| 简介 |
A downhole source of electrical power for drilling tools is driven by the rotation of a drillstring or, if the drillstring is not rotating, by the rotation of a drilling sub. The source consists of an alternator having a rotor made up of a series of coils that are mounted on, and spaced around the circumference of, a rotating drilling sub, and a stator made up of a multiplicity of permanent magnets also spaced around the circumference of the drilling sub. The stator is mounted on the drilling sub by means of bearings and incorporates a counterweight that holds the rotor relatively stationary with respect to the sub and with respect to the coils, so that rotation of the sub, either by rotation of the drillstring or by rotation of a drill motor in the drilling sub, produces relative rotation of the rotor and stator to generate an AC electrical output. The rotor and stator surround an axial fluid passage through the center of the alternator so that they do not impede the flow of the drilling fluid, and the motor is totally sealed to prevent damage to the bearings. The alternator stator is an annular permanent magnet structure which surrounds the drilling axis, and which may include two sets of rare earth disc magnets spaced axially to form an axial gap, with the counterweight holding the magnets stationary with respect to the alternator coils. The coil structure is also annular and also surrounds the drilling axis, with the coil structure preferably including two sets of offset coils positioned to rotate in the gap between the permanent magnets. The two sets of coils make up a two-phase system; if desired, a single set of coils may be used to provide a single-phase system. |
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