Motor Power/Current Measurement for Improving Rod Pump Efficiencies

Abstract

The development of accurate digital measurement of instantaneous power during a pump stroke has made possible a very quick and detailed analysis of the efficiency of the pumping system. The efficiency is then used as the benchmark for determining whether a complete well performance analysis is warranted from the standpoint of making best use of personnel and economic resources to increase oil production. In addition, power measurement provides direct information about lifting cost per barrel of fluid and barrel of oil produced, electrical and mechanical loading of the prime mover, peak power demand, power factor and minimum required ratings. These results give operating personnel information regarding potential problems and give to management a complete picture of the distribution of pumping costs. The power measurements are also converted, by the software, to instantaneous torque and presented as continuous torque curves for the upstroke and downstroke. This allows determination of the existing level of counterbalance and provides the most rapid and accurate method for counterbalance adjustment to achieve lower torque loading on the gear box and reduced energy utilization. One of the principal advantages of this balancing method is that counterbalance adjustment can be made without need for an accurate description of the pumping unit's geometry which is often unknown or inaccurate. The effect of counterweight displacement on torque and power is observed immediately by repeating the power measurement after relocating the counterweights. This paper presents a series of case studies showing the application of power measurement to a variety of pumping systems and components, including conventional, Mark II, Rotaflex units and high efficiency motors. INSTANTANEOUS MOTOR POWER MEASUREMENT A system was designed and implemented to undertake quantitative measurement of instantaneous power using sensors consisting of two current probes and three voltage leads, which are connected to the three phase leads inside the units switch box. Special purpose integrated circuits process the sensors data so as to generate an analog signal which is proportional to the instantaneous power. The sensors are calibrated so as to determine the power with an accuracy better than 5% provided the probes are correctly installed. The measurement procedure must be followed closely in order to obtain data of good and repeatable quality. In general the user is interested in establishing the power use of the pumping system when it is operating under steady state conditions. In the case that the well is pumping a full barrel and then begins to pump a partial barrel of liquid, the measured power will vary and will not be representative of the normal operating conditions. Therefore it is advisable to insure that the well being tested is produced while testing at the same conditions as normal operations. This can easily be undertaken by running quick dynamo meter measurements. When power measurements are to be made with the purpose of comparing the efficiency of different motor wiring options (low, medium or high torque for example) it is important not to move the current sensors after installation so as not to change the relative position of the wire within the current sensor. Such change would cause small variations in the readings which might invalidate the conclusions of the test. The data for two successive pump cycles are acquired with a high speed, high precision A/D converter and processed by a portable PC. The software then generates graphic and tabular output screens which are saved on disk for subsequent printing. Figure 1 presents the information related to energy utilization. P. 815^

Study on Balance Criterion of Pumping Unit

“…In practical application the current method can not judge the balance of pumping unit exactly. So the power method and peak torque method of crank shaft are discussed widely, but there is no common opinion [2,3,4]. In order to save energy, prolong life of reduction gearbox, operate easily, the basic principle of balance criterion is analyzed.…”