The linear induction motor ( LIM ) is an alternating current (AC), asynchronous linear motor acting on the same general principle as any other induction motor but usually designed to produce directly motion in a straight line. Characteristically, a linear induction motor has a limited primary or secondary length, which produces a final effect, whereas a conventional induction motor is arranged in an endless circle.
Regardless of their name, not all linear induction motors produce linear motion; some linear induction motors are used to produce large diameter rotations where continuous primary use will be very expensive.
Like rotary motors, linear motors often run on three phase power supplies and can support extremely high speeds. However, there is a final effect that reduces motor force, and it is often impossible to install a gearbox to swap power and speed. The linear induction motor is thus often less energy efficient than the normal rotary motor for any required force output.
LIMITS, unlike their rotary counterparts, can give effect to levitation. It is therefore often used where contactless strength is required, where low maintenance is desired, or where the work cycle is low. Their practical uses include magnetic levitation, linear propulsion, and linear actuators. They have also been used for pumping molten metal.
Video Linear induction motor
History
The history of linear electric motors can be traced back at least as far back as the 1840s to the work of Charles Wheatstone at King's College in London, but the Wheatstone model is too inefficient to be practical. A decent linear induction motor is described in US patent 782312 (1905; inventor Alfred Zehden of Frankfurt-am-Main), and for riding a train or an elevator. German engineer Hermann Kemper built a working model in 1935. In the late 1940s, professor Eric Laithwaite from Imperial College in London developed the first full-size work model.
In one-sided versions, the magnetic field can create a force of repulsion that pushes the conductor away from the stator, dilates it and carries it along the direction of the moving magnetic field. Laithwaite calls these later versions a magnetic river. This version of the linear induction motor uses a principle called transversal flux in which two opposite poles are placed side by side. This allows very long poles to be used, and thus enables high speed and efficiency.
Maps Linear induction motor
Construction
A primary linear electric motor typically consists of a flat magnetic core (generally laminated) with a transverse slot which is often straight cut with a coil placed into the slot, with each phase providing alternating polarity so that different phases are physically overlapping.
The secondary is often an aluminum sheet, often with an iron support plate. Some LIMs have two sides with one primer on each secondary side, and, in this case, no iron support is required.
Two types of linear motors exist: a short prime , in which the coil is cut shorter than the secondary, and the secondary short , where the conductive plate is smaller. Shorter secondary LIMs are often wrapped as parallel connections between coils on the same phase, while short preliminaries are usually wrapped in series.
The introduction of the flux of transverse flux has a series of twin poles lying transversely side-by-side with opposite winding direction. These poles are usually made with a suitable laminated retaining plate or a series of cross-U-cores.
Principles
In the design of this electric motor, the force is generated by a linearly moving magnetic field acting on a conductor in the field. Any conductor, whether it is a loop, a coil, or just a piece of metal plate, placed in this field will have an eddy current induced in it thus creating an opposing magnetic field in accordance with Lenz's law. The two opposing fields will repel each other, creating a movement when the magnetic field sweeps the metal.
where f s is the supply frequency in Hz, p is the number of poles, and n s is the sync speed of the magnetic field in the second revolution.
Polish medan perjalanan memilize kecepat:
where v s is the linear velocity of the plane traveled in m/s, and t is the pole pitch.
Untuk slip s , kecepatan sekunder dalam motor linear diberikan oleh
Pasukan
Thrust
Drives generated by linear induction motors are somewhat similar to conventional induction motors; the driving force exhibits a characteristic form almost identical to the slip, although modulated by the final effect.
The equation exists to calculate the motor's thrust.
End effect
Unlike circular induction motors, linear induction motors exhibit a 'final effect'. These final effects include losses in performance and efficiency that are believed to be caused by magnetic energy being carried and lost at the primary end by the relative movement of primary and secondary.
With a short secondary, the behavior is almost identical to the rotary engine, provided at least two long poles but with a short primary reduction in the thrust that occurs at low slip (below about 0.3) to eight poles or longer.
However, due to the final effect, linear motors can not 'run light' - normal induction motors can run motors with near sync field under low load conditions. In contrast, the final effect creates a much more significant loss with linear motors.
Levitation
In addition, unlike rotary motors, electrodynamic levitation forces are indicated, these are zero at zero slip, and provide a constant amount of force/slot as the slip increases in both directions. This happens on a one-sided motor, and levitation usually will not happen when the iron support plates are used on the secondary, as this causes the attraction that dominates the lifting force.
Performance
Linear induction motors are often less efficient than conventional induction rotating motors; the relatively large end effect and relatively large air gap usually will reduce the resulting force for the same electrical power. Similarly, efficiency during generator operation (electrical braking/recovering) with linear induction motors is reported relatively low due to the final effect. The larger air gap also increases motor inductance which can require larger and more expensive capacitors.
However, linear induction motors can avoid the need for gearboxes and similar drivetrains, and these have their own disadvantages; and knowledge working on the importance of the good factor can minimize the effects of a larger air gap. In any case the use of electricity is not always the most important consideration. For example, in many cases linear induction motors have far fewer moving parts, and have very low maintenance. Also, using a linear induction motor instead of a rotating motor with a rotary-to-linear transmission in a motion control system, allows for higher bandwidth and control system accuracy, since rotary-to-linear transmission introduces backlash, static friction and/or mechanical compliance in the system control.
Usage
Because of these properties, linear motors are often used in maglev propulsion, as in the magnetic Linimo railway line near Nagoya.
The world's first commercial automatic maglev system is a low-speed maglev shuttle that runs from the Birmingham International Airport terminal to Birmingham International railway station between 1984-1995. The length of the track is 600 meters (2,000 feet), and the train "fly" at a height of 15 millimeters (0.59 inches), lifted by an electromagnet, and driven by a linear induction motor. It operated for almost eleven years, but the obsolete problems with electronic systems made it unreliable in subsequent years. One of the original cars is now on display at Railworld in Peterborough, along with a floating RTV31 train vehicle.
However, linear motors have been used separately from magnetic levitation, such as the Toei Tokyo Line edo. Bombardier's Innovia Metro is an example of an automated system that utilizes LIM propulsion; the longest fast transit system using such technology is the Vancouver SkyTrain, with about 60 km (37 mi) of tracks compatible with the Innovia Metro train.
Linear induction motor technology is also used in some roller coasters that are launched. It's still not practical for street-trams, although this, in theory, can be done by burying it in a hollow channel.
Beyond public transport, vertical linear motors have been proposed as lifting mechanisms in deep mines, and the use of linear motors grows in motion control applications. They are also often used on sliding doors, such as low floor trams like Citadis and Eurotram.
A double-axis linear motor also exists. This special tool has been used to provide direct X-motion for precision laser cutting of fabric and sheet metal, automatic drafting, and cable formation. Also, a linear induction motor with a secondary cylinder has been used to provide linear motion and rotate simultaneously for the installation of electronic devices on printed circuit boards.
Most linear motors used are LIM (linear induction motor) or LSM (linear sync motor). Linear DC motors are not used because they include more costs and the linear SRM suffers from poor thrust. So for the long term in traction, LIM is preferred and for short-term NGOs is preferred.
The linear induction motor has also been used to launch the aircraft, the Westinghouse Electropult system in 1945 is an early example and the Electromagnetic Aircraft Launch System (EMALS) is scheduled to be delivered in 2010.
Linear induction motors are also used in looms, magnetic levitation allows bobbins to float between fibers without direct contact.
The first ropeless elevator created by ThyssenKrupp uses linear induced driving power.
See also
- Goodness factor
- Maglev
- Tracked Hovercraft
References
Source of the article : Wikipedia
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