One method of producing oil from a subterranean reservoir is through the use of submersible pumps located in deep wells. Such pumps typically are powered by alternating current (A.C.) motors, which are often automatically stopped for a variety of reasons, such as momentary overloads due to power fluctuations and well shut-downs occurring as a result of pressure variations or equipment problems. When this happens the momentum of the spinning motor causes it to continue to spin in a forward direction for a brief period of time. Thereafter the liquid in the casing above the pump begins to flow back down into the well, turning the vanes of the pump in the reverse direction, thereby causing the motor to rotate in a counter direction. Depending upon the height of the liquid column, which often may be as much as several thousand feet above the pump, the motor may continue to backspin for quite a long period of time, as long as an hour in some cases. By waiting for the backspinning to stop before starting the pumping operation once more, valuable production time is lost. The alternative of starting the pump motor before backspinning terminates is too risky, however, in view of the possibility that this could cause the motor or pump shaft to twist off due to excessive torque or could burn out the motor as a result of the extra time that locked motor amperage will be applied.
In addition to recommending against premature starting of a backspinning motor, pump manufacturers have attempted to solve the problem by incorporating delay timers into the circuitry of the unit to prevent restarting of the motor for a period of time calculated to allow the backspinning to cease. Since all situations cannot be foreseen, the time delay selected can be quite a bit longer than is actually necessary or, in some cases, less than that actually required. Another preventive measure has been to incorporate backspin detection systems in the pump unit to prevent restarting as long as the motor continues to backspin. While this in theory prevents the motor from being harmed due to premature starting, it does nothing to alleviate the long production delays resulting from the backspinning phenomenon. Moreover, such measures have not proven reliable in practice. The use of downhole or surface check valves to prevent downward flow of the column of liquid in the production tubing has also been employed, but this too is unreliable due to leaking and plugging of the check valves, which allows the downward passage of fluid to cause backspinning of the motor to occur.
In view of the problems outlined above, it would be highly desirable to be able to not only prevent premature starting of a submersible pump motor, but also to reduce the duration of the backspinning of the motor. It is therefore a primary object of the invention to provide a means for stopping the backspinning of a motor prior to restarting it.