Adaptive rotor flux observer for direct field oriented induction motor drives

At the present time the direct field oriented (DFO) control is the widely used control technique in high-performance induction motor (IM) drive. It requires the implementation of a state vector observer for the detection of rotor flux vector. The IM drive performance is strictly connected to that of the rotor flux observer. The motor parameter variations cause the observer mismatching and hence, the IM drive performance degradation. In the present paper an adaptive observer is proposed which allows the on-line rotor time-constant tuning. The identification of the rotor time constant is performed by an MRAS approach. The adaptive observer is synthesized using the Lyapunov stability theory. To improve the speed convergence of the identifier and the selectivity characteristic of the proposed adaptive observer, a time-varying gain has been introduced. The stability of the overall system has been verified using the Lyapunov stability theory. The simulation results evidence the good performance of the proposed observer and of the overall DFO controlled drive. In particular, the interactive effects between observer and identifier gains are evidenced and, thus, used for the selection of the identifier time-varying gain.