One of the drawbacks of going anywhere fast is
that you always have to stop sooner or later. In an emergency, when
you have to brake quickly, the only thing that comes between safe
stopping and disaster is the simple science of friction: you slow to
a halt when two surfaces rub together. Now friction brakes have
more than proved their worth: you'll find them in every car, bicycle,
airplane, and most factory machines. But they have a big drawback
too: every time you use them, they wear out a little bit, and that
means they're relatively expensive. What's the alternative? One
option is to slow things down with the force of electromagnetism instead
of friction. It sounds like something out of Flash Gordon or Buck
Rogers, but it's the basic idea behind eddy-current brakes,
which can cost half as much to run over their lifetime as
traditional, friction brakes. What are they are how do they work?
Let's take a closer look!
Photo: Eddy-current brakes in action! Relatively few trains use electromagnetic braking, but this is an exception: the Japanese Shinkansen 700 bullet train running the Nozomi service. You can see a photo of the brakes used in this train further down the page. Photo by Sui-setz published on
Wikimedia Commons in 2008,
licensed as public domain.
Moving things have kinetic energy and, if you want
to stop them, you have to get rid of that energy somehow. If you're
on a bicycle going fairly slowly, you can simply put your feet down
so they drag on the ground. The soles of your feet act as brakes.
Friction (rubbing) between the rough ground and the grip on your
soles slows you down, converting your kinetic energy into heat energy
(do it long enough and your shoes will get hot). Brakes on vehicles work pretty much the same way, with "shoes" that press rubber pads (brake blocks) against discs mounted to the wheels. (Find out more about this in our main article on brakes.)
Photo: Motorcycle brakes: Like most vehicles, this bike brakes with friction. When you pull on the brake handle, fluid flowing down a hydraulic pipe applies the brake pads to the brake rotor disc, slowing the machine down by converting
your kinetic energy to heat. The tire doesn't normally play much part in braking unless you brake really hard: then
the wheel will lock completely and friction between the tire and the road will bring you to a sudden halt, leaving a rubber skid mark on the road. That's not a good way to brake: it'll wear out your tires very quickly.
Even if you make brakes from super-strong,
hard-wearing materials like Kevlar®, they're still going to wear out
sooner or later. But there are other problems with friction brakes.
The faster you go, the harder they have to work to get rid of your
kinetic energy, and the quicker they'll wear out. Use your brakes
too often and you may suffer a problem called brake fade,
where heat builds up too much in the brakes or the hydraulic system that operates
them and the brakes can no longer work as effectively.
What if your brakes can't stop you in time?
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What are eddy currents?
Before we can understand eddy current brakes, we
need to understand eddy currents! They're part of the science of
electromagnetism: electricity and
magnetism aren't two separate
things but two sides of the same "coin"—two different aspects of
the same underlying phenomenon.
Electricity and magnetism go hand in hand
Wherever you get electricity, you get magnetism as
well, and vice-versa. This is the basic idea behind
electricity
generators and electric motors. Generators use some kind of movement
(maybe a wind turbine rotor spinning around) to make an
electric current, while motors do the opposite, converting an
electric current into movement that can drive a machine (or propel
something like an electric car or
electric bike).
Both kinds of machine (they are virtually
identical) work on the idea that you can use electricity to make
magnetism or magnetism to make electricity. To make electricity, all
you have to do is move an electrical conductor (like a copper wire)
through a magnetic field. That's it! It's called Faraday's law of
induction after English scientist
Michael Faraday, who discovered the
effect in the early 19th century. If you connect the wire up to a meter, you'll see the needle flick every time you move the wire (but only when you move it). If you were clever, you could figure out some
way of removing the electricity and storing it: you'd have made
yourself a miniature electric power plant.
How eddy currents are made
What if the conductor you're moving through the
magnetic field isn't a wire that allows the electricity to flow
neatly away? You still get electric currents, but instead of
flowing off somewhere, they swirl about inside the material.
These are what we call eddy currents. They're electric
currents generated inside a conductor by a magnetic field that can't
flow away so they swirl around instead, dissipating their energy as heat.
