US Patent 7057316 Downhole torque generator

A torque-generating apparatus includes a first assembly (10, 25) including a generally cylindrical member of magnetically soft material and having a longitudinal axis and a second assembly (2) arranged coaxially within the first assembly and including an electromagnetic winding (4). The first assembly (10, 25) and the second assembly (2) are rotatable relative to each other about the axis. Relative rotation between the first (10, 25) and second (2) assemblies induces a magnetic field which generates rotational torque between the first and second assemblies.