Research And Development For Overloading Soft Starting System Based On DSP

Thyristorized motor soft starter is being used in many fields with its advantages. By using it the starting current surge can be reduced remarkably with the small voltage applied to the motor. But it has a severe demerit that a small reduction in voltage produces a considerable drop in electromagnetic torque, which may cause the motor with load failure to start. This implies that the traditional soft start method has limited applications where the starting torque requirement of the mechanical load is not high. Unfortunately, there are many applications that the motor needs to start with full load, such as Ball mill, Grinder, Mine pit hoist crane, wire-drawing equipment, conveyor and so on. In order to boost the starting torque and improve the starting performance, the research on discrete variable frequency method is carried out in detail based on lots of literatures investigation and present status analysis of motor starting technique research & development.The content of this research falls into the following several parts:1. The method of reducing power supply frequency in discrete steps is proposed by simulation in this paper. Without changing the main circuit of conventional motor soft starter, the strategy defines the triggering instants of thyristors to make the thyristors conduct or block Half cycles or partial half cycles of the ac power supply can pass or not. Therefore, the discrete frequency voltage, which is lower than the power supply voltage frequency, is applied to the motor. For analyzing the speciality of new system under control of the method, lots of simulation has been done. Put forward a principle of choose the optimal fire angle combination when the system is not balance. it means not only considering the main discrete frequency ,which can produce the most positive torque, but others discrete frequency.2. The research on how to choose the best frequencies used to variable frequency control and how to change from one frequency to another are made in this paper. In addition, A new discrete variable voltage variable frequency equivalent sinusoidal control method is proposed.It has been verified experimentally and by simulation to have good high starting torque and small current performance.3. The design and implementation of discrete variable frequency soft starting control system for three phase ac induction motor and the experimental research are given. The system adopts the high speed DSP-F2812 with single chip EPM1270T144, which makes the system process detected data in the real-time matter. In the meantime adopting Intel 16-bit SCM 80196KC to dispose the keyboard response and LCD display. So DSP can complete the control algorithm excellently. The system not only has good performance of soft starters, but also meets the need of heavy load starting.