wiki/Atmospheric_railway
https://en.wikipedia.org/wiki/StarTram
https://en.wikipedia.org/wiki/Category:Magnetic_propulsion_devices
https://en.wikipedia.org/wiki/Category:Electrodynamics
https://en.wikipedia.org/wiki/Halbach_array
https://en.wikipedia.org/wiki/Earnshaw%27s_theorem
https://en.wikipedia.org/wiki/Drag_(physics)
https://en.wikipedia.org/wiki/Linear_stage
https://en.wikipedia.org/wiki/Electromagnetic_Aircraft_Launch_System
https://en.wikipedia.org/wiki/Aircraft_catapult#Steam_catapult
https://en.wikipedia.org/wiki/Spacecraft_propulsion
https://en.wikipedia.org/wiki/Escape_velocity
https://en.wikipedia.org/wiki/Electric_generator
https://en.wikipedia.org/wiki/Flywheel
https://en.wikipedia.org/wiki/Spindle_(textiles)
https://en.wikipedia.org/wiki/Flywheel_energy_storage
https://en.wikipedia.org/wiki/Rotational_energy
https://en.wikipedia.org/wiki/Rotational_speed
https://en.wikipedia.org/wiki/Inductor
https://en.wikipedia.org/wiki/Accumulator_(energy)
https://en.wikipedia.org/wiki/Reciprocating_engine
https://en.wikipedia.org/wiki/Reaction_wheel
https://en.wikipedia.org/wiki/Gyroscope
Disc-type generator[edit]
This device consists of a conducting flywheel rotating in a magnetic field with one electrical contact near the axis and the other near the periphery. It has been used for generating very high currents at low voltages in applications such as welding, electrolysis and railgun research. In pulsed energy applications, the angular momentum of the rotor is used to accumulate energy over a long period and then release it in a short time.
In contrast to other types of generators, the output voltage never changes polarity. The charge separation results from the Lorentz force on the free charges in the disk. The motion is azimuthal and the field is axial, so the electromotive force is radial. The electrical contacts are usually made through a "brush" or slip ring, which results in large losses at the low voltages generated. Some of these losses can be reduced by using mercury or other easily liquefied metal or alloy (gallium, NaK) as the "brush", to provide essentially uninterrupted electrical contact.
A recent suggested modification is to use a plasma contact supplied by a negative resistance neon streamer touching the edge of the disk or drum, using specialized low work function carbon in vertical strips. This would have the advantage of very low resistance within a current range possibly up to thousands of amps without the liquid metal contact.[citation needed]
If the magnetic field is provided by a permanent magnet, the generator works regardless of whether the magnet is fixed to the stator or rotates with the disc. Before the discovery of the electron and the Lorentz force law, the phenomenon was inexplicable and was known as the Faraday paradox.
Drum-type generator[edit]
A drum-type homopolar generator has a magnetic field (B) that radiates radially from the center of the drum and induces voltage (V) down the length of the drum. A conducting drum spun from above in the field of a "loudspeaker" type of magnet that has one pole in the center of the drum and the other pole surrounding the drum could use conducting ball bearings at the top and bottom of the drum to pick up the generated current.
Astrophysical unipolar inductors[edit]
Unipolar inductors occur in astrophysics where a conductor rotates through a magnetic field, for example, the movement of the highly conductive plasma in a cosmic body's ionosphere through its magnetic field. In their book, Cosmical Electrodynamics, Hannes Alfvén and Carl-Gunne Fälthammar write:
- "Since cosmical clouds of ionized gas are generally magnetized, their motion produces induced electric fields [..] For example the motion of the magnetized interplanetary plasma produces electric fields that are essential for the production of aurora and magnetic storms" [..]
- ".. the rotation of a conductor in a magnetic field produces an electric field in the system at rest. This phenomenon is well known from laboratory experiments and is usually called 'homopolar ' or 'unipolar' induction.[6]
Unipolar inductors have been associated with the aurorae on Uranus,[7] binary stars,[8][9] black holes,[10][11][12] galaxies,[13] the Jupiter Io system,[14][15] the Moon,[16][17] the Solar Wind,[18]sunspots,[19][20] and in the Venusian magnetic tail.[21]
Physics[edit]
Like all dynamos, the Faraday disc converts kinetic energy to electrical energy. This machine can be analysed using Faraday's own law of electromagnetic induction. This law, in its modern form, states that the full-time derivative of the magnetic flux through a closed circuit induces an electromotive force in the circuit, which in turn drives an electric current. The surface integral that defines the magnetic flux can be rewritten as a line integral around the circuit. Although the integrand of the line integral is time-independent, because the Faraday disc that forms part of the boundary of line integral is moving, the full-time derivative is non-zero and returns the correct value for calculating the electromotive force.[22][23] Alternatively, the disc can be reduced to a conductive ring along the disc's circumference with a single metal spoke connecting the ring to the axle.[24]
The Lorentz force law is more easily used to explain the machine's behaviour. This law, formulated thirty years after Faraday's death, states that the force on an electron is proportional to the cross product of its velocity and the magnetic flux vector. In geometrical terms, this means that the force is at right-angles to both the velocity (azimuthal) and the magnetic flux (axial), which is therefore in a radial direction. The radial movement of the electrons in the disc produces a charge separation between the center of the disc and its rim, and if the circuit is completed an electric current will be produced.[25]
https://en.wikipedia.org/wiki/Homopolar_generator
https://en.wikipedia.org/wiki/Electrolysis
https://en.wikipedia.org/wiki/Angular_momentum
https://en.wikipedia.org/wiki/Spectrum_(functional_analysis)#Point_spectrum
https://en.wikipedia.org/wiki/Synchrotron
https://en.wikipedia.org/wiki/Coilgun
https://en.wikipedia.org/wiki/Carl_Friedrich_Gauss
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