Eddy currents are not always welcome: often they are pure loss.
When Foucault is an enemy
In transformers and electric motors the eddy currents in the iron cores are unwanted: they dissipate precious energy as heat. This is why real cores are made of thin laminations (0.3–0.5 mm) stacked and insulated from each other with varnish or oxide. The laminations interrupt the possible circuits for the currents, exactly as the radial slots did in the “broken” Waltenhofen pendulum. Same physics, opposite side of the coin.
The design criterion is therefore the opposite of that for eddy current brakes: there dissipation is maximised, here it is minimised. Since the dissipated power grows with the square of the plate thickness, splitting the core into many thin insulated laminations drastically cuts the losses while leaving the useful magnetic flux path intact.
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Topics: Electromagnetic induction Concepts: Eddy currents · Joule heating
Related exercises: Problem — The magnet in the copper tube · Worked exercise — magnet in a tube, terminal velocity · Worked exercise — Waltenhofen’s disc