Ac Motor Direct Torque Control System Performance Improvement

Motors are the device that can realize the conversion between mechanical energyand electric energy. Nowadays the society can't go without them. As we all known, DCmotors belong to linear objects and can be well controlled, so they lie in dominance in theadjusting-speed occasions for a long time. Unfortunately, their applications have beenlimited because of the complex structure, expensive cost and maintenance, andnon-inherent anti-explosive structure. Comparatively speaking, AC motors have manysuperiorities such as the simple structure,low make cost, credible run, convenientmaintenance and easy anti-explosive performance. However, they are mostly used asdriving forces for a long time because they belong to non-linear, multi-variables objectsand are difficult in the adjusting-speed occasions.With the development of control theory, high-speed switch devices andmicro-processor technique, the Field Oriented Control (FOC) of AC motors came intobeing, which has been urging AC motors to adjusting speed field. And AC motors aretending to dominating in this field. Nowadays higher performance AC speed controlsystems are brought forward such as wide running range, servo characteristic and highefficiency. Direct torque control (DTC) was born as a novel high performance controltechnique after FOC. It can realize the direct control of the torque of motors. Comparingwith FOC, DTC has a simpler controller structure, good dynamic and static performances,and is not sensitive to the rotor parameters. Henceforth it is gaining attention and favor inAC driving field.There are some problems unsolved well for DTC of AC motors such asfluctuant torque in low speed running (for imprecise u-i model of stator flux andincompact six-sector of magnetic flux), and lower efficiency in low load rate. Deeplystudying the DTC of AC motors is full of challenge, and engineering significance.This research on the DTC of AC motors is partly sponsored by the Nature ScienceFund of Shandong Province, which main work is as follows: (â…°) a dynamic simulationplatform for AC speed control systems is established by using Matlab/Simulink tooland self-complied S-function; (â…±) a novel twelve-sector control strategy is presentedbased on the traditional magnetic flux six-sector control of DTC; (â…²) an efficiencycontrol strategy of the DTC of permanent magnetic synchronous motors (PMSM) isproposed; (â…³) sufficient simulations and comparison experiments are done aiming at theaspects above. The results indicate that (â…°) both the six-sector and twelve-sector controlstrategies show better dynamic and statistic performance in common speed range, but thelatter clearly shows an obvious advantage in low running speed; (â…±)using iron loss andcopper loss model of PMSMthe optimal magnetizing current can be got, and then theoptimal reference stator flux is given. Compared with constant flux, the optimal reference flux strategy can raise running efficiency of PMSM, which is calculated based on themodel considering motor iron loss and copper loss. The research results are separatelypublished in Journal of Qingdao University (engineering & technology edition) andElectric Machines and Control.The main contents of this thesis are as follows:Chapter 1 gives a brief survey on AC drives systems. Chapter 2 explains the principleof DTC. Both dynamic simulation models for induction motor (IM) and PMSM are builtusing Matlab/Simulink tool and self-complied S function. And furthermore, asystem simulation platform of DTC is established. Chapter 3 presents a noveltwelve-sector control strategy based on the traditional six-sector control of magnetic fluxof DTC. Chapter 4 proposes the loss-minimum efficiency optimization control strategyconsidering the iron loss and copper loss of PMSM. Sufficient simulation experiments toDTC system for IM and PMSM are separately done based on the embedded analysis oftheory results in chapters 1-3. Consequently the correctness of the proposed controltheory is validated. Chapter 5 summarizes the whole thesis, and prospects some furtherresearch issues.