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Q & A: how to increase eddy current braking

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Most recent answer: 03/26/2014
Q:
In regard to eddy current braking, which will provide more improvement to braking strength: increasing the thickness of the conductive material passing between two magnets (recognizing that a thicker material requires the magnets to be further apart, thus decreasing their magnetic pull to each other) or decreasing the thickness of conductive material passing between two magnets (allowing the magnets to be closer together, thus increasing their magnetic pull)?
- John (age 35)
San Diego
A:

The eddy current braking doesn't directly come from the pull between the magnets. In fact, you can get braking with just a magnet on one side. But we'll see that having magnets on both sides does matter for the thickness dependence. 

The eddy currents are set up because, from the reference frame of the part of the metal near the magnets, the magnets are in motion. That means that what looks like a magnetic field from our point of view looks partly like an electric field from the metal's point of view, so it drives electrical currents around in the metal. From our point of view we say that the the electrons in the metal are moving at right angles to a magnetic field and thus experience a Lorentz force that drives the currents.

So now let's look at how the braking changes as you make the rotating conductor thicker. Say that the conductor is thin compared to the size of the magnet poles. making it thicker doesn't change the field pattern much but does increase the volume of conductor in which the currents are flowing. So it increases the braking.  If the metal gets very thick compared to the magnet size, then the fields from the magnets on each side don't overlap much. The fields are only half as big as the case where they're close together. You get the sum of the braking effects from each magnet. It's twice as big as from one magnet. Since the energy loss from the currents goes as the square of the current density, which is proportional to the field, you'd get four times the single-magnet braking if the two fields completely overlapped. So as the thickness increased to the point where it reduced the field overlap, you lost some braking power. That's the effect  you were wondering about. 

The bottom line is that the braking should increase as the metal gets thicker until the thickness is comparable to the size of the magnet poles. Further thickening will reduce the braking. If you just had a magnet on one side, then the thicker the conductor the better.

Mike W.


(published on 03/26/2014)

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