Study Of Heavy Soft-starter Based On Discrete Variable Frequency

the soft starter have clear advantages in adjustable voltage and current,saving some hard switches and reducing the size of start-up equipment. However, it has a serious flaw for a small reduction in voltage produces a considerable drop in electromagnetic torque. This implies that the traditional soft start method has restricted 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 or heavy load, such as Ball mill,Grinder,Mine pit hoist crane and so on. In order to boost the starting torque and improve the starting performance, the paper research on discrete variable frequency theory and present status analysis of heavy load soft starter.The main contents of the study are as follows:(1) Without measuring sensors, the paper introduces the new methods of the motor speed. According to the thyristor blocking voltage is related to motor speed, a new motor speed detecting method which applying for conventional soft starter is proposed. Based on characteristics of discrete variable frequency, another novel measuring method using residual voltage after AC dump is also proposed for heavy load soft starter. These methods do not need to additional speed sensors and obtains the speed information, which is the basis of many functions of the soft-starter, only based on the inherent characteristic of the soft-starter main circuit.(2) In order to increase the motor torque, a method of reducing power supply frequency in discrete steps is proposed. The proposed method, with the main circuit of motor soft starter unchanged, only changes the motor stator voltage frequency through the control of the firing angle of the thyristor and controls the motor with the fundamental voltage. For analyzing the characteristics of new system under discrete frequency control, A principle of choose the optimal trigger angle combination is determined under three system of positive sequence asymmetry,negative sequence asymmetry and imbalance. The optimal combination of trigger angle is determined by phase diagram. Furthermore, this paper study in-depth on frequency select,trigger angle and the frequency switching process .(3) Benefiting from the fast data processing capability and enriched peripheral resource of DSP, A prototype of heavy -duty soft starter used in heavy load system is designed. The hardware and software development are also given, the prototype which is analyzed and simulated by MATLAB in the start-up process verifies the correctness of the theory.