PUMP-OFF CONTROL FOR PUMPING JACK

摘要

A pumping jack drive system using an induction motor whose speed is controlled by a variable-voltage, variable-frequency power supply comprises means for detecting the speed of the induction motor and an instantaneous value of the secondary current; means for detecting the down stroke time for every cycle of each pumping jack; means for detecting the maximum value of the instantaneous secondary current at each down stroke; means for detecting and storing the time from each down stroke reference point to the maximum value of instantaneous secondary current, and means for setting the reference time to the maximum value of instantaneous secondary current to be compared with the stored value wherein the two are compared at the end of the down stroke for each cycle and, if the time that the instantaneous value of the second current reaches the maximum value is later than the set reference value, then a pump-off condition is considered to occur. Thus, an accurate pump-off and continuous production in an oil well can be performed.