Patent ID: 8444393

Claim:
A method of continuously determining operating parameters of a rod pump used in oil or gas production, the rod pump including a rod string carrying a down hole pump and a drive system including an AC electrical drive motor having a rotor coupled to the rod string through a transmission unit, the rod string including a polished rod, said method comprising the steps of: using voltage sensors and current sensors associated with the drive motor to continuously measure electrical voltage applied to the drive motor and electrical current drawn by the drive motor, and using the measured values of electrical voltage applied to the drive motor and electrical current drawn by the drive motor to calculate nearly instantaneous torque and nearly instantaneous velocity inputs to the rode pump without external instrumentation, wherein the external instrumentation includes rod load sensor, rod position sensor, down hole sensor, echo meter or flow sensor; and using parameters related to the geometry of the rod pump and said torque and velocity values to calculate values in real-time, of polished rod load, and wherein calculating said values of polished rod load includes the steps of deriving a crank angle value from the motor velocity values, using the crank angle value and the values of electrical torque in calculating values of polished rod load, and including using parameters representing the geometry of the rod pump together with the crank angle value to calculate a torque factor, and including using an overall gear ratio of the transmission unit to calculate from the electrical torque value, the output torque of the transmission unit, and further including the steps of: using parameters representing the geometry of the rod pump and the motor velocity values to derive a value of rotary weight torque for the crankshaft; using parameters representing the geometry of the rod pump and a torque factor derived from the parameters representing the geometry of the rod pump and the crank angle to obtain a value of load inertia for the drive motor; deriving from the motor velocity values, values of instantaneous acceleration of the rotor of the drive motor; and using the rotary weight torque value, the load inertia value, the motor acceleration values, the torque factor, the electrical torque value and at least one characteristic value of the rod pump to calculate instantaneous load for the polished rod, wherein all the calculations and derivations are performed by a processing unit.