What are linear motors?

Abstract

A description about Linear motors; history, how they work, etc.

Full text

What are linear motors?
Linear motors are electric induction motors that produce motion in a
straight line rather than rotational motion. In a traditional electric
motor, the rotor (rotating part) spins inside the stator (static part); in a
linear motor, the stator is unwrapped and laid out flat and the "rotor"
moves past it in a straight line. Linear motors often use
superconducting magnets, which are cooled to low temperatures to
reduce power consumption.
By Ghaseminejad, M Eng
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
Top: Normal motor: The rotor
spins inside the stator and the
whole motor is fixed in place.

Bottom: A linear motor is like a
normal electric motor that has
been unwrapped and laid in a
straight line. Now the rotor
moves past the stator as it turns.
By Ghaseminejad, M Eng
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The basic principle behind the linear motor was discovered in 1895, but practical
devices were not developed until 1947. During the 1950s, British electrical
engineerEric Laithwaite started to consider whether linear motors could be used in
electric weaving machines. Laithwaite's research at Imperial College, London attracted
international recognition in the 1960s following a speech to the Royal Institution
entitled "Electrical Machines of the Future."
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Picture courtesy of NASA Marshall Space
Flight Center (NASA-MSFC)
NASA tests a linear motor on a
prototype Maglev railroad, 1999.
Tracks like this could be used to
launch vehicles into space in future.
According to NASA: "A full-scale,
operational track would be about
1.5-miles long and capable of
accelerating a vehicle to 600 mph in
9.5 seconds."
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Linear motors are now used in all sorts of machines that require linear (as
opposed to rotational) motion, including overhead traveling cranes and beltless
conveyors for moving sheet metal. They are probably best known as the source
of motive power in the latest generation of high-speed "maglev" (magnetic
levitation) trains, which promise safe travel at very high speeds but are
expensive and incompatible with existing railroads. Most research on maglev
trains has been carried out in Japan and Germany.
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In the 1960s, Eric Laithwaite's research into linear motors led to renewed
interest in the idea of a magnetically levitated or "maglev" train. Around this
time MIT scientist Henry Kolm proposed a "magnaplane" running on rails
that could carry 20,000 people at 200 mph (320 kph). This prompted a US
research program and led to a working prototype that was tested in Colorado
in 1967. However, the US program ran into political difficulties and was
shelved in 1975. The early 1990s brought an ambitious proposal to link Las
Vegas, Los Angeles, San Diego, and San Fransisco with a maglev railroad, but
that project has since run into more political problems.
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By contrast, maglev has been enthusiastically developed by Germany
and Japan. German engineers first produced a working prototype in
1971 and developed the Transrapid system a year later. With
considerable support from the German government, this has been
progressively refined into a viable train that has been tested at speeds
of up to 271 mph (433 kph). Strictly speaking, the Transrapid uses
magnetic attraction rather than the magnetic repulsion normally
associated with maglev: the copper magnets are fixed to a "skirt" that
runs underneath, and is attracted up toward, the steel track.
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The Japanese have been even bolder and
plan to develop a high-speed maglev train
that can travel the 320 miles (515 km) from
Tokyo to Osaka in just one hour. Unlike the
German Transrapid system, the Japanese
system is genuine maglev: the train floats on
the repulsive force between the copper or
aluminum coils in the track and a series of
helium-cooled, niobium-titanium
superconducting magnets in the cars. The
Japanese prototype ML-500 train achieved a
train speed record of 321 mph (513 kph) in
1979. A later prototype known as the
MLU002 was destroyed by fire in 1991; a
firefighter apparently found his axe pulled
from his hand by one of the superconducting
magnets as he approached the burning train!
A Maglev train using linear motor technology.
Picture courtesy of US Department of
Energy/Argonne National Laboratory
Although maglev technology continues to generate a
great deal of interest around the world, it is still
more expensive mile-for-mile than building a
traditional high-speed railroad. For this reason (and
also because it is completely incompatible with
existing railroads), it is unlikely to be widely used
for some years.
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Eric Laithwaite
Eric Roberts Laithwaite (14 June 1921 27 November 1997) was a British electrical engineer,
known as the "Father of Maglev" for his development of the linear induction motor and maglev
rail system.
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Eric Roberts Laithwaite was born in Atherton, Lancashire on 14 June 1921, raised in the Fylde, Lancashire and
educated at Kirkham Grammar School. He joined the Royal Air Force in 1941. Through his service in World
War II he rose to the rank of Flying Officer, becoming a test engineer for autopilot technology at the Royal
Aircraft Establishment in Farnborough.
On demobilization in 1946, he attended the University of Manchester to study electrical engineering His work
on the Manchester Mark I computer earned him his master's degree. His subsequent doctoral work started his
interest in linear induction motors. He derived an equation for 'goodness' which parametrically describes the
efficiency of a motor in general terms, and showed that it tended to imply that large motors are more efficient.
