An electromagnetic clutch includes an armature plate connected to a shaft and being movable axially with respect to the shaft, a rotor being rotatable relative to the shaft, and a plurality of electromagnetic coils, wherein when at least one of the electromagnetic coils are energized a magnetic field...http://www.google.fr/patents/US7467701?utm_source=gb-gplus-shareBrevet US7467701 - Multiple-stage magnetic clutch coil
