Abstract
Electrical traction using induction machine sensorless control requires high observer performance for all speed ranges, even for low speed or regenerative braking conditions which appear frequently during long time. It is well known that the speed of induction motors is unobservable at very low stator frequencies. This paper uses an observability index to continuously analyze speed observability for sensorless control of induction machines. The correlation between observability-index and observer performance is illustrated in a Hardware in the Loop (HIL) experimental test-bench combining the well-known vector control with an extended Kalman filter. Thanks to the observability-index information, an optimal strategy is proposed to design controllers to guide the system away from undesirable behavior and avoid the weak observability-index region by taking into account all working constraints. A simplified case is presented to improve the speed observer performance, which was tested in the same conditions with the same HIL test-bench to experimentally validate the proposed sensorless control for traction applications.
1. INTRODUCTION
To reduce the number of physical sensors, and the direct and maintenance costs they induce, industrial solutions have been based on state observers for years. For an induction machine drive, the speed sensor is the most critical because of its cost and its high failure rate compared to current and voltage sensors. In the case of a traction application, the high failure rate of the speed sensor is mainly caused by the harsh environment (vibration, impacts, temperature...). However, in this application, it is compulsory torque range. The problem of the speed sensorless induction machine drive has been first addressed in the 1980s (Tamai et al. (1985)).
6. CONCLUSION
This paper proposed to go deeper in the observability analysis of induction machine sensorless in order to define an observability continuous measurement. Its application to induction machine sensorless drive evidences two main results: first, the direct link between the observation accuracy and the observability continuous measurement is highlighted; second, this measurement has been considered in the control in order to avoid the weak observability region. The proposed control strategy is illustrated through a simplified case named observability-index based control applied to induction machine sensorless drive. Experimental results on a HIL test-bench validate the proposed control for realistic railway traction conditions. It shows that the proposed control can ensure the accuracy and the dynamic of the speed observation, particularly for low speed conditions during long time. The proposed control approach has the potential to be applied easily to any kind of induction machine.
