Observability-Index-Based Control Strategy for Induction Machine Sensorless Drive at Low Speed

This paper focuses on the observer-based methods for induction machine sensorless drive. It is well known that the use of speed observer is limited at a very low stator frequency due to speed unobservability. Some existing methods propose to avoid the zero-stator-frequency working points. These methods are nonetheless limited to high-torque operations. The first contribution of this paper is to analyze finely the speed observability, which leads to the definition of an observability index. The correlation between this index and the observer performance is analyzed and illustrated. The second contribution is to propose a control strategy that maximizes the observability index and improves the observer performance. The proposed strategy is tested first in open loop to evaluate the speed observer performance, and then, in closed loop to illustrate its pertinence for induction machine sensorless drive. All these results are validated by an experiment. The performed experimentation shows that the proposed control strategy gives better results than the existing method that avoid the zero-frequency working points. It offers a new path for induction machine sensorless drives.