Patent ID: 7057316

Claim:
A down-hole torque-generating apparatus comprising: a first assembly ( 10 , 25 ) including a generally cylindrical member of magnetically soft material and having a longitudinal axis and a wall thickness; 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 ) being rotatable relative to each other about the axis; the arrangement being such that relative motion between the first ( 10 , 25 ) and second ( 2 ) assemblies induces a magnetic field which generates rotational torque between the first and second assemblies; a substantially annular space defined between the first and second assemblies for the passage of drilling mud therethrough; a plurality of longitudinal grooves ( 20 ) provided in an inside surface of the cylindrical member of magnetically soft material, the magnetically soft material of the first assembly being positioned adjacent to the substantially annular space between the first assembly and the second assembly and the longitudinal grooves ( 20 ) extending only partially through the wall thickness of the cylindrical member of magnetically soft material; and control means for controlling energisation of the electromagnetic winding ( 4 ) whereby the electromagnetic winding ( 4 ) is temporarily de-energised to allow the torque-generating apparatus to run without electromagnetic excitation such that magnetic particles are expelled from the torque-generating apparatus.