Jim McCoy & Bill Drake, Echometer Co.; Jim Jennings, Consultant; A.L. Podio, University of Texas at Austin
A power/current transducer for measuring both power and apparent current permits an operator to obtain a more complete analysis of the performance of a motor powered pumping unit system. Power and apparent current are acquired and displayed during a single stroke to aid in the analysis of pumping unit efficiency, cost of electricity, proper balance and torque. Power measurement, for induction motors, showing both consumption and generation are much easier to analyze than apparent current data from a conventional clamp-on amp meter which does not distinguish between consumption and generation. Power transducers require the measurement of current in two legs of a 3-phase system. In addition, voltage probes must be attached to each of the three phases. The phase relationship between the current and voltage is processed electronically to obtain a voltage signal proportional to power. This signal is digitized at a rate of 20 times per second and displayed as a plot of power per single pumping unit stroke. Apparent current is also obtained and plotted for additional analysis. The power data is easily processed to obtain gearbox torque without the need for pumping unit geometry nor polished rod loads. Field data are given for examples of motor performance including cost of electricity with and without generation credit, an under balanced unit, an overbalanced unit, an under loaded motor and other conditions.