| The content of this thesis is an important part of the project " Scraper Conveyor Drive System"(serial number:110148032), which is the College Students’ Innovation and Entrepreneurship Project sponsored by Taiyuan Science and Technology Bureau. There are many advantages of the three-phase asynchronous motors, such as simple structure, high stability, easy maintenance, low-cost products and so on, which is widely used in all aspects of national life and production. However, three-phase asynchronous motors has larger starting current, which has some disadvantages such as serious damage to the motor, affecting the normal work of other equipment and so on. In view of this, a three-phase asynchronous motor soft start control system which can reduce motor starting current and lessen the impact of the power grid was developed.In this paper, a3kW three-phase asynchronous motor was used as research subject and thyristor was used as main power component. A soft-starter based on C8051F040was developed. Tests results show that the system can effectively reduce the starting current of the three-phase induction motor and the impact of the grid. achieve the purpose of three-phase asynchronous motor soft start. The purpose of soft start for the three-phase asynchronous motor is achieved. Main contests of the thesis are as follows.Referring to a large number of relevant literatures, research status of three-phase asynchronous motor soft start control system was summarized. Conventional conditions of three-phase induction motor soft start technology are elaborated and traditional soft start methods are cited. Modern soft-start control methods based on thyristor are introduced. Three-phase full-controlled bridge composed of three sets of two-way anti-parallel thyristors is chosed as the main circuit.Principle of discrete frequency starting was studied. The method of which can reduce the starting current and starting torque was analyzed. Effects to motor torque, the optimal band selection, the minimum frequency bands, trigger mode selection and control of power factor angle are discussed. Precautions of band switching and harmonic suppression were elaborated.The main power components and chip models were selected. The hardware circuits of the control system are designed, which are composed of synchronizing voltage signal detecting circuit, current detection circuit, thyristor drive circuit, minimum system circuit, human-computer interaction circuit etc. Tests showed the system has a favorable stability and stability performances and it can meet the design requirements.Modular programming methods have been adopted. Software of the system was developed in C, including main program, initialization program, signal acquisition program, thyristor trigger program, start-up mode program and human-computer interaction program. The debugging and upgrade of the system were more convenient and the running efficiency of the system was promoted.The platform of three-phase induction motor soft start control system was founded. Hardware and software for the system have been debugged. Under no-load conditions, sync signal waveform voltage, phase voltage and current waveforms, and trigger pulse phase waveforms were tested. The results showed that the system can meet the requirements of high accuracy, high real-time reliability.The loading platform of Magnetic brakes was founded. Loading experiments of three-phase asynchronous electric motor has been done. Soft-start current waveform and direct starting current waveform were compared. Motor starting current is much smaller than the direct starting current under certain load, which can reached the purposes of limiting starting current. |
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