Power section consists of stator and rotor. Stator is made by squeezing rubber sleeve on the wall of steel tube. There forms spiral structure with a certain geometric parameter. Rotor is a chrome-plated screw rod.
Stator and rotor matches with each other, to form spiral line and seal cavity through their guide rail difference. With rotor running in the stator, the seal cavity is moving along its axial direction, countinuously forms and disappears to complete its energy conversion. This is the basic principle of downhole motor.
Spiral seal line on rotor is divided into single end and multi-end (stator with one more end than rotor). The less ends the motor has, the higher speed and the lower torque is. The more ends it has, the lower speed and the higher torque is.
A guide rail forms a seal cavity in motor stator. This is called one stage. When the rated working pressure of each stage lows down to 0.8Mpa, the maximum pressure loss will be 1.3 times of the rated pressure. For example, for 4-stage motor, the rated pressure loss shall be 4.2Mpa. As the pressure loss is over such value, motor may bring about leakage and speed will quickly slow down. More seriously, it may cause operation into complete stop, even cause the motor to be damaged. This is the caution for the customers.
In order to ensure rotor seal in good condition, the matching size between rotor and stator relates closely to depth temperature. Therefore, correct downhole temperature shall be provided to the manufacturer for reliable selection of appropriate motor. The actual mud flow rate is required in the recommended range. Otherwise incorrect flow rate may affect the motor efficiency, even cause speeding up motor wear.
Motor torque is proportional to its pressure loss, but its outlet speed is inversely proportional to mud flow rate. With load increasing, screw drill speed decreases. Thus, pressure shall be controlled by the pressure gauge and flow rate shall be controlled by the flow rate gauge on surface. This may control both torque and speed of downhole motor.
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