| Summary: | 碩士 === 國立成功大學 === 工程科學系碩博士班 === 94 === The direct torque control (DTC) is one of the actively researched control schemes which is based on the decoupled control of flux and torque. DTC allows a very quick and precise control of the flux and torque without calling for complex field-oriented algorithms and the inner current regulation loop. However, this switching-table-based DTC approach needs a very high sampling frequency for calculations of torque and flux in order to provide good tracking performance and limit the errors of torque and flux within the specified bands. Sometimes, the sampling frequency of control is too high as compared with the switching frequency of inverter, and therefore gives rise to significant speed and torque ripples. For coping with above-mentioned issues, three novel switching strategies for DTC-based drives have been proposed: the 9-level hysteresis torque comparator, the 24-sector flux comparator and the proposed hybrid switching strategy.
By increasing the output of torque comparator from three to nine choices, the 9-level torque comparator makes the control scheme having more flexibility in positive/negative accelerations. On the contrary, by increasing the choices of mixed voltage vectors, the 24-sector flux comparator leads to a smoother tracking of flux linkage command. Applying the 9-level torque comparator to the 24-sector case, the proposed DTC can significantly reduce the torque ripples and speed ripples in comparison with the conventional switching strategy. Furthermore, the proposed switching strategy can be easily designed by a table composed of hybrid space voltage vectors. Simulated and experimental results will confirm that, without losing the fast torque response of conventional switching-table-based DTC, the proposed switching strategy provides lower speed and torque ripple in very wide speed range despite load condition.
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