A Discrete Variable Frequency-Based High Torque Starting Technique Using Multi-Mode Control

With prominent features of structural simplicity, performance stability and good cost effectiveness,AC asynchronous motors have wide applications in industry, agriculture, national defense and household appliances. Meanwhile, the supporting control system designed for AC asynchronous motors has also undergone a rapid development. The motor starter buck regulator, for example, developed from level regulating to step less regulating, effectively solves the jitter and current impact problem during the starting process. Possessing the advantages of a compact structure, a light weight, and ease of operation, electronic soft starters have been widely used in occasions without regulating speed. As the motor output torque is proportional to the square of the voltage on the stator side, by using the electronic soft starting technique, the role of limit currents is strengthened and the motor starting torque becomes smaller, which makes it only suitable for small load or non-loading starting situations. Variable frequency technology can help solve the problem in motor starting, and achieve an excellent starting performance with a small current and a high torque, this can generally apply for governing speed situations, but it's quite uneconomical if it is only used as a starter. Therefore, it is of great practical significance to study the soft starter that can be used in a heavy-load starting situation.To deal with the drawback of a small starting torque with electronic soft starters, the discrete variable frequency-based soft start technique that can provide a high starting torque at the same time is studied in the research. First, by browsing relevant dissertations in this area, the principle of discrete variable frequency was analyzed, and the three-phase relationship after discrete variable frequency conversion obtained, the symmetrical frequency division level of positive sequence and frequency division methods summarized. Second, the initial phase angle frequency formula for each phase has been derived by using Fourier analysis, and they were verified by simulations. Furthermore, based on the symmetrical component method, the initial and transition frequencies were chosen to find two optimized asymmetric binary frequency divisions and three asymmetric binary frequencies. Finally, based on the characteristics of the discrete fractional frequency conversion, an artificial intelligence multiple mode control was first applied to the frequency control. With a criterion of rotating speed and phase time, the study built a frequency switching characteristic model and implemented the adaptive switching frequency band.In the simulation trials, Matlab/Simulink toolbox was employed based on which a model was built to verify the effectiveness of the control scheme. Simulation results show that, the control scheme can adjust the running time for each of the crossover frequencies according to different loads, which effectively improves the comprehensive performance of the discrete fractional frequency soft starting technique. This research yields a high torque soft starter which has self-adaptive switching ability. It is of great significance in industrial occasions which heavy-loaded starting or constant-loaded starting is essential but speed control is absent, it also can benefit the development of intelligent and multi-functional thyristor based soft starter.