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The influence of the rotor time constant mismatch on the global stability of induction motors under indirect field oriented control (IFOC) is analyzed. A test for global stability is provided, based on necessary and sufficient conditions for the existence of quadratic Lyapunov functions for IFOC drives. This test yields robustness margins with respect to rotor time constant mismatches. A typical example is presented to show that the robustness margins obtained are significantly less conservative than the ones obtained with previously proposed stability criteria. Index Terms--Global asymptotic stability, indirect field-oriented control (IFOC), induction motors, robustness.

A nonlinear robust output feedback control is designed for a sixth-order model of an induction motor. The control uses only measurement of the rotor position and stator currents. It contains two observers, a second-order observer to estimate the rotor flux from the stator current and a third-order high-gain observer to estimate the rotor speed and acceleration from its position. The control is robust to uncertainties in the rotor and stator resistances, and a bounded time-varying load torque. It guarantees that the speed tracking error can be made arbitrarily small by choice of certain design parameters. Moreover, when the speed reference and load torque are constant, it ensures asymptotic regulation. Simulation results agree well with the analysis.

In a recent note Ortega and Espinosa (8) presented a globally stable controller for torque regulation of a complete induction motor model with partial state feedback, i.e., no assumption of flux measurement. The result was established under the assumptions that both the desired and load torques are constant, that the former does not exceed certain bounds which depend on the systems natural damping, and that the motor parameters are known. In the present contributions we extend these results in several directions. First, by 'adding mechanical damping' to the closed-loop system we relax the upper bound condition on the desired torque. Finally, a new estimator is proposed to handle time-varying (linearly parameterized) unknown loads.

This work develops the use of dynamic state feedback to achieve feedback linearization of the induction motor. It is shown that the sixth-order nonlinear dynamical model of the induction motor consisting of rotor speed, two stator currents, two rotor fluxes and an integrator in the feedback controller can be made equivalent to two third-order decoupled linear systems by nonlinear state feedback through the stator input voltages. It is further shown that a nonsingular dynamic feedback linearizing transformation exists as long as the (electromagnetic) torque put out by the motor is nonzero.

A nonlinear adaptive state feedback input-output linearizing control is designed for a fifth-order model of an induction motor which includes both electrical and mechanical dynamics under the assumptions of linear magnetic circuits. The control algorithm contains a nonlinear identification scheme which asymptotically tracks the true values of the load torque and rotor resistance which are assumed to be constant but unknown. Once those parameters are identified, the two control goals of regulating rotor speed and rotor flux amplitude are decoupled, so that power efficiency can be improved without affecting speed regulation. Full state measurements are required.

Practitioners in the electric drives community have recently introduced a novel torque control strategy for high power induction motor applications called direct torque control (DTC), which has three distinctive features: 1) it focuses on stator (instead of rotor) flux regulation; 2) in contrast to classical field-oriented control, it does not aim at an asymptotic system inversion (hence it does not require additional current loops); and 3) it explicitly takes into account the discrete nature of the control actions, which are simply points of a finite set obtained from a switching logic. Besides its simplicity, it is claimed that the achieved performance is (in some instances) superior to field oriented strategies because of the digital form of the control structure and the reduced dependence on parameter variations of the stator flux calculations. Our objective in this paper is twofold, first to carry out a mathematical analysis of the stabilization mechanism of DTC, which helps us assess the achievable performance of the current scheme and provide guidelines for its tuning. Second, to propose a modified DTC to improve its dynamical behavior. Experimental results are presented to illustrate the main points of our paper.