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A new method to the sliding-mode control of single-phase uninterruptible-power-supply inverters is introduced. The main idea behind this new method is to utilize a time-varying slope in the sliding surface function. It is shown that the sliding line with the time-varying slope can be rotated in the phase plane in such a direction that the tracking time of the output voltage is improved during load variations. The adjustment of the time-varying slope is achieved dynamically by employing a simple function which involves the error variables of the system. This function is obtained from the input/output relationship of the single-input fuzzy logic controller operating on the error variables. When a newly computed slope value is applied to the system, the position of the representative point is changed so as to achieve the desired response. The performance of the proposed control method has been tested through computer simulations and experiments using a triac-controlled resistive load and a diode bridge rectifier load. The results of the proposed method are compared with a classical sliding mode controller and a standard controller. It has been shown that the proposed method is capable of shortening the tracking and sliding times, resulting in a smaller total harmonic distortion in the output voltage.
Date of Publication: Oct. 2012