Power or motor section
A drilling mud motor has three main parts: the housing, a steel stator, and an elastomeric rotator. Essentially, it operates as a reverse Moineau pump. Drilling fluid is pumped into the motor’s progressive cavities. As the fluid moves, it forces the shaft to rotate inside the stator.
In short, the power section transforms the hydraulic energy of the drilling fluid into mechanical power. This mechanical power, produced by a multi-lobe rotor/stator system, results from a combination of high torque and rotational speed.
Stator
The elastomeric stator is carefully bonded inside a steel tubular component, an integral component of the motor
casing. The number of lobes varies from 4 to 11.
Rotator
Mud motor manufacturers design the steel rotor with lobe profiles and a helical pitch that match the stator but with one fewer lobe. This allows the rotor to fit and move inside the stator. When drilling fluid flows through, it generates hydraulic pressure, causing the rotor to precess and rotate within the stator.
Drilling mud motors are categorized into three types: slow-speed, medium-speed, and high-speed. These classifications depend on the pitch of the motor stages and the number of lobes and cavities in the stator. The speed and torque output of the power section are directly influenced by the number of lobes on both the rotor and stator, as shown in figure 1 &2 below.
Different rotor/stator lobe configurations allow adjustments to torque and rotational outputs. Configurations with more lobes generally produce higher torque. The differential pressure across the power section directly determines the output torque. However, the mechanical strength of the stator elastomer limits the maximum torque.
Increasing the power section length and the number of lobes boosts the total chamber volume and torque output. A longer power section improves volumetric efficiency but may slightly lower mechanical efficiency.
Drilling Mud Motor Rotor Catch Device (RCD)
Rotor Catch in down-hole Drilling Motors secures the lower parts of the mud motor in case of a twist-off or backoff.
On rare occasions, motors, mainly the power section, have parted below the Stator. When this occurs
generally, the stator emerges from the hole, leaving the Rotor facing up the hole and resulting in a fishing
job over an uneven surface.
Also, if the drill bit is free, it is hard to rotate over the Rotor to fish it. The Rotor catch sits at the top of the Power
Section and screws directly into the Rotor. If the tool should part, the Rotor Catch will contact an internal
shoulder and retrieve the Rotor.
The RCD includes a retainer rod with a mushroom-shaped retaining disk that hangs up on a stop ring inside the
stator tube. The disk retains all inner parts of the motor and all outer parts below the back-off or twist-off,
minimizing the probability of a fish remaining in the hole.
