Eddy currents are built up by passing a conductive material such as copper, 
aluminum, etc. through an alternating magnetic field.  This can be an 
electro-magnetic or permanent magnetic field.  The faster you move the 
conductive material, the stronger the resistance to that movement becomes 
because you are building more current.  If you stay at a constant rpm, you 
will have a constant resistance.  At zero rpm, you will have no resistance 
because you are not building any Eddy Current.  

Other factors that increase the resistance are the strength of the magnets 
and how close the conductive material is to the alternating magnetic field.

The materials will however, build up a lot of heat during the process due to 
the resistance between the magnets and the conductive material.  I have built 
several at work with permanent rare earth magnets that get so hot, you can't 
touch the surface of the metal.  

John
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