Starting Characteristics of Electric Submergible Oil Well Pumps

The electric submergible pump (ESP) consists of a centrifugal pump powered by a medium-voltage three-phase induction motor. Being constrained to operate within common well casings, the ESP is rather unique; for example, a 500-hp motor may be less than 6 in in diameter and more than 100 ft long. As such, its mechanical rotational behavior under starting conditions can be severe, and some actions normally thought to attenuate the severity can actually amplify it. Both analytic and experimental approaches to understanding the starting incident and its effects are presented. A model was developed to predict the electrical and mechanical conditions prevalent when starting with various types of reduced-voltage fixed-frequency starters. These include the solid-state soft starter, reduced-voltage starters using variable or switched series impedances and stepped-voltage starters. Direct on-line full-voltage starting was used as a base case. Model predictions for the various starting methods were compared against tests measuring transient speed, voltage, current, and torque. Test results indicate that some starting techniques can produce unstable operation, i. e., torsional vibration. In large horsepower ESP´s shaft strength safety factor is reduced due to space limitations and torsional vibrations must be minimized to avoid breakage due to fatigue. Depending on the application, configuration of the ESP and adequacy of available power, good motor starter design can be crucial to reliability. Finally, it is believed that this work has direct applicability to the starting of many more common large induction motors.