Stalled Drilling Motors produce no torque
The classic motor theory is that torque output increases with differential pressure up-to the moment a motor stalls - i.e. is incapable of providing enough torque to maintain shaft rotation.
I was taught that at stall the motor produces max differential pressure, maximum torque, but no RPM. But I think this basic statement is flawed.
This is illustrated on the generic Motor Performance Chart shown below;
However, the definition of torque is defined by Google as follows;
In physics, torque is the tendency of a force to turn or twist. ... The force applied to a lever, multiplied by the distance from the lever's fulcrum, multiplied again by the sine of the angle created, is described as torque. This is also known as "r cross f," or "force times fulcrum distance times sine theta."
When the motor is in a stalled condition, it produces no rotation and so there is no "theta". Hence a stalled motor produces no torque.
So - would it not make more sense to amend the Performance Chart as follows;
This indicates that a stalled motor still produces differential pressure, but no torque.
As the motor approaches stall the drill stem is twisted more and more. When stalled, the motor does not add anymore torque to the drill string but there is a lot of torque trapped in the pipe, and it is the stalled motor that is preventing the string from unwinding. Anyone who has ever run a motor has seen this over and over.?
Ex-senior Directional Driller, retired from Halliburton. Open to short term consultancy work.
6 年Hi Chris, Happy New Year mate.? I have to disagree with your conclusions above. If there is differential there has to torque of a kind. The closest analogy I can think of to a stalled motor is a watch spring. Clearly a watch spring has torque, initially applied by turning a key and then held until some pendulum swings or whatever moves a lever which releases some of the torque stored in the spring. Until that point the torque is held in a static equilibrium. When a motor stalls the bit no longer has sufficient power to cut the formation, it still has torque which causes the big increase in reactive. akin to the winding of a watch spring, driven by the force of the mud on the motor. This almost instantly reaches a maximum as the string resists the reactive torque twisting it. The mud is then forced to deform and chunk out the rubber of the stator and bypass the stalled rotor. If the flow rate doesn't change you now have a state of equlibrium where nothing moves but there is plenty of potential energy stored in the wound up string (a massive torsion spring) and some element of differential that would rather turn the motor than bypass it. This stored energy is still applying torque even though it is unmoving. Torsion springs, according to Wiki, 'obey an angular form of Hooke's Law'? (I'm not going to pretend I know what that is) ie?? T= -Ksigma Where T= the torque exerted by the spring, sigma is the angle of the twist (eg the total amount of reactive and might be the equivalent of your theta above) and K is a constant (the 'spring constant'). Torque is there and I think in total it is similar to the graph vs diff, it's just held in equilibrium like a watch spring waiting for a change and a chance to release. I'm not an engineer but this is what makes sense to me....? Mike
Technoion - Waste Recycling Plant (Hydro-Metallurgy)
6 年When you start applying WOB (loading the motor) the RPM will be dropping
Technoion - Waste Recycling Plant (Hydro-Metallurgy)
6 年Chris, you are wrong! The chart is for free running motor.
Senior Directional Driller at Schlumberger
6 年HI Chris; While running 3 3/4'' Ultra HS Motor for slimhole with lower torque 275 ft.lb as per data sheet; What would be the reactive Torque when the motor is getting Stall; Double; 03 times.. .....10 times ????? How can this reactive torque can break the connection ( unscrew ) with 3Kft.lb; while reducing pumps & Picking up.