Combined Positive-Sequence Flux Estimation and Current Balancing for Sensorless Motor Control Under Imbalanced Conditions

Abstract

Sensorless motor control under imbalanced conditions, commonly caused by the use of an imbalanced cable, poses a number of challenges relating to stability and power quality. With reference to stability, concerns arise because the angle and frequency estimates of conventional phase-locked loops (PLLs) and observers deteriorate in the presence of imbalance, which in turn degrades the response of synchronous-reference-frame current controllers. Power quality is also degraded due to the asymmetry of the currents supplied to the motor, which results in torque ripple and increased motor losses. In this article, an adaptive positive-sequence flux estimator based on second-order generalized integrators is presented to solve these problems. The balanced fluxes generated by the estimator are suitable for PLL-based sensorless control of a motor over an imbalanced cable. With negligible additional computational effort, the flux estimator also provides negative-sequence current estimates, which are then controlled to balance the motor currents. Simulation and experimental results with a permanent magnet synchronous motor run by a commercial motor drive via a flat long electrical submersible pumping cable are presented. It is shown that the proposed method can prevent instabilities that occur when using conventional flux estimation methods and reduce current imbalance by approximately 10 to 20 times, to less than 1%.