Accuracy limitations of velocity and flux estimation in direct field oriented induction machines

The authors focus on the accuracy and robustness limitations of direct field oriented (DFO) systems incorporating velocity estimation that are based solely upon measured stator voltages and currents. The analysis is limited to integrated velocity and flux estimation methods that rely upon the induction machine back EMF. After assessing the limitations of a model reference adaptive controller (MRAC) approach, an improved topology utilizing an integrated closed-loop flux observer and a mechanical system model is proposed and experimentally verified. Given some knowledge of the machine load, the proposed observer can potentially improve both the velocity estimation dynamics and the transient low speed field orientation. However, it does not overcome the fundamental lack of robustness at zero speed and loss of accuracy due to parameter sensitivity. Finally, the existence of the same limitations in alternative approaches to velocity estimation and field orientation is discussed