Linear motor

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Free-body diagram of a U-channel synchronous linear motor. The view is perpendicular to the channel axis. The two coils at centre are mechanically connected, and are energized in "quadrature" (meaning a phase difference of 90deg (p/2 radians) between the flux of the magnets and the flux of the coils). The bottom and upper coils in this particular case have a phase difference of 90deg, making this a two phase motor. (Not to scale) Linear motor U-tube.svg
Free-body diagram of a U-channel synchronous linear motor. The view is perpendicular to the channel axis. The two coils at centre are mechanically connected, and are energized in "quadrature" (meaning a phase difference of 90° (π/2 radians) between the flux of the magnets and the flux of the coils). The bottom and upper coils in this particular case have a phase difference of 90°, making this a two phase motor. (Not to scale)
Synchronous linear motors are straightened versions of permanent magnet rotor motors Linearmotorprinzip.png
Synchronous linear motors are straightened versions of permanent magnet rotor motors

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.

Contents

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.

Types

Brushless

Brushless linear motors are members of the Synchronous motor family. They are typically used in standard linear stages or integrated into custom, high performance positioning systems. Invented in late 1980s by Anwar Chitayat at Anorad Corporation, now Rockwell Automation, and helped improving throughput and quality of industrial manufacturing processes. [4]

Brush

Brush (electric) linear motors were used in industrial automation applications prior to the invention of Brushless linear motors. Compared with three phase Brushless motors, which are typically being used today, brush motors operates with a single phase. [5] Brush linear motors have lower cost since they do not need moving cables and three phase servo drives. However, they require higher maintenance since their brushes wear out.

Synchronous

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

A typical 3 phase linear induction motor. An aluminium plate on top often forms the secondary "rotor". Three phase linear induction motor.gif
A typical 3 phase linear induction motor. An aluminium plate on top often forms the secondary "rotor".

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

Railgun schematic Railgun-1.svg
Railgun schematic

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

Piezoelectric motor action Piezomotor type inchworm.gif
Piezoelectric motor action

Piezoelectric drive is often used to drive small linear motors.

History

This Line 6 Guangzhou Metro train manufactured by CRRC Sifang and Kawasaki Heavy Industries propels itself using an aluminium induction strip placed between the rails. Running on the Xunfeng Gang To Hengsha section of Guangzhou Metro Line 6.jpg
This Line 6 Guangzhou Metro train manufactured by CRRC Sifang and Kawasaki Heavy Industries propels itself using an aluminium induction strip placed between the rails.

Low acceleration

The history of linear electric motors can be traced back at least as far as the 1840s, to the work of Charles Wheatstone at King's College London, [7] but Wheatstone's model was too inefficient to be practical. A feasible linear induction motor is described in the U.S. Patent 782,312 (1905 - inventor Alfred Zehden of Frankfurt-am-Main), for driving trains or lifts. The German engineer Hermann Kemper built a working model in 1935. [8] In the late 1940s, Dr. Eric Laithwaite of Manchester University, later Professor of Heavy Electrical Engineering at Imperial College in London developed the first full-size working model. In a single sided version the magnetic repulsion forces the conductor away from the stator, levitating it, and carrying it along in the direction of the moving magnetic field. He called the later versions of it magnetic river.

A linear motor for trains running Toei Oedo Line Linear Motor of Toei Oedo Line.jpg
A linear motor for trains running Toei Ōedo Line

Because of these properties, linear motors are often used in maglev propulsion, as in the Japanese Linimo magnetic levitation train line near Nagoya. However, linear motors have been used independently of magnetic levitation, as in the Bombardier Advanced Rapid Transit systems worldwide and a number of modern Japanese subways, including Tokyo's Toei Ōedo Line.

Similar technology is also used in some roller coasters with modifications but, at present, is still impractical on street running trams, although this, in theory, could be done by burying it in a slotted conduit.

Outside of public transportation, vertical linear motors have been proposed as lifting mechanisms in deep mines, and the use of linear motors is growing in motion control applications. They are also often used on sliding doors, such as those of low floor trams such as the Alstom Citadis and the Socimi Eurotram. Dual axis linear motors also exist. These specialized devices have been used to provide direct X-Y motion for precision laser cutting of cloth and sheet metal, automated drafting, and cable forming. Most linear motors in use are LIM (linear induction motor), or LSM (linear synchronous motor). Linear DC motors are not used due to higher cost and linear SRM suffers from poor thrust. So for long run in traction LIM is mostly preferred and for short run LSM is mostly preferred.