One of the interesting things about eddy currents is that they're not completely random: they flow in
a particular way to try to stop whatever it is that causes them. This
is an example of another bit of electromagnetism called Lenz's law
(it follows on from another law called the conservation of energy, and it's built into the four equations summarizing electromagnetism that were set out by James Clerk Maxwell).
Animation: 1 and 2) Drop a coin-shaped magnet (blue) through a plastic pipe (purple) and it falls
very quickly, just as you'd expect. 3) Drop the same magnet through a copper pipe (gray), and it falls extremely slowly.
4) As it falls, its magnetic field induces swirling eddy currents (red) in the pipe. 5) The currents produce a magnetic
field that opposes their cause (the field of the falling magnet). This causes an upward force on the magnet (white arrow)
that slows it down like an invisible parachute.
Here's an example. Suppose you drop a coin-shaped magnet down the inside of a
plastic pipe. It might
take a half second to get to the bottom. Now repeat the same
experiment with a copper pipe and you'll find your magnet takes much
longer (maybe three or four seconds) to make exactly the same
journey. Eddy currents are the reason. When the magnet falls through
the pipe, you have a magnetic field moving through a stationary
conductor (which is exactly the same as a conductor moving through a
stationary magnetic field). That creates electric currents in the
conductor—eddy currents, in fact. Now we know from the laws of
electromagnetism that when a current flows in a conductor, it
produces a magnetic field. So the eddy currents generate their own
magnetic field. Lenz's law tells us that this magnetic field will try
to oppose its cause, which is the falling magnet. So the eddy
currents and the second magnetic field produce an upward force on the
magnet that tries to stop it from falling. That's why it falls more
slowly. In other words, the eddy currents produce a braking effect on
the falling magnet.
It's because eddy currents always oppose whatever
causes them that we can use them as brakes in vehicles, engines, and
other machines.
How does an eddy current brake stop something moving?
Suppose we have a railroad train that's actually a
huge solid block of copper mounted on wheels. Let's say it's hurtling
along at high speed and we want to stop it. We could apply friction
brakes to the wheels—or we could stop it with eddy currents. How?
What if we put a giant magnet next to the track so the train had to
pass nearby. As the copper approached the magnet, eddy currents would be
generated (or "induced") inside the copper, which would produce
their own magnetic field. Eddy currents in different parts of the copper
would try to work in different ways. As the front part of the train approached the magnet, eddy currents in that
bit of the copper would try to generate a repulsive magnetic field (to slow down the copper's approach to the magnet).
As the front part passed by, slowing down, the currents would start generating an attractive
magnetic field that tried to pull the train back again (again, slowing it down).
The copper would heat up as the eddy currents swirled inside it, gaining
the kinetic energy lost by the train as it slowed down. It might sound like a strange way
to stop a train, but it really does work. You'll find the proof of it
in many rollercoaster cars, which use magnetic brakes like this,
mounted on the side of the track, to slow them down.
Artwork: Simplified concept of a linear Eddy-current brake.
Here's our simple copper block train moving from right to left, and I've embedded a giant bar magnet in the track to stop it. As the train approaches, eddy currents are induced in the front of it that produce a repulsive magnetic field, which slows the train down. If the train is moving really fast, this magnet might not stop it completely, so it'll keep moving beyond the magnet. As it moves past the other end of the magnet, the induced eddy currents will work to produce an attractive magnetic field that tries to pull the train backward, but still tries to slow it down. The basic point is simple: the eddy currents are always trying to oppose whatever causes them. (Note that eddy currents are actually induced through the whole of the copper block, but I've drawn only a few of them for clarity.)
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Types of eddy current brakes
Real eddy current brakes are a bit more
sophisticated than this, but work in essentially the same way. They
were first proposed in the 19th century by the brilliant French
physicist Jean-Bernard Léon Foucault (also the inventor of the Foucault pendulum and one of the first people to measure the speed of
light accurately on Earth). Indeed, that's why, in older books, you'll sometimes see
Eddy currents referred to as Foucault currents.
Eddy current brakes come in two basic flavors—linear and circular.
Linear brakes
Linear brakes feature on things like train tracks and rollercoasters, where the track itself (or something
mounted on it) works as part of the brake.
