Methods and apparatus for controlling long-stroke pumping units using a variable-speed drive

A method for driving a downhole pump lifting system with a variable-speed motor comprising the steps of:

• (a) reciprocating the lifting system so that it completes pumping cycles comprised of an upstroke and a downstroke;
• (b) detecting a select portion of the lifting system as the portion passes a first select position in the upstroke of each pumping cycle and again as the portion passes a second select position in the downstroke of each pumping cycle;
• (c) generating a signal when the detection occurs;
• and
• (d) using the signal to control the motor by varying its speed each time the signal is generated.

An apparatus for controlling a pump lifting system comprising:

• (a) a lifting string including a rod string and a fluid pump;
• (b) means for vertically reciprocating the lifting string through a pump stroke, the means including a vertical drive and a reversing mechanism coupled to the drive;
• (c) a variable-speed motor driving the reciprocating means, comprising an electric drive motor with a variable-frequency power source;
• (d) first switch means for sensing when the reversing mechanism is approaching a top portion of the vertical drive and for generating a first signal;
• (e) second switch means for sensing when the reversing mechanism is approaching a bottom portion of the vertical drive and for generating a second signal;
• and
• (f) control means for receiving the first signal and for using it to slow the motor for a first selected portion of the stroke, and for receiving the second signal and for using it to alter the motor speed for a second selected portion of the stroke.