Close-up of the flat passive conductor surface of a motion control Sawyer motor Linear motor platen surface.jpg
Close-up of the flat passive conductor surface of a motion control Sawyer motor

High acceleration

High-acceleration linear motors have been suggested for a number of uses. They have been considered for use as weapons, since current armour-piercing ammunition tends to consist of small rounds with very high kinetic energy, for which just such motors are suitable. Many amusement park launched roller coasters now use linear induction motors to propel the train at a high speed, as an alternative to using a lift hill. The United States Navy is also using linear induction motors in the Electromagnetic Aircraft Launch System that will replace traditional steam catapults on future aircraft carriers. They have also been suggested for use in spacecraft propulsion. In this context they are usually called mass drivers. The simplest way to use mass drivers for spacecraft propulsion would be to build a large mass driver that can accelerate cargo up to escape velocity, though RLV launch assist like StarTram to low earth orbit has also been investigated.

High-acceleration linear motors are difficult to design for a number of reasons. They require large amounts of energy in very short periods of time. One rocket launcher design [9] calls for 300 GJ for each launch in the space of less than a second. Normal electrical generators are not designed for this kind of load, but short-term electrical energy storage methods can be used. Capacitors are bulky and expensive but can supply large amounts of energy quickly. Homopolar generators can be used to convert the kinetic energy of a flywheel into electric energy very rapidly. High-acceleration linear motors also require very strong magnetic fields; in fact, the magnetic fields are often too strong to permit the use of superconductors. However, with careful design, this need not be a major problem. [10]

Two different basic designs have been invented for high-acceleration linear motors: railguns and coilguns.

Usage

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

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.

Machine tools

Synchronous linear motor actuators, used in machine tools, provide high force, high velocity, high precision and high dynamic stiffness, resulting in high smoothness of motion and low settling time. They may reach velocities of 2 m/s and micron-level accuracies, at short cycle times and a smooth surface finish. [12]

Train propulsion

Conventional rails

All of the following applications are in rapid transit and have the active part of the motor in the cars. [13] [14]

Bombardier Innovia Metro

Originally developed in the late 1970s by UTDC in Canada as the Intermediate Capacity Transit System (ICTS). A test track was constructed in Millhaven, Ontario, for extensive testing of prototype cars, after which three lines were constructed:

ICTS was sold to Bombardier Transportation in 1991 and later known as Advanced Rapid Transit (ART) before adopting its current branding in 2011. Since then, several more installations have been made:

All Innovia Metro systems use third rail electrification.

Japanese Linear Metro

One of the biggest challenges faced by Japanese railway engineers in the 1970s to the 1980s was the ever increasing construction costs of subways. In response, the Japan Subway Association began studying on the feasibility of the "mini-metro" for meeting urban traffic demand in 1979. In 1981, the Japan Railway Engineering Association studied on the use of linear induction motors for such small-profile subways and by 1984 was investigating on the practical applications of linear motors for urban rail with the Japanese Ministry of Land, Infrastructure, Transport and Tourism. In 1988, a successful demonstration was made with the Limtrain at Saitama and influenced the eventual adoption of the linear motor for the Nagahori Tsurumi-ryokuchi Line in Osaka and Toei Line 12 (present-day Toei Oedo Line) in Tokyo. [16]

To date, the following subway lines in Japan use linear motors and use overhead lines for power collection:

In addition, Kawasaki Heavy Industries has also exported the Linear Metro to the Guangzhou Metro in China; [17] all of the Linear Metro lines in Guangzhou use third rail electrification:

Monorail

  • There is at least one known monorail system which is not magnetically levitated, but nonetheless uses linear motors. This is the Moscow Monorail. Originally, traditional motors and wheels were to be used. However, it was discovered during test runs that the proposed motors and wheels would fail to provide adequate traction under some conditions, for example, when ice appeared on the rail. Hence, wheels are still used, but the trains use linear motors to accelerate and slow down. This is possibly the only use of such a combination, due to the lack of such requirements for other train systems.
  • The TELMAGV is a prototype of a monorail system that is also not magnetically levitated but uses linear motors.

