Abstract: A method and apparatus for controlling the operation of an oil well pumping unit in which the prime mover is a polyphase AC electric motor. The system comprises means for generating a plurality of signals representative of motor fault parameters including a motor winding overtemperature fault, undervoltage fault, and a phase imbalance fault. Upon occurence of a fault, a contactor in the motor supply leads is open to shut down the motor. After such shutdown because of undervoltage or phase imbalance faults the fault parameters are continuously scanned and once the fault has been cleared the motor is restarted. Where motor shutdown occurs because of an over temperature fault, the motor is maintained in a latched off shut down condition until an operator implemented reset occurs. An accessible counting memory is provided having separate registers for each of the fault conditions.

Abstract: Method and apparatus for providing an electric motor such as used on an oil well pumping unit with stall protection. The system comprises means for generating a signal representative of motor speed and for establishing at least two stall speed set points corresponding to upper and lower stall speeds of the motor. Upon the motor speed signal reaching the upper speed set point for one time duration or the lower stall speed set point for a second short time duration, a motor control function is generated. The system may be used for motors having a plurality of operating modes. In this case, at least two stall condition suites, comprising at least two upper and lower stall speed set points as described above, are established corresponding to at least two motor modes, e.g., high torque and low torque modes. The motor winding leads are monitored to identify whether the motor is in the high torque or low torque mode and the appropriate stall condition suite selected.