In order to increase the average road speeds of heavy duty commercial vehicles on flat roads and during uphill passages more powerful engines are continuously installed by the vehicle manufacturers. During downhill passages the average road speed has to be increased just as well, whereby the service braking system of a vehicle is subjected to extreme performance requirements. This fact leads to an intensified continuation in the development work and the augmented use of wear-free brakes. The most diversified retarder types have been known for some time now, however, the requirements demanded on these wear-free retarders have risen quite drastically during the last years due to economic and safety related reasons.Not only a conceivably simple integration into the vehicle braking system is demanded on the retarders, but also high braking power covering the entire road speed range. Low weight ratings and simple operation by the driver are only a few additional characteristics which have to be added to the list.