How does an eddy-current brake work ?
When moving a metal plate in the air gap of a permanent magnet, current will be induced in this metal plate. This current will flow back in a closed loop – like a whirl or an eddy.
A flow of current always means that there is a magnetic field as well. Due to Lenz‘ law, the two magnetic fields (the primary of the permanent magnet and the secondary, created by the eddy-current) interact. This interaction leads to forces that work against the movement direction of the metal plate. The metal plate will slow down and eventually stop. Instead of mechanical friction, we have created ‚magnetic friction‘.
The image to the right is a computer simulation of eddy-currents and their loops, induced in a metal plate by two permanent magnets.
School experiment for eddy-current brakes.
The braking force is a function of the movement speed (i.e. the entrance speed of a gondola into the brake) – and the material, the gondola’s braking fin is made from. As shown in the drawing below, the braking force rapidly increases with speed. However, it will decline when the gondola gets too fast.
Finally, what happens to the gondola’s kinetic energy? It will be transformed into thermal energy.
The braking fin will heat up. Because its specific resistance is in the eddy-currents‘ way – and current flowing across a resistor always heats up the resistor. This effect is used e.g. in induction ovens. However, you may also know this effect as ‚iron loss‘ of transformers.