These are symilar machines to parasitic current brakes, in which the air gap is ocupaid by iron and magnetic dust limaduras, where its particles are oriented and "compressed" acording to the strength of the magnetic field that crosses them.

These limaduras are basically the ones that work as union fluid between rotor and strator. The behavior is symilar to the one of the electric brakes, but they can restrain from much more low regimes (almost from the stop). Against them there is its limited capacity to dissipate calorific energy, reason why they are used in low power applications.