# Damping ratio from Eddy current breaking

1. Jan 20, 2014

### breen155

Hello All,

I have been asked to design and build a rig that will introduce the concept of a mass spring damper system to mechanical engineers in their early years of university. To make it slightly more complex, the rig needs to be torsional rather than linear.

My design so far is to use a torsional pendulum (an aluminium disk attached to a wire) and then slide a Neodynium magnet over the disc to increase the damping force via Eddy currents.

I have a desired angular velocity for the disc and calculated the appropriate spring constant for the wire and inertia for the disc, I require a high inertia so the disc will be heavy (~10kg).

What I am struggling to find however, is a method of determining the damping force that the magnet would exert on the disc. If anyone has any ideas or could point me in the right direction, that would be fantastic! :)

Liam

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2. Jan 20, 2014

### Ryoko

Eeks. That's a tough question. I wish I still remembered more of my e-mag coursework. The answer depends on the magnets shape, spacing, and how the magnetic flux flows around them and thru the disk. In short, you need to solve for the B-field that passes thru the disk.

You could cheat and determine the damping force by solving the equations of motion using measured data from your apparatus.

**edit
You may want to post this question over on the Electrical Eng forum. Somebody there who still remembers their e-mag courses could probably give you more insight.