Magnetic levitation

The Birmingham International Maglev shuttle Birmingham International Maglev.jpg
The Birmingham International Maglev shuttle

Amusement rides

There are many roller coasters throughout the world that use LIMs to accelerate the ride vehicles. The first being Flight of Fear at Kings Island and Kings Dominion, both opening in 1996. Battlestar Galctica: Human VS Cylon & Revenge of the Mummy at Universal Studios Singapore opened in 2010. They both use LIMs to accelerate from certain point in the rides. Revenge of the Mummy also is located at Universal Studios Hollywood and Universal Studios Florida.The Incredible Hulk Coaster and VelociCoaster at Universal's Islands of Adventure also uses linear motors. Rock 'n' Roller Coaster Starring Aerosmith was opened in 1999 in Disney's Hollywood Studios that uses an LSM to launch the vehicles into the indoor, blacklight ride enclosure.

Aircraft launching

Proposed and research

See also

Related Research Articles

Electric motor Machine that converts electrical energy into mechanical energy

An electric motor is an electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a wire winding to generate force in the form of torque applied on the motor's shaft. Electric motors can be powered by direct current (DC) sources, such as from batteries, or rectifiers, or by alternating current (AC) sources, such as a power grid, inverters or electrical generators. An electric generator is mechanically identical to an electric motor, but operates with a reversed flow of power, converting mechanical energy into electrical energy.

Transrapid German developed high-speed monorail train

Transrapid is a German-developed high-speed monorail train using magnetic levitation. Planning for the Transrapid system started in 1969 with a test facility for the system in Emsland, Germany completed in 1987. In 1991 technical readiness for application was approved by the Deutsche Bundesbahn in cooperation with renowned universities.

Coilgun Artillery using coils to electromagnetically propel a projectile

A coilgun, also known as a Gauss rifle, is a type of mass driver consisting of one or more coils used as electromagnets in the configuration of a linear motor that accelerate a ferromagnetic or conducting projectile to high velocity. In almost all coilgun configurations, the coils and the gun barrel are arranged on a common axis. A coilgun is not a rifle as the barrel is smoothbore. The name "Gauss" is in reference to Carl Friedrich Gauss, who formulated mathematical descriptions of the magnetic effect used by magnetic accelerator cannons.

Linear induction motor

A linear induction motor (LIM) is an alternating current (AC), asynchronous linear motor that works by the same general principles as other induction motors but 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.

Synchronous motor Type of AC motor

A synchronous electric motor is an AC motor in which, at steady state, the rotation of the shaft is synchronized with the frequency of the supply current; the rotation period is exactly equal to an integral number of AC cycles. Synchronous motors contain multiphase AC electromagnets on the stator of the motor that create a magnetic field which rotates in time with the oscillations of the line current. The rotor with permanent magnets or electromagnets turns in step with the stator field at the same rate and as a result, provides the second synchronized rotating magnet field of any AC motor. A synchronous motor is termed doubly fed if it is supplied with independently excited multiphase AC electromagnets on both the rotor and stator.

Bombardier Innovia Metro

Innovia Metro is an automated rapid transit system manufactured by Bombardier Transportation. Innovia Metro systems run on conventional metal rails and pull power from a third rail, but are powered by a linear induction motor that provides traction by pulling on a "fourth rail" placed between the running rails. A new version of the technology being marketed by Bombardier is compatible with standard electric rotary propulsion.

Electrodynamic suspension Magnetic levitation by time-varying fields

Electrodynamic suspension (EDS) is a form of magnetic levitation in which there are conductors which are exposed to time-varying magnetic fields. This induces eddy currents in the conductors that creates a repulsive magnetic field which holds the two objects apart.

Electromagnetic propulsion (EMP) is the principle of accelerating an object by the utilization of a flowing electrical current and magnetic fields. The electrical current is used to either create an opposing magnetic field, or to charge a field, which can then be repelled. When a current flows through a conductor in a magnetic field, an electromagnetic force known as a Lorentz force, pushes the conductor in a direction perpendicular to the conductor and the magnetic field. This repulsing force is what causes propulsion in a system designed to take advantage of the phenomenon. The term electromagnetic propulsion (EMP) can be described by its individual components: electromagnetic – using electricity to create a magnetic field, and propulsion – the process of propelling something. When a fluid is employed as the moving conductor, the propulsion may be termed magnetohydrodynamic drive. One key difference between EMP and propulsion achieved by electric motors is that the electrical energy used for EMP is not used to produce rotational energy for motion; though both use magnetic fields and a flowing electrical current.

