Adaptive full order observer based stable speed sensorless scheme for vector controlled induction motor drives

The full-order Luenberger flux observer (FOLO) combined with adaptive techniques is the most widely used method for sensorless induction machine (IM) vector controlled drives. However, IM model-based sensorless schemes have an inherently unstable region under regenerating conditions. Additionally, the stator resistance is also required at low speeds, which leads to a complicated multiple-input multiple-output (MIMO) system. In this paper, the simplification of the MIMO system into a single-input single-output (SISO) system by the two-time scale approach is proposed for a simple, but systematic, design procedure of the sensorless scheme. The stabilization of the regenerating region is solved by the "observer gains retuning" method and a novel "augmented error signal" for the stator resistance estimator. In addition, the IM model adopted allows combining the stabilization strategy with a simple and flexible observer gains tuning method based on the current and voltage estimator models. This method has significant promise for improving observer characteristics such as estimation convergence speed, damping and sensitivity against parameter mismatch. The suggested scheme is suitable for any vector controlled drive and it has been successfully validated in a 7.5kW experimental testbench with the rotor flux oriented vector control (RFOC) drive and the indirect self-control (ISC) drive.