The Research On Soft Starter For Induction Motor Based On Discrete Variable Frequency Theory

There are lots of over-loading transport equipments in industry, such as disintegrators, belt conveyers, drawing conveyers, etal. These equipments will become more difficult to restart once they occur to break down. Starting directly will produce a big current impact and short the useful time of the motor. Reducing voltage start and traditional soft starter can not provide big enough starting torque and not carry heavy load to start. Using inverter in start will reduce start current and impact to power system, but bring grate cost. Aiming at these problems, a high torque soft starter based on discrete variable has been researched on. According to the inverse proportion relation between frequency and electro-magnetic torque, this soft starter uses the three groups of inversely parallel-connecting thyristors, which are the same hardware configuration as the traditional soft starters, and introduces the AC/AC variable frequency theory to the soft starter. Then we can select some half cycles of the line frequency and omit the others to obtain sub-frequency of the line frequency. When changing the frequency, switching the trigging angel can adjust the stator voltage, so that the starting current comes down and will not cause the impact to the electrical power system, and the starting torque will be increased which can fit the equipment with heavy load.This paper researches on the discrete viable frequency principle, and proposes the most optimal combination of discrete frequency initial phase angels which can produce the biggest electro_magnetic torque of the induction motor. Studing on the simulation and expriment of the soft starter , the results show the proposed strategy can improve start torque and reduce impact to the power system, and possess some ingenuity on settling over-loading start. The paper sets up the system simulation model by using Matlab/Simulink, comparing with other starting ways, the results of simulation verify the feasibility of this control strategy. Based on the simulation, the soft starter has been designed and implemented. The system adopts the high speed DSP chip TMS320LF2407A as the central control cell, and can select the best starting strategy from the traditional starting manners or discrete viable frequency starting for different occasions. System hardware configuration, block diagram of the system software are also introduced in detail. The simulation and experiment results approves that the soft starter based on discrete frequency theory can improve the starting torque of the motor effectively and fit the over_load starting suitably.