He became professor of heavy electrical engineering at Imperial College London in 1964 where he continued his
successful development of the linear motor. He was involved in creating a self-stable magnetic levitation system
called Magnetic river which appeared in the film The Spy Who Loved Me where it levitated and propelled a tray
along a table to decapitate a seated dummy.
He also worked at applying linear motors on the Tracked Hovercraft until its cancellation.
In the 1980s, he was involved in creating a device to extract energy from sea waves; although the technology was
successful in trials, it could not be made storm proof, hence it never became a commercial success.
By Ghaseminejad, M Eng
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Linear induction motor
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A linear induction motor (LIM) is an 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 length primary or
secondary, which generates end-effects,
whereas a conventional induction motor is
arranged in an endless loop.
A transverse flux linear induction
motor (here the primary is at top of
picture) and has two sets of
opposite poles side by side. (Picture
from US Patent 3824414 by Eric
Laithwaite)
By Ghaseminejad, M Eng
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LIMs are 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.
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. They also, unlike their rotary counterparts,
can give a levitation effect.
By Ghaseminejad, M Eng
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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 in London, but Wheatstone's model was too inefficient
to be practical. A feasible linear induction motor is described in
the US patent 782312 ( 1905 - inventor Alfred Zehden of
Frankfurt-am-Main ), for driving trains or lifts. The German
engineer Hermann Kemper built a working model in 1935. In
the late 1940s, professor Eric Laithwaite of Imperial College in
London developed the first full-size working model.
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In a single sided version, the magnetic field can create repulsion
forces that push the conductor away from the stator, levitating it,
and carrying it along in the direction of the moving magnetic
field. Laithwaite called the later versions of it magnetic river.
These versions of the linear induction motor use a principle
called transverse flux where two opposite poles are placed side
by side. This permits very long poles to be used, which permits
high speed and efficiency.
FEMM simulation of a Cross-section
of Magnetic River, coloured by
electric current density
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Construction
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A linear electric motor's primary typically consists of a flat magnetic core (generally
laminated) with transverse slots which are often straight cut with coils laid into the
slots, with each phase giving an alternating polarity and so that the different phases
physically overlap.
The secondary is frequently a sheet of aluminum, often with an iron backing plate.
Some LIMs are double sided, with one primary either side of the secondary, and in this
case no iron backing is needed.
Two sorts of linear motor exist, short primary, where the coils are truncated shorter
than the secondary, and a short secondary where the conductive plate is smaller. Short
secondary LIMs are often wound as parallel connections between coils of the same
phase, whereas short primaries are usually wound in series.
The primaries of transverse flux LIMs have a series of twin poles lying transversely
side-by-side, with opposite winding directions. These are typically made either with a
suitably cut laminated backing plate or a series of transverse U-cores.
By Ghaseminejad, M Eng
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Construction of a Linear Induction Motor
By Ghaseminejad, M Eng
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Construction wise a LIM is similar
to three phase induction motor in
more ways than one as it has been
depicted in the figure.
If the stator of the poly phase
induction motor shown in the figure
is cut along the section aob and laid
on a flat surface, then it forms the
primary of the LIM housing the
field system, and consequently the
rotor forms the secondary consisting
of flat aluminum conductors with
ferromagnetic core for effective flux
linkage.
By Ghaseminejad, M Eng
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There is another variant of LIM also
being used for increasing efficiency
known as the double sided linear
induction motor or DLIM, as shown
in the figure.
Which has a primary winding on
either side of the secondary, for
more effective utilization of the
induced flux from both sides.
By Ghaseminejad, M Eng
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Principles
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In this design of electric motor, the force is produced by a linearly moving
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. The two opposing fields will repel each other, thus creating
motion as the magnetic field sweeps through the metal.
By Ghaseminejad, M Eng
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Working of a Linear Induction Motor
When the primary of an LIM is excited by a balanced three phase power supply, a
traveling flux is induced in the primary instead of rotating 3 φflux, which will
travel along the entire length of the primary. Electric current is induced into the
aluminum conductors or the secondary due to the relative motion between the
traveling flux and the conductors. This induced current interacts with the traveling
flux wave to produce linear force or thrust F. If the secondary is fixed and the
primary is free to move, the force will move the primary in the direction of the
force, resulting in the required rectilinear motion.
By Ghaseminejad, M Eng
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When supply is given, the synchronous speed of the field is given by the equation :
=


