A Simple Torque Estimator for In-Service Efficiency Determination of Inverter-Fed Induction Motors

This article deals with air-gap torque estimation in inverter-fed induction motors. Torque estimation is an important stage in the methods used to evaluate energy consumption and efficiency. Those methods are essential to monitoring electric motors and processes in mining and metal industries. As some of the drives have voltage and current sensors already installed in them for condition monitoring, it is very attractive to use measurement data available to perform torque and efficiency estimation. It is highly desirable to implement these functions without process interruption and/or requiring minimum changes in the drive configuration. Most of the torque estimation algorithms require two or three voltage and current measurements to be implemented. Aiming to reduce the number of sensors and obtain a lower cost setup, a strategy to estimate torque that requires data acquisition of the current and voltage of only one phase suited to be used in algorithms of efficiency estimation of in-service induction motors was presented in a previous article by the same authors. There, the performance of the method was investigated when applied to induction motors directly connected to the power grid. In a second published article, the performance of the method was additionally investigated for induction motors supplied by means of pulsewidth-modulated frequency inverters. The effects of unbalance in voltage supply were also described and analyzed there. In this article, in addition to the analysis performed in the last published article, a measurement uncertainty analysis is done. Simulation and experimental results are used to check the validity and performance of the proposed technique.