Postive displacement motor (PDM) is a downhole motor, the high drilling fluid enter into the downhole motor making the high fluid power converted into mechanical energy, and the rotor drive the principal shaft and bit rotating, drilling. It meets to API.

Mud stream from the outlet of mud pump flows through a by-pass valve into the motor. This stream produces pressure loss between inlet and outlet of the pump, to push the rotor into rotating, and to transmit the torque and speed to the bit.