Downhole Mud Motors
There are two major types of downhole motors powered by mud flow;
1) The turbine, which is basically a centrifugal or axial pump and

2) The positive displacement mud motor (PDM). The principles of operation are shown in Figure and the design of the tool are totally different.
Turbines were in wide use a number of years ago and are seeing some increased use lately but the PDM is the main
Four configurations of drilling motors provide the broad range of bit speeds and torque outputs required satisfying a multitude of drilling applications. These configurations include:
High Speed / Low Torque

Medium Speed / Medium Torque

Low Speed / High Torque

Low Speed / High Torque -Gear Reduced

The high speed drilling motor utilizes a 1:2 lobe power section to produce high speeds and low torque outputs. They are popular choices when drilling with a diamond bit, tri-cone bit drilling in soft formations and directional applications where single shot orientations are being used.

The medium speed drilling motor typically utilizes a 4:5 lobe power section to produce medium speeds and medium torque outputs. They are commonly used in most conventional directional and horizontal wells, in diamond bit and coring applications, as well as sidetracking.

The low speed drilling motor typically utilizes a 7:8 lobe power section to produce low speeds and high torque outputs. They are used in directional and horizontal wells, medium to hard formation drilling, and PDC bit drilling applications.

The gear reduced drilling motor combines a patented gear reduction system with a 1:2 lobe high speed power section. This system reduces the output speed of the 1:2 lobe power section by a factor of three, and increases the output torque by a factor of three. The result is a drilling motor with similar performance outputs as a low speed drilling motor, but with some significant benefits. The 1:2 lobe power section is more efficient at converting hydraulic power to mechanical power than a multi-lobe power section and also maintains more consistent bit speed as weight on bit is applied. This motor can be used in directional and horizontal wells, hard formation drilling, and PDC bit drilling applications.

Some other motor selections are also available including a tandem and modified motor. These variations are described below.

Tandem Drilling Motor – The drilling motor utilizes two linked power sections for increased torque capacity.

Modified Drilling Motor – The bearing section of the drilling motor has been modified to provide different drilling characteristics (ie. change bit to bend distance, etc.).

All drilling motors consist of five major assemblies:

Dump Sub Assembly
Power Section
Drive Assembly
Adjustable Assembly
Sealed or Mud Lubricated Bearing Section.