Indirect field-oriented control of induction motors is robustly globally stable

It is generally accepted that eld orientation, in one of its many forms, is an established control method for high dynamic performance AC drives. In particular, for induction motors indirect eld oriented control is a simple and highly reliable scheme which has become the de facto industry standard. In spite of its widespread popularity no rigorous stability proof for this controller was available in the literature. In a recent paper 15] we have shown that, in speed regulation tasks with constant load torque and current fed machines, indirect eld oriented control is globally asymptot-ically stable provided the motor rotor resistance is exactly known. It is well known that this parameter is subject to signiicant changes during the machine operation, hence the question of the robustness of this stability result remained to be established. In this paper we provide some answers to this question. First, we use basic input-output theory to derive suucient conditions on the motor and controller parameters for global boundedness of all solutions. Then, we give necessary and suucient conditions for uniqueness of the equilibrium point of the (nonlinear) closed loop, which interestingly enough allow for a 200% error in the rotor resistance estimate. Finally, we give conditions on the motor and controller parameters, and the speed and rotor ux norm reference values that insure (global or local) asymptotic stability or instability of the equilibrium. This analysis is based on a linear time varying change of coordinates and classical Lyapunov stability theory.