Photo: The linear eddy-current brakes from a roller coaster. (The brakes are the black
things mounted on the side of the track.) Photo by Stefan Scheer courtesy of
Wikimedia Commons
published under a Creative Commons Licence.
The simplest linear, eddy-current brakes have two
components, one of which is stationary while the other moves past it
in a straight line. In a rollercoaster ride, you might have a series
of powerful, permanent magnets permanently mounted at the end of the
track, which produce eddy currents in pieces of metal mounted on the
side of the cars as they whistle past. The cars move freely along the
track until they reach the very end of the ride, where the magnets
meet the metal and the brakes kick in.
This kind of approach is no use for a conventional
train, because the brakes might need to be applied at any point on
the track. That means the magnets have to be built into the structure
that carries the train's wheels (known as the bogies) and they have
to be the kind of magnets you can switch on and off (electromagnets,
in other words). Typically, the electromagnets move a little less
than 1cm (less than 0.5 in) from the rail and, when activated, slow the train by
creating eddy currents (and generating heat) inside the rail itself.
It's a basic law of electromagnetism that you can only generate a
current when you actually move a conductor through a magnetic
field (not when the conductor is stationary); it follows that you can
use an eddy current brake to stop a train, but not to hold it
stationary once it's stopped (on something like an incline). For that reason,
vehicles with eddy current brakes need conventional brakes as well.
Circular brakes
Like linear eddy current brakes, circular brakes also have one static part and one moving part.
They come in two main kinds, according to whether the electromagnet moves or stays still.
The simplest ones look like traditional brakes, only with a
static electromagnet that applies magnetism and creates eddy currents in a rotating metal disc (instead of simple pressure and friction) that moves through it. (The Shinkansen brakes work like this.)
Artwork: A simple, circular eddy current brake.
1) Magnetic core; 2) Electric coil wrapped around core; 3) Brake disc; 4) Electromagnetism ("magnetic flux") produced by coil in core induces eddy currents in brake disc, slowing it down.
In the other design, the electromagnets move instead: there's a series of electromagnet coils mounted on an outer wheel that spins around (and applies magnetism to) a fixed, central shaft. (Telma frictionless "retarder" brakes, used on many trucks, buses, and coaches, work this way.)
Photo: Close-up of the circular eddy-current brake from the Shinkansen 700 train in our top photo.
Although this resembles the motorcycle friction brake up above, it works in a totally different way. You can see the brown brake disc and the electromagnet that surrounds it at the top. When the brake is applied, the electromagnet switches on and induces eddy currents in the disc, which create opposing magnetic fields and stop it rotating. Unlike in the motorcycle brake,
there is no contact at all between the electromagnet and the brake disc: there's an air gap of a few millimeters
between them. Photo by Take-y courtesy of Wikimedia Commons published under a Creative Commons Licence.
How do these things work in practice? Suppose you have a high-speed factory machine that
you want to stop without friction. You could mount a metal wheel on
one end of the drive shaft and sit it between some electromagnets.
Whenever you wanted to stop the machine, you'd just switch on the
electromagnets to create eddy currents in the metal wheel that bring
it quickly to a halt. Alternatively, you could mount the
electromagnet coils on the rotating shaft and have them spin around
or inside stationary pieces of metal.
With a linear brake, the heat generated by the
eddy currents can be dissipated relatively easily: it's easy to see
how it would disappear fairly quickly in a brake operating outdoors over a
relatively long section of train track. Getting rid of heat is more of an issue with circular brakes, where the eddy
currents are constantly circulating in the same piece of metal. For
this reason, circular eddy current brakes need some sort of cooling
system. Air-cooled brakes have metal meshes, open to the air, which
use fan blades to pull cold air through them. Liquid-cooled brakes
use cooling fluids to remove heat instead.
Where are eddy current brakes used?
Despite being invented over a century ago, eddy
current brakes are still relatively little used. Apart from
rollercoasters, one area where they're now finding applications is in
high-speed electric trains. Some versions of the German
Inter City Express (ICE) train and Japanese
Shinkansen ("bullet train")
have experimented with eddy-current brakes and future versions
of the French TGV are expected to use them as well.
