Eddy current brakes
by Chris Woodford. Last updated: September 15, 2014.
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 Doug Bowman published on Flickr in 2006 under a Creative Commons Licence.
How ordinary (friction) brakes work
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.)
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?
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 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.
Photo: A basic electric motor has an axle (the silver rod in the middle) that rotates when you feed electricity into the motor’s copper coil 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 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).
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 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 many rollercoaster 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 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 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 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). Eddy current brakes come in two basic flavors—linear and circular.
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 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.
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.
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.) 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 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.
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 of Wikimedia Commons published under a Creative 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, and require 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 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, 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.