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

Monitoring energy consumption and condition of electric motors is important for economical purposes and safe operation of drives and processes in the industry. These activities are especially valid in processes requiring high availability such as those found in mining and metals industry. Methods able to implement these functions without process interruption and/or requiring minimum changes on the drive configuration are extremely welcome. The air-gap torque estimator is at the core of the main efficiency-estimation methods used today; therefore, it is crucial to its successful implementation. Most of the torque estimation algorithms require two or three voltage and current measurement to be implemented. Unfortunately, this number of sensors is not always available for all the motors used in industrial processes. Aiming to face this problem, a solution requiring 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 paper. The installation in the motor control center cabinet of a pair of sensors to measure these variables is more feasible and, sometimes, it is done for monitoring and protection purposes. In this paper, the method governing equations are simplified further and the performance under unbalanced voltage condition investigated and improved. Simulation and experimental results are used to check the validity and performance of the proposed technique.