Özet:

Direct Torque Control (DTC) drive allows direct and independent control of flux linkage and electromagnetic torque by the selection of optimum inverter switching tables. There is no need for any complex transformation of current or voltage. However in the conventional DTC induction motor drive there are torque and flux ripples, since none of the inverter switching vectors is able to generate the exact stator voltage required to produce the desired chances in the electromagnetic torque and stator flux linkage. In this study, we propose a method to reduce torque and flux ripples. Two steps were carry out for this purpose. The first step is to use duty-ratio. A selected inverter switching vector not for the entire switching period as in conventional DTC, but only for a part of the switching period which is defined as the duty ratio, δ and by using zero switching vector for the rest of period. The duty ratio determines the average input voltage to the induction motor during the application of each switching vector as δVdc. The duty ratio of each switching state is a non-linear function of the electromagnetic torque error, stator flux-linkage error and a function of the position of the stator flux-linkage space vector. In that case fuzzy-logic based duty-ratio control can be used because of it is difficult to model its non-linear structure. The second step is to use hybrid flux observers. The hybrid flux observer uses two model for high and low speeds. Accurate stator flux estimation is obtained by using the stator voltage equation at high speeds, but low speeds accurate stator flux estimation is obtained by the rotor voltage equation. The proposed method is simulated via using MATLAB Simulink and compared to conventional DTC results. Comparing results suggest that the proposed method reduced the ripples in electromagnetic torque, stator flux linkage, stator currents and speed. Motor parameters, reference flux and torque values of motor and load torque value of conventional and proposed methods are arranged to be same for the comparison.

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