Research And Design On Soft Starter Of Discrete Frequency

Electronic soft starting can realize the function of reducing the starting current by improving the asynchronous motor starting voltage gradually with the method of shifting the thyristor phase.Compared with the traditional soft starter; it has small structure and can be intelligently controlled.But it can not solve the problem that the starting torque is reduced yet. The frequency converter has superior performance and is widely applied into the industrial AC speed regulation.It can not only reduce the starting current of the asynchronous motor, but also make the motor maintain high starting torque. However, its structure is very complex and its price is very expensive.It is very uneconomical to start the motor, which does not need to change the speed, with the frequency converter into the application.Since Ginart Antonio proposed the discrete frequency theory with others in1997,namely triggering the positive half wave or the negative of the fundamental frequency power signal selectively and realizing the function of changing the power frequency discretely, it has become the focus of experts’ studies, that applying the theory into the electronic soft starter. With this theory, only changing the thyristor triggering strategy by the software, electronic soft starter can have the similar performance of frequency converter under the condition of keeping the original major loop topological structure. It has the advantages of simple structure, low cost, large starting torque.However, the discrete frequency technology has been in the study and research of theory. It has not been applied to the actual motor. In this thesis, by studying the discrete frequency theory, doing a large number of simulating experiments, summarizing the experience of predecessors, the discrete frequency theory is applied to the low voltage soft starter prototype in the laboratory. Good starting performance of discrete frequency soft starter has proved by the theory and practice. It can greatly increase the starting torque, solving the bottleneck of electronic soft starter. Based on this, a soft starter with discrete variable frequency for high voltage was successfully designed.In this thesis, the discrete frequency principle, the sequence relationship after frequency division, and how to produce the maximum torque, were studied in depth; Based on the selection of phases, frequency steps, voltages on the same frequency, the calculation of trigger angle, the best trigger strategy was achieved; The studies of discrete frequency theory are simulated with the software of Simulink; The advice on how to reduce the harmonic content produced by frequency division was given; The hardware circuit design of the soft starter with discrete variable frequency for high voltage was discussed, which mainly including the whole structure design of the system, the design of major loop topological structure, the design of control loop, such as synchronous signal generating circuit, the thyristor triggering circuit, protection circuit and communication circuit; The programming flow charts of the entire system software and the triggering strategy was given; Finally, the Profibus fieldbus was applied into the soft starter with discrete variable frequency,which is then able to communicate with the PLC directly, in the form of intelligent slave station.Due to the complexity of the research task and the limition of my time and capabilities, there is some distance to put the the soft starter with discrete variable frequency for high voltage into the market. Combined with my own research, some suggestions were put forward for further research of later generations. Through the theory and simulation analysis, discrete variable frequency soft starter can not only reduce the starting current, but also can maintain larger torque.Practice has proved that it has good starting performance.