Where, 
is supply frequency in
Hz
,
and p = number of poles,
is the synchronous speed of the rotation of magnetic field in revolutions per
second.
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The developed field will results in a linear traveling field, the velocity of which is
given by the equation,

 

where, 
is velocity of the linear traveling field, and t is the pole pitch.
For a slip of s, the speed of the LIM is given by
  󰇛  󰇜

[𝑚/𝑠𝑒𝑐]
By Ghaseminejad, M Eng
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Forces
By Ghaseminejad, M Eng
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Thrust
The drive generated by linear induction motors is somewhat similar to conventional
induction motors; the drive forces show a roughly similar characteristic shape relative to
slip, albeit modulated by end effects.
Thrust generated as a function of slip
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End effect
Unlike a circular induction motor, a linear induction motor shows 'end effects'. These
end effects include losses in performance and efficiency that are believed to be caused by
magnetic energy being carried away and lost at the end of the primary by the relative
movement of the primary and secondary.
With a short secondary, the behaviour is almost identical to a rotary machine, provided
it is at least two poles long, but with a short primary reduction in thrust occurs at low
slip (below about 0.3) until it is eight poles or longer.
However, because of end effect, linear motors cannot 'run light'- normal induction
motors are able to run the motor with a near synchronous field under low load
conditions. Due to end effect this creates much more significant losses with linear
motors.
By Ghaseminejad, M Eng
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Levitation
In addition, unlike a rotary motor, an electrodynamic levitation force is shown, this is zero at
zero slip, and gives a roughly constant amount of force/gap as slip increases in either direction.
This occurs in single sided motors, and levitation will not usually occur when an iron backing
plate is used on the secondary, since this causes an attraction that overwhelms the lifting force.
Levitation and thrust force curves of a linear motor
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How linear motors work
In a traditional DC electric motor, a central core of tightly wrapped magnetic material (known as the
rotor) spins at high speed between the fixed poles of a magnet (known as the stator) when an electric
current is applied. In an AC motor, electromagnets positioned around the edge of the motor are used to
generatinduction e a rotating magnetic field in the central space between them. This "induces"
(produces) electric currents in a rotor, causing it to spin. In an electric car, DC or AC motors like these
are used to drive gears and wheels and convert rotational motion into motion in a straight line.
By Ghaseminejad, M Eng
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A linear motor is effectively an AC
induction motor that has been cut open
and unwrapped. The "stator" is laid out
in the form of a track of flat coils made
fromaluminumorcopper and is known
as the "primary" of a linear motor. The
"rotor" takes the form of a moving
platform known as the "secondary."
When the current is switched on, the
secondary glides past the primary
supported and propelled by a magnetic
field.
Linear motors have a number of
advantages over ordinary motors. Most
obviously, there are no moving parts to
go wrong. As the platform rides above
the track on a cushion of air, there is no
loss of energy to friction or vibration
(but because the air-gap is greater in a
linear motor, more power is required and
the efficiency is lower). The lack of an
intermediate gearbox to convert
rotational motion into straight-line
motion saves energy. Finally, as both
acceleration and braking are achieved
through electromagnetism, linear motors
are much quieter than ordinary motors.
By Ghaseminejad, M Eng
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Superconducting magnets
The main problem with linear motors has
been the cost and difficulty of developing
suitable electromagnets. Enormously
powerful electromagnets are required to
levitate (lift) and move something as big
as a train, and these typically consume
substantial amounts of electric power.
Linear motors often now use
superconducting magnets to solve this
problem.
If electromagnets are cooled to low
temperatures using liquid helium or
nitrogen their electrical resistance
disappears almost entirely, which reduces
power consumption considerably. This
helpful effect, known as
superconductivity, has been the subject of
intense research since the mid 1980s and
makes large-scale linear motors that
much more viable.
By Ghaseminejad, M Eng
