Question

What are eddy currents and how do eddy current brakes work?

Asked by: Matt Behre

Answer

An eddy current is a swirling current set up in a conductor in response to a changing magnetic field. By Lenz's law, the current swirls in such a way as to create a magnetic field opposing the change; to do this in a conductor, electrons swirl in a plane perpendicular to the magnetic field.

Because of the tendency of eddy currents to oppose, eddy currents cause energy to be lost. More accurately, eddy currents transform more useful forms of energy, such as kinetic energy, into heat, which is generally much less useful. In many applications the loss of useful energy is not particularly desirable, but there are some practical applications. One is in the brakes of some trains. During braking, the metal wheels are exposed to a magnetic field from an electromagnet, generating eddy currents in the wheels. The magnetic interaction between the applied field and the eddy currents acts to slow the wheels down. The faster the wheels are spinning, the stronger the effect, meaning that as the train slows the braking force is reduced, producing a smooth stopping motion.

Answered by: Jason Heidecker, Physics Undergrad, Occidental College, Los Angeles



An eddy current is a swirl (like a whirlpool) of current that is induced in a solid conducting mass.

Imagine a square loop of wire being pulled out of an area through which a uniform magnetic field runs perpendicular to the plane of the loop. What is going to happen?

The inductance of the loop is going to resist the change in magnetic field within it and an emf will be generated. In other words, as the loop moves out of the field a current will be induced in the loop which causes another magnetic field of the same polarity as the surrounding field. This will cause the loop to get 'attracted' to the surrounding field; the loop will feel as if it is being pulled back into the field. If you look at the forces on each individual section of wire moving in the field, you'll see that the one perpendicular to the motion of the loop causes a force contrary to that motion.

Mechanical energy being used to move the loop will be turned into electrical energy driving current in the loop. The faster the loop is pulled, the harder the loop will pull back.

I think that most people can get this far on their own, yet eddy currents still seem strange, though the idea of an eddy current is no different than this.

Just like current being induced in a loop of wire, current 'swirls' or 'eddies' -- little whirlpools of current -- can be induced inside a solid conductive slab. While there is no "wire" inside the slab, the inductance (all conductors are inductors, including capacitors before transients die out; imagine a capacitor acting like a short for high frequencies) of the slab causes nature to move current along the same way it would if there was a circular wire.

So if you moved a slab into a magnetic field or out of a magnetic field, eddy currents would be induced inside the slab that would cause an equal an opposite reaction to the forces being applied to the slab.

So, if you've followed that, then perhaps you'll see that I have answered the question backwards.

Eddy current brakes just make excessive use of the example I gave above. Perhaps I need another example to make this clear . . .

I remember in my Frosh E&M class my professor had built an interesting pendulum. I later found that this was a common experiment in E&M classes.

The pendulum could swing back and forth, and could attach to a number of different types of conductive rings at the end of it. As it swung, the ring on the end of it would pass between two poles of a very strong very huge magnet (when rolled into a small physics lab, this magnet would discolor all CRTs in the room) . . .

So the professor would draw the pendulum back, let it swing, and we would observe what would happen when the conductive ring passed into (and perhaps out of) the magnetic field.

The first example would be to put a conductive ring (it didn't have to be a ring -- the ring was just how it attached to the pendulum; only the solid part went through the strong part of the magnetic field) at the end of the pendulum and let it swing through. This conductive ring was solid -- like a large dough-nut. Even if the professor threw the pendulum hard into the field, once the ring came in between the two poles, it stopped immediately. All of its kinetic energy went into moving current inside the conductive ring (and I'm sure the ring's resistance gave that energy off as heat).

Now, the next example was to take a very similar ring, but this ring had a number of slits cut in it. It was still a ring, but the area that passed between the two poles looked like frayed edges.

This time, the pendulum swung through the poles with no trouble -- it swung back and forth a few times.

The slits cut in the second example had the same effect as breaking the 'loop' in the very first example I gave in this answer. If you break the loop, an emf will be generated, but no current will be able to flow. Without any current, no field can be generated.

So by making slits, if there are going to be any eddy currents, they are going to be very small currents that provide very little bleeding off of the kinetic energy.

This type of 'dynamic breaking' can be extended further. Imagine a motor. If you replace the battery of that motor with a light bulb and turn the motor manually, the light bulb may light. If you continue to add more loads than just a light bulb, they might all start to operate, but with more loads, it will become harder and harder to rotate the "backwards motor generator." Removing all of these loads so that there is nothing connecting the two previous battery leads will make the motor rotate easily (assuming low friction/appropriate gear ratio/etc.).

Similarly, you can put an infinite load across those empty leads by simply shorting them together. Providing the magnets are strong enough (my professor's magnet was not strong enough when he cut slits in the ring), the motor will not be able to move.

This is how a hand drill automatically stops when you let go of the trigger. If you look closely at the motor housing, you may see a spark when you let go of the trigger. This happens when the battery is disconnected from the leads and replaced with a short circuit. The motor's own kinetic energy attempts to drive it in the opposite direction (that's one way to look at it) and ends up slowing itself very quickly down to zero velocity.

Now let's start to get creative . . . what if you replace the short circuit with an uncharged capacitor. With the short circuit, the kinetic energy causes a strong current in the motor, which causes a force opposing the motor's motion . . . But that kinetic energy has a limited supply, and once it stops building up the field, the energy will be given off as heat from the resistance of the wire. HOWEVER, if you place a capacitor there, you can start to catch some of that energy and store it there. HOWEVER, if you're not careful, the energy from the capacitor will start to discharge into the motor and try to spin it . . . and you have a very odd little oscillator.

So what if you had a smart way of dynamic braking which would allow you to charge the capacitor as much as possible and then disconnect the capacitor and replace it with a short (or similar) . . . and what if you replaced the capacitor with a battery . . .

Then not only do you have a really interesting braking system that doesn't require friction to work, but instead of wasting all that energy going into heat, you get to store a lot of it back in the battery that makes you move in the first place . . .

You can extend this farther. Devices like the Segway actually use this same sort of technology to charge their batteries in more than just braking -- in going downhill as well.

Now, in my last few examples, they had nothing to do with eddy currents -- if you want to capture that energy you have to build the circuit yourself -- you can't let nature do it for you in eddies. It's also hard to dynamically brake with eddy currents -- you can't just add slits and take them away on a whim.

Eddy currents actually do play a part in things like transformers, which is why transformers are usually made of laminated cores that prevent eddy currents.

When a coil of wire surrounds a ferromagnetic core, like iron, for example, of a transformer, the CHANGING magnetic field induces an eddy current in the core, which happens to be conductive as well. If you build the core out of a great deal of layers all separated by an insulator, you can prevent eddy currents. Preventing eddy currents in transformers prevents power loss (currents in resistive materials cause power loss) in the transformer. There are other sources of power loss in a transformer, but those are much more complicated. Even the simplest [moderately large] transformer (not necessarily chokes) will most likely have a laminated core.

Answered by: Ted Pavlic, Electrical Engineering Undergrad Student, Ohio St.

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