Brushless DC electric motor Synchronous electric motor powered by an inverter

A brushless DC electric motor, also known as an electronically commutated motor or synchronous DC motor, is a synchronous motor using a direct current (DC) electric power supply. It uses an electronic closed loop controller to switch DC currents to the motor windings producing magnetic fields which effectively rotate in space and which the permanent magnet rotor follows. The controller adjusts the phase and amplitude of the DC current pulses to control the speed and torque of the motor. This control system is an alternative to the mechanical commutator (brushes) used in many conventional electric motors.

Maglev Train system using magnetic levitation

Maglev 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, maglev can compete favourably with high-speed rail and airplanes.

Launched roller coaster Modern form of roller coaster

The launched roller coaster is a modern form of roller coaster. A launched coaster initiates a ride with high amounts of acceleration via one or a series of linear induction motors (LIM), linear synchronous motors (LSM), catapults, tires, chains, or other mechanisms employing hydraulic or pneumatic power. This mode of acceleration powers many of the fastest rollercoasters in the world.

Linear actuator Actuator that creates motion in a straight line

A linear actuator is an actuator that creates motion in a straight line, in contrast to the circular motion of a conventional electric motor. Linear actuators are used in machine tools and industrial machinery, in computer peripherals such as disk drives and printers, in valves and dampers, and in many other places where linear motion is required. Hydraulic or pneumatic cylinders inherently produce linear motion. Many other mechanisms are used to generate linear motion from a rotating motor.

Electromagnetic suspension

Electromagnetic suspension (EMS) is the magnetic levitation of an object achieved by constantly altering the strength of a magnetic field produced by electromagnets using a feedback loop. In most cases the levitation effect is mostly due to permanent magnets as they don't have any power dissipation, with electromagnets only used to stabilize the effect.

SCMaglev Japanese maglev system

The SCMaglev is a magnetic levitation (maglev) railway system developed by Central Japan Railway Company and the Railway Technical Research Institute.

Linimo

Linimo, formally the Aichi High-Speed Transit Tobu Kyuryo Line is a magnetic levitation train line in Aichi Prefecture, Japan, near the city of Nagoya. While primarily built to serve the Expo 2005 fair site, the line now operates to serve the local community.

AC motor Electric motor driven by an AC electrical input

An AC motor is an electric motor driven by an alternating current (AC). The AC motor commonly consists of two basic parts, an outside stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft producing a second rotating magnetic field. The rotor magnetic field may be produced by permanent magnets, reluctance saliency, or DC or AC electrical windings.

skyTran

skyTran is a personal rapid transit system concept first proposed by inventor Douglas Malewicki in 1990, and under development by Unimodal Inc. A prototype of the skyTran vehicle and a section of track have been constructed. The early magnetic levitation system, Inductrack, which SkyTran has replaced with a similar proprietary design, has been tested by General Atomics with a full-scale model. In 2010, Unimodal signed an agreement with NASA to test and develop skyTran. skyTran has proposed additional projects in France, Germany, India, Indonesia, Malaysia, the United Kingdom, and the United States.

Incheon Airport Maglev

The Incheon Airport Maglev is a maglev line in South Korea opened on February 3, 2016. It is the world's second commercially operating unmanned urban maglev line after Japan's Linimo. The trains are lighter, cutting construction costs in half. The majority of construction was completed by November 2012.

Magnetic levitation Method by which an object is suspended with no support other than magnetic fields

Magnetic levitation (maglev) or magnetic suspension is a method by which an object is suspended with no support other than magnetic fields. Magnetic force is used to counteract the effects of the gravitational force and any other forces.

High performance positioning system

A high performance positioning system (HPPS) is a type of positioning system consisting of a piece of electromechanics equipment that is capable of moving an object in a three-dimensional space within a work envelope. Positioning could be done point to point or along a desired path of motion. Position is typically defined in six degrees of freedom, including linear, in an x,y,z cartesian coordinate system, and angular orientation of yaw, pitch, roll. HPPS are used in many manufacturing processes to move an object smoothly and accurately in six degrees of freedom, along a desired path, at a desired orientation, with high acceleration, high deceleration, high velocity and low settling time. It is designed to quickly stop its motion and accurately place the moving object at its desired final position and orientation with minimal jittering.

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