Counterweight Torque (and Counterbalance Equivalent) Versus Gearbox And Motor Loads

Capital and power consumption costs for sucker-rod pumps depend, in part, on the counterweight torque. Counterweight torque is the primary variable because peak torque depends on how well the well is balanced. The secondary variables affect the ideal counterweight torque (motor slip, pumpjack efficiency, net lift, and torque equation). High slip motors allow more crank speed variation. Pumpjack efficiency (preventive maintenance) affects power consumption. Net lift changes the net crankshaft torque as the well pumps off. The general form of the net torque equation more accurately converts polished rod forces into equivalent moments at the crankshaft than the simplified counterbalance equivalent equation. Each combination of the primary and secondary variables may have a unique counterweight torque that minimizes the peak net crankshaft torque by equalizing the largest upstroke and downstroke torques. The peak torque affects capital costs for the assumed operating and preventive maintenance conditions by determining the smallest acceptable gearbox and motor sizes. Fully loaded motors use less electricity because power factor and electrical efficiency go up with the ratio of brake horsepower to motor horsepower. In short, it is possible to cut capital and power consumption expenses by managing the difference between the ideal and the actual counterweight torques throughout the life of the well.