Abstract: An electromagnetic coupler couples a first rotating element with a second element with a torque transfer between the first and second elements. The coupling is solely by a magnetic field which can be fully coupled, fully uncoupled or modulated to provide a variable transmission of power between the first and second elements. The electromagnetic coupler comprises a set radially disposed electromagnets and a concentric set of pole pieces separated by an air gap. The magnetic field between the electromagnets and the pole pieces is increased as the current through the electromagnets is increased. At low current, there is substantial slippage between the respective driving and driven elements. With full magnetic coupling, the electromagnets and pole pieces are fully coupled or without any slippage therebetween. When the magnetic field is eliminated, there is no torque transfer between the driving and driven elements.