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 industry. Those activities are especially valid in processes requiring high availability such as those founded 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 and 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. Therefore, a solution employing a reduced number of sensors is necessary. In order to meet this requirement, this paper presents a new and simple technique to estimate torque that is able to be used in algorithms of efficiency estimation of in-service induction motors. The technique requires data acquisition of current and voltage of only one phase for its implementation. Sensors for measuring these variables are usually available in the Motor Control Center - MCC - cabinet for monitoring and protection purposes. From one phase current and voltage samples, instantaneous values of torque, angular velocity, power and efficiency are calculated. Simulation and experimental results are used to check the validity and performance of the proposed technique.