Eddy Brakes

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Eddy current brakes

by Chris Woodford. Last updated: January 10, 2012.

One of the drawbacks of going anywhere fast is that you always have tostop 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 Doug Bowman published

onFlickrin 2006 under aCreative Commons Licence.

How ordinary (friction) brakes work

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Moving things have kinetic energy and, if you want to stop them, you have to get ridof 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.)

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 yourkinetic 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?

Photo: Motorcycle brakes: Like most vehicles, this bike brakes with friction. When you pull on the

brake handle, a hydraulic cable 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.

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 magnetismaren'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

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Wherever you get electricity, you get magnetism as well, and vice-versa. This is the

basic idea behind electricity generators andelectric motors. Generators use some

kind of movement (maybe a wind turbinerotor spinning around) to make an electriccurrent, while motors do the opposite, converting an electric current into movement

that can drive a machine (or propel something like anelectric 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 miniatureelectric power plant.

Photo: A basic electric motor has anaxle(the silver rod in the middle) that rotates when you feed

electricity into the motor'scoppercoil through two wires. A generator is similar, but you turn the axle

manually and get electricity out of the wires instead.

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 amagnetic 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).

Here's an example. Suppose you drop a coin-shaped magnet down the inside ofa plastic pipe. It might take a half second to get to the bottom. Now repeat the same

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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 wheelsor we could stop it with eddy currents.

How? What if 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 thefront 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 there would

reverse, 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

manyrollercoaster cars, which use magnetic brakes like this, mounted on the side of

the track, to slow them down.

Artwork: Here's our simple copper block train moving from right to left, and I've embedded a giant barmagnet in the track to stop it. As the train approaches, eddy currents are induced in the front of it that

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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 the opposite way. Now they'll produce

an attractive magnetic field that tries to pull the train backward, but still trying 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.)

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 physicistJean-Bernard Lon Foucault(also the inventor of the

Foucault pendulum and one of the first people to measure the speed

of light accurately on Earth). Eddy current brakes come in two basic flavorslinear

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.

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 theyreach 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 movea conductor through a magnetic

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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.

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 ofWikimedia Commonspublished under

aCreative Commons Licence.

Circular brakes

Like linear eddy current brakes, circular brakes also have one static part and onemoving 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.) 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.)

How do these things work in practice? Suppose you have a high-speed factorymachine 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 constantlycirculating in the same piece of metal. For this reason, circular eddy current brakes

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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.

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

courtesy ofWikimedia Commonspublished under aCreative Commons Licence.

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. You'll also find eddy current brakes in all kinds of machines, such as circular

saws and other power equipment. And they're used in things like rowing machines

and gym machines to apply extra resistance to the moving parts so your muscles

have to work harder.

Advantages and disadvantages of eddy current

brakes

On the plus side, eddy-current brakes are quiet, frictionless, and wear-free, andrequire little or no maintenance. They produce no smell or pollution (unlike friction

brakes, which can release toxic chemicals 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 Gazettein 2008, the electromagnetic

parts of eddy current brakes have sometimes caused problems by interfering withtrain signaling equipment. Although heat dissipation in rails should not, theoretically,

be an issue, 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.

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Further reading

On this website

Brakes

Electric motors

Electricity generators

Energy

Magnetism

Regenerative brakes

Books

Electromagnetic fields in electrical engineering by Slawomir Wiak. IOS

Press, 2002. An introduction to the various practical applications of

electromagnetism.

Articles

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

Lenz's law and eddy currents: An explanation of how eddy currents

oppose the thing that causes them. Quite a good introduction to the

theory.

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.

Sponsored links

Please do NOT copy our articles onto blogs and other websites

Text copyright Chris Woodford 2012. All rights reserved. Full copyright notice and terms of use.

Kevlar is a registered trademark of E. I. Du Pont de Nemours and Company.

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