Utility of Motor-Speed Measurements in Pumping-Well Analysis and Control

Abstract

Summary. This paper describes how, motor speed can be used to deduce operating conditions in rod-pumped wells. Through the use of motor performance data, speed can be used to infer gearbox torque, unit balance, performance data, speed can be used to infer gearbox torque, unit balance, motor load, qualitative dynamometer cards, and power consumption. Pumpoff control and continuous monitoring are also applications of the methods discussed. Introduction The electrical prime mover on a rod-pumped well is responsive to the load imposed on it. When load increases, the prime mover slows down. Similarly, when load decreases, the motor responds by speeding up. By virtue of the motor's reaction to load, motor speed can be used to diagnose operating conditions in the equipment, to perform pumpoff control, and to monitor the well perform pumpoff control, and to monitor the well continuously. This paper describes how motor speed can be used to deduce items of practical interest. With respect to equipment loading, gearbox torque can be deduced from motor speed and the relationship between speed and motor torque. Unit balance can be sensed and counterbalance can be adjusted to minimize loading and to conserve power. An unexpected result is that reasonably precise power. An unexpected result is that reasonably precise dynamometer cards can be inferred from motor speed and unit geometry, even without use of a dynamometer. When suitable motor performance data are available (relationships between motor speed. torque, current. and efficiency), a qualitative electrical analysis can be performed. even without electrical instrumentation. Power performed. even without electrical instrumentation. Power consumption, power factor, and current levels can all be inferred from motor speed and suitable manufacturer-supplied data. In continuous monitoring, motor speed can be used to infer. The integrated motor output during a stroke is computed by use of speed and mathematical relationships between instantaneous output power and speed. Because motor output is used to lift fluid and to overcome friction, it is reasonable that a decrease in motor output indicates a decrease in fluid lifted because friction is approximately constant whether the well is pumped off or not. The well is sensed to be pumped off when motor output power (derived from speed) drops sufficiently below output power required when the well is on the verge of pumping off. In continuous monitoring, motor speed can be used to infer rod parts, worn pumps, tubing leaks, and responding wells. This paper presents several comparisons of actual and inferred performance to demonstrate the utility and validity of the concepts. Electrical and Mechanical Performance of Induction Motors Most beam-pumping units are powered by three-phase induction motors. Their electrical and mechanical behavior is represented graphically in Fig. 1, which shows example curves for a 100-hp [75-kW] Nema D motor. In principle, all performance parameters can be inferred from three speed relationships with torque, line current, and efficiency. A key item is motor output torque. As shown in Fig. 1 a large torque is produced at zero speed (locked rotor condition), which provides the starting torque required for the pumping unit. Output torque is zero at synchronous speed. Should the motor be driven past synchronous speed by the counterweights, a negative braking torque is produced. In the operating range near synchronous, speed produced. In the operating range near synchronous, speed is seen to decrease as higher output torque demands are place on the motor. place on the motor. The current is another important parameter shown vs. motor speed in Fig. 1. At locked rotor conditions, a large current is drawn. As motor speed increases, current decreases to a minimum value at synchronous speed (called the magnetizing current). If the motor is driven by the pumping unit into the braking region, line current again begins to increase. The ratio of- output to input (efficiency) is the final parameter. At locked rotor conditions, efficiency is zero parameter. At locked rotor conditions, efficiency is zero because output is zero (the rotor is not turning. At synchronous speed, efficiency is again zero because output torque is zero. Efficiency rises to a maximum value between locked rotor and synchronous speeds. As Fig. 1 shows, the efficiency can be very low at speeds near synchronous. This is why oversized motors that run too near synchronous speed tend to be less efficient than smaller motors that tend to run in a more optimum range of their efficiency curves. If the motor is driven past synchronous speed, efficiency against rises to a local maximum and then declines at higher speeds. SPEPE p. 199