Design of sliding mode controller for a three phase AC motor

Abstract

The chattering phenomenon associated with the use of Sliding Mode Controllers (SMC) is considered a major drawback of the robust controllers. Ideally, SMCs are to exhibit infinite switching frequency but this is always inhibited by the existence of parasitic dynamics which introduces delays to the system and leads to high yet finite switching frequency. In a sense, the performance and robustness of the controller can be closely linked to the resulting switching frequency. SMCs have often been a source of confusion in control problems. Frequently, these allegedly discontinuous controllers are presumed to generate smooth continuous control signals which is, in fact, untrue. Therefore, when controlling the speed of induction motors, the outputs of these controllers are commonly used as modulation inputs for various modulation schemes whether Pulse Width Modulation (PWM) or Space Vector Modulation (SVM) all of which require a slow input signal. In first fraction of this thesis study, the control behaviour of SMCs when used with PWM was examined. This was done by first developing a better representative model for the PWM operation. Frequency domain analysis was then done via the Locus of a Perturbed Relay System (LPRS) method to determine the controller's switching frequency and evaluate its performance. It was shown that the controllers experience depreciation in performance with the inclusion of PWM. In the second fraction of this thesis, a SMC strategy which omits the need for modulation was developed. The topology is based on an interior SMC control loop with an exterior model based loop. The designed control scheme was first verified through simulations, which demonstrated good performance of the developed control strategy, after which, was implemented experimentally on hardware.

Date of Award	2015
Original language	American English
Supervisor	IGOR Boiko (Supervisor)