Linear motors
Last updated: June 6, 2009.
Have you seen those extraordinary
"maglev" (magnetic levitation) trains that float on
air instead of rolling on wheels?
They're entirely electric but, instead
of using ordinary electric motors that spin around, they use
a kind of "unwrapped" motor called a linear motor that causes them to
move at high speed in a straight line. What are linear motors and how do they work?
Let's take a closer look!
Photo: A Maglev train using linear motor
technology.
Picture courtesy of US Department of
Energy/Argonne National Laboratory
What are linear motors?
Linear motors are electric 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.
Photo: 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." Picture courtesy of NASA
Marshall Space Flight Center (NASA-MSFC)
Linear motors (more correctly known as linear induction motors) are
electric motors that produce straight-line rather than rotational
motion. The basic principle behind the linear motor was discovered in 1895,
but practical devices were not developed until 1947. It was at this
time
that British electrical engineer Eric 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."
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.
How linear motors work
In a traditional 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 electric car, motors like this are used to drive
gears and wheels and convert
rotational motion into motion in a straight line.
Photo: An ordinary electric motor is all about rotation: the rotor (the coils in the center)
turns inside the stator (the outer magnetic case).
A linear motor is effectively a normal electric motor that has been
cut open and unwrapped. The "stator" is laid out in the form of a track
of flat coils made from aluminum or
copper 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.
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.
Maglev - "A Closer Look"
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 1914 by French engineer Emile
Bachelet,
but soon abandoned due to the enormous amount of electrical power
required.
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.
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.
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!
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.
Photo: NASA tests a prototype Maglev railroad, 2001.
Picture courtesy of NASA
Marshall Space Flight Center (NASA-MSFC)
