Downhole motors consist of a section of drill string that contains a progressive cavity displacement pump (PCDP). The helical rotor of the PCDP is driven by the flow of drilling fluid, commonly referred to as mud, through the motor. This eccentric motion of the rotor is transferred to concentric power that drives the drill bit.
The power of the downhole motor is dependent on the number of lobes and the length of the rotor, as well as the viscosity of the drilling fluid. They are described by the number of stages, lobe ratio, and external diameter. Performance specifications include flow rate, rotational speed of the drill bit, and drilling torque. Specialized downhole motors are used for directional drilling.