| The field-oriented speed control of induction motors without mechanical sensors is considered. A sliding mode speed control algorithm is developed for the speed control that replaces the traditional PI controller. The traditional approach for the sensorless speed control of using flux and speed observers is augmented with a sixth-order nonlinear induction machine model that describes the motor in field-oriented coordinates. The model takes into consideration the errors in flux estimation. The flux regulation problem is addressed by following the traditional approach of using PI controllers. For the speed regulation problem, the machine model is simplified by assuming that the flux regulation takes place relatively fast and by employing a sliding mode controller that, presents good performance against un-modeled dynamics, insensitivity to parameter variations, external disturbance rejection and fast dynamic response. A performance comparison of the developed sliding mode controller with the traditional PI controller is presented. The simulation presents the edge that the developed sliding mode controller has over its counterpart, a traditional PI controller. The analysis reveals the conditions under which the sliding mode controller provides effective speed control while preserving the closed-loop system stability under uncertain external load disturbances and reference speed variations. |