(The ICE3 train, for example, uses high-power, linear brakes on one of the bogies to induce
eddy currents in the rails beneath.)
There are eddy current brakes in wind turbines.
They're better at braking the rotors at high speeds and easier to
maintain (wind turbines are often in inaccessible places where friction brakes
would be too hard to service or repair). You'll also find eddy current
brakes in all kinds of machines, such as circular saws and other
power equipment. They're used in things like rowing machines (including ones made by Hydrow)
and gym machines to apply extra resistance to the moving parts so your
muscles have to work harder. And on dynamometers, which
measure the force, power, or speed produced by engines and machines.
Artwork: Old-fashioned exercise bikes use springs or pulleys to drag on the back wheel,
providing an adjustable amount of resistance to make your muscles work harder; newer machines are as likely to do
the same job with eddy-current brakes. Instead of being packed with spokes and sprockets, the back wheel has a copper disc attached that spins between the poles of a magnet, generating eddy currents that slow you down.
On some machines, you can slide the magnet back and forth to adjust the amount of braking.
You can find a much more detailed description in US Patent 6,964,633: Exercise device with an adjustable magnetic resistance arrangement by Clint D. Kolda et al, November 15, 2005.
Advantages and disadvantages
On the plus side, eddy-current brakes are quiet,
frictionless, and wear-free, and require little or no maintenance.
They produce no smell or air pollution, unlike friction brakes, which can
release toxic "particulates" (microscopic bits of dust and metallic fragments) into the environment.
All this makes them much more attractive than noisy friction brakes that need regular
inspection and routinely wear out. It's been estimated that switching
an electric train from friction brakes to eddy-current brakes could
halve the cost of brake operation and maintenance over its lifetime.
The drawbacks of eddy current brakes are more to
do with how little experience we have of using them in real-world
settings. As Jennifer Schykowski
noted
in an excellent review of the technology for Railway Gazette in 2008,
the electromagnetic parts of eddy current brakes have sometimes
caused problems by interfering with train signaling equipment. Although heat
dissipation in rails should not, theoretically, be an issue, the brakes
on high-speed trains need to be very powerful, so the heating effect
can be significant. If there's a busy section of track where many trains brake in quick
succession (something like the approach to a station), the
heating and expansion of rails could prove to be an issue, either
reducing the effectiveness of the brakes or leading to structural
problems in the rails themselves.
Another important question is whether
eddy-current brakes will ever become widespread, given the
growing interest in regenerative brakes that
capture and store the energy of moving vehicles for reuse (a much more energy-efficient approach
than turning energy into useless heat with eddy currents). Some of the latest
Shinkansen trains (series E5) use regenerative brakes where earlier
models used eddy-current technology. You could argue that the two technologies are
polar opposites: eddy current brakes squander energy where regenerative brakes save it.
Both types of braking can add considerable weight to a vehicle (an eddy current train
brake can weigh almost a tonne), which reduces fuel economy and efficiency,
but at least regenerative brakes claw some of that wasted energy back again.
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Don't want to read our articles? Try listening instead
Clutches and Brakes: Design and Selection
by William C. Orthwein. CRC Press, 2004. Chapter 7 goes into some detail about eddy-current (and other electromagnetic) brakes.
Eddy-current braking: a long road to success by Jennifer Schykowski. Railway Gazette, 2 June 2008. An interesting survey of how railroad companies (notably Deutsche Bahn, DB) have tried to deploy eddy current brakes, with mixed results.
Mysterious forces: Eddy Currents by Dave Ansell, Naked Scientists. In this short article, Dave demonstrates why a magnet falls more slowly down a copper pipe due to eddy currents, and goes on to explain what eddy currents actually are.
Videos
Eddy current brake in action: Watch how this children's horse slows down as it reaches the eddy-current brake at the end of the track.
Eddy current exercise machine: Another interesting application of eddy current brakes: providing varying amounts of resistance in an electronically controlled exercise bicycle.
Patents
If you're looking for more technical detail, here's a selection of real-world inventions that use eddy-current braking technology. There are lots more examples on Google Patents.
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