Eddy Current Speedometer Experiment

[Thomas Rugh]

Trying to create an experiment that utilizes a motor driven shaft to measure the angular velocity of another shaft via eddy currents. We made a mock experiment that uses 8mm shafts, pillow bearings, and ceramic magnets from home depot, but don't see a torque on a non-ferrous metal attached to a staff.

We tried using aluminum cans, copper coils wrapped around the diameter, and are about to try aluminum plating. The ceramic magnets have their poles orientated as seen in the diagram. The motor spins at 550 rpm, but we can't see the slightest torque being produced nor feel anything if we hold it by hand.

Do we need thicker sheet metal, stronger magnets, geometry, and/or have the magnets closer? Are we overestimating the force that can be generated? Thanks in advanced.

 

[sophiecentaur]

Hi and welcome to PF. I have never tried to make one of these but I have (distant past) taken a car speedo apart so I recognise the parts.
It is easy to underestimate the design effort in an everyday bit of old technology and there are some important features required for the system to work. You need a good magnetic circuit, involving the bar magnet and a steel cup (for a 'return path') with a small gap in which the aluminium cup can rotate. It is the field, sweeping across the thickness of the aluminium cup that generates the eddy currents.
You don't show a diagram of your actual setup so I am assuming that this is what could be wrong. You mention magnets (plural). The classic arrangement is a single bar magnet so what is your design? Do you have your ceramic magnets orientated the right way and do you have a very small gap between magnet and aluminium disc?
The eddy current effect with powerful modern magnets is usually very easily demonstrated. If you drop a magnet down through a copper tube, the eddy currents will produce a definite braking effect - so the force is significant. (If the tube has an insulating slot running down it, there is no braking effect.
I suggest you Google around for "eddy current experiments" and that should give you some ideas.
Edit: PS I googled around and, of course, modern super magnets are strong enough to show eddy current effects all over the place. Much more than the magnets I played with as a boy,