High speed positive displacement motor

According to the present invention, a downhole motor converts hydraulic energy from drilling fluid passing through the motor into mechanical power that is useful to rotate a drillbit. The downhole motor includes a tubular housing and a stator mounted within the tubular housing. A rotor is positioned within the stator for rotation relative thereto. The stator has a central first axis and the rotor has a central second axis. The rotor is rotated about the second axis and nutated about the first axis within the stator by drilling fluid passing through the stator. The rotor further has a concentric lower end portion thereon. A cylindrical coupling is disposed within the tubular housing beneath the rotor and has an axis substantially aligned with the first axis and an inner cylindrical opening the axis of which is substantially aligned with the second axis, the opening being adapted for receiving the lower end portion of the rotor. A first bearing assembly is disposed in the opening of the coupling for rotatably receiving and supporting the lower end portion of the rotor, whereby nutation of the lower end portion of the rotor about the first axis induces rotation of the coupling about the first axis. A second bearing assembly is disposed between the coupling and the tubular housing for supporting rotation of the coupling relative to the tubular housing. A transmission shaft and bit box are connected to the coupling for rotation. In this manner, nutation of the rotor about the first axis results in rotation of the transmission shaft and the bit box. A related method is also provided.