Sliding mode control of induction machines by use of nonlinear switching surfaces and sliding mode state estimation

A variable structure controller and discontinuous observer is developed for the three-phase symmetrical induction motor by use of the theory of sliding modes and variable structure systems. The theoretical development of the controller and observer is based on the two-axis representation of the induction machine in the stationary reference frame. The switching surfaces used in the variable structure motor controller are orthogonal, nonlinear functions composed of the electromagnetic machine torque and a new function introduced as the orthogonal, or reactive, torque. The variable structure input to the state estimator is discontinuous on the speed estimation error. The only physical measurement required of the complete control system is the mechanical rotor speed.;A new observer-controller system structure is introduced which is amenable to sliding mode analysis by the method of the equivalent control. In the new structure, the process and observer dynamics are combined into a single system. The process control is discontinuous on functions that are composed of the estimated variables, while the observer control is discontinuous on functions composed of both the estimated and the process variables. The new structure, although formulated for analysis, allows the controller and observer to be designed independently. The theoretical basis is thus akin to the separation principle of linear systems.;The sliding mode dynamics demonstrate that both the motor and observer exhibit sinusoidal behavior in the stator currents and rotor flux linkages. Also demonstrated in the solution is the first-order dynamic behavior of the estimated speed and the machine rotor speed. The theoretical analyses show that the speed controller is robust with respect to parameter variations and load torque disturbances.;The theoretical results are verified through simulations and by an experimental implementation of the controller and observer. The experimental results include transient rotor speed trajectories, plots of the estimated current and flux components, and oscillograms of the motor phase currents. The simulation and experimental results verify the sinusoidal characteristics and first-order speed dynamics predicted by the sliding mode analysis. The controller has a wide range of operating speeds and is capable of operation at speeds well in excess of the rated synchronous speed.