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Application of Linear Induction Motor
By Ghaseminejad, M Eng
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A linear induction motor is not that widespread compared to a conventional
motor, taking its economic aspects and versatility of usage into consideration. But
there are quite a few instances where the LIM is indeed necessary for some
specialized operations.
Few of the applications of a LIM have been listed below.
1. Automatic sliding doors in electric trains.
2. Mechanical handling equipment, such as propulsion of a
train of tubs along a certain route.
3. Metallic conveyor belts.
By Ghaseminejad, M Eng
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Maglev
By Ghaseminejad, M Eng
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NASA tests a prototype Maglev railroad, 2001.
Picture of NASA Marshall Space Flight Center
(NASA-MSFC).
Everyone knows that the "like" poles of
two magnets repel one another. With a
little ingenuity, it is possible to make one
magnet levitate (float) above another one
using this repulsive force and (crucially)
some additional external support. The
idea of using electromagnetic levitation
to support a moving vehicle was first
proposed in 1912 by French engineer
Emile Bachelet, but soon abandoned due
to the enormous amount of electrical
power required.
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UNDERWATER LINEAR
ACTUATORS
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INDUSTRY SPOTLIGHT
-filled aluminum actuators offer high performance linear
motion at depths up to 20,000 feet below seawater. The power density of this subsea actuator is
extremely high due to enhanced thermal dissipation in the motor coils caused by fluid motion.
leading to an increase in the effective thermal mass of the motor and a decrease in thermal
resistance between windings and case. When operating underwater, these actuators are exposed
to a near infinite thermal reservoir and a much higher convective heat transfer coefficient than
overdriven to 2 3 times the standard power rating, allowing for high power density
underwater linear actuation.
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Features
115 lbf continuous @ 8 in/s , 250 lbf peak (in air)
Nema 17 BLDC motor
Oil filled cabling
Pressure balanced oil-filled compatible connectors (PBOF)
Electrical conductivity throughout the entire actuator and sacrificial zinc
anodes for corrosion resistance.
Type 316 Stainless steel and type III hard coat anodized aluminum
components
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Performance specifications
NEMA 17 models (forces up to 100 lbs)
NEMA 23 models (forces up to 500 lbs)
Depth ratings up to 20,000 ft
Temperature ranges from -30°C to 65°C
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Design and Manufacturing Capabilities
MIL-A-8625 Type III (Hardcoat) anodized aluminum enclosures
316L stainless steel hardware and shaft
Galvanic corrosion mitigation with proper isolation, and sacrificial anodes
Underwater connectors (Seacon, Subcon, Birns)
Pressure compensated oil filled designs
Customizable motor types (brushless and stepper motors)
Optional oil-filled cables designs
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A very brief video demonstration of
Laithwaite's "magnetic river" concept,
in which a moving object is held on a
steady path and propelled by a
magnetic field.
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Magnetic levitation
By Ghaseminejad, M Eng
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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 and any other accelerations.
The two primary issues involved in magnetic levitation are lifting force:
providing an upward force sufficient to counteract gravity, and stability: ensuring
that the system does not spontaneously slide or flip into a configuration where
the lift is neutralized.
Magnetic levitation is used for maglev trains, contactless melting, magnetic
bearings and for product display purposes.
By Ghaseminejad, M Eng
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Lift
Magnetic materials and systems are able to attract or press each other apart or together with
a force dependent on the magnetic field and the area of the magnets, For example, the
simplest example of lift would be a simple dipole magnet positioned in the magnetic field of
another dipole magnet, oriented with like poles facing each other, so that the force between
magnets repels the two magnets.
Essentially all types of magnets have been used to generate lift for magnetic levitation;
permanent magnets, electromagnets, ferromagnetism, diamagnetism, superconducting
magnets and magnetism due to induced currents in conductors.
To calculate the amount of lift, a magnetic pressure can be defined.
For example, the magnetic pressure of a magnetic field on a superconductor can be
calculated by:

=
 = 4π×10−7 N·A
[Pascal]
By Ghaseminejad, M Eng
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Magnetic river
Magnetic river is a electrodynamic magnetic levitation (maglev) system designed by Eastham
and Eric Laithwaite in 1974. It consists of a thin conductive plate on an AC linear induction
motor. Due to the transverse flux and the geometry, this gives it lift, stability and propulsion as
well as being relatively efficient. The name refers to the action that provides stability along the
longitudinal axis, which acts similar to the flow of water in a river.
By Ghaseminejad, M Eng
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Goodness factor
By Ghaseminejad, M Eng
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Goodness factor is a metric developed by Eric Laithwaite to determine the
'goodness' of an electric motor.
Using it he was able to develop efficient magnetic levitation induction motors.
  
   

Where
G is the goodness factor (factors above 1 are likely to be efficient)
Am, Ae are the cross sections of the magnetic and electric circuit
lm, le are the lengths of the magnetic and electric circuits
μis the permeability of the core
ωis the angular frequency the motor is driven at
σis the conductivity of the conductor
By Ghaseminejad, M Eng
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Sources
Gieras, J. et al. Linear Synchronous Motors:
Transportation and Automation Systems. CRC Press,
2011.
Laithwaite, E. R. A History of Linear Electric Motors.
Macmillan, 1987.
Livingston, J. Driving Force: The Natural Magic of
Magnets. Harvard University Press, 1996.
Vranich, J. Supertrains: Solutions to America's
Transportation Gridlock. St Martin's, 1991.
Electromechanical Actuator Specialists
The Magnetic Attraction of Trains, BBC News, 9
November 1999. A basic overview of maglev, including
a brief look at the ill-fated Birmingham Airport maglev
and the role of Eric Laithwaite.
China maglev budget 'may double', BBC News, 4
January 2008. Covers the huge cost of extending the
Shanghai maglev railroad. Includes some nice diagrams
(at the bottom) showing how maglev works.
German plans for maglev derailed, BBC News, 27
March 2008. Soaring costs kill off maglev again!
Linear Laithwaite, New Scientist, 20 September 1973.
An early interview with Eric Laithwaite, in which the
engineer explains his fascination with linear motors
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ER Laithwaite (1965). "The Goodness of a Machine".
Electronics and Power 11 (3): 101 103.
doi:10.1049/ep.1965.0071
DJ Patterson, CW Brice, RA Dougal, D Kovuri (2003).
"The "Goodness" of Small Contemporary Permanent
Magnet Electric Machines". Proceedings of the
International Electric Machines and Drives Conference
2: 1195 1200. doi:10.1109/IEMDC.2003.1210392
ER Laithwaite (1965). "Electromagnetic levitation".
Electronics and Power 11 (12): 408 410.
doi:10.1049/ep.1965.0312
Patent number 3585423, 1971 Laithwaite et al
"Charles Wheatstone - College History - King's College
London". Kcl.ac.uk. Retrieved 2010-03-01.
Force Analysis of Linear Induction Motor for Magnetic
Levitation System 14th International Power Electronics
and Motion Control Conference, EPE-PEMC 2010
"The magnetic attraction of trains". BBC News. 9
November 1999.
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