Research On Current Control Of High-power Inverter Resistance Spot Welding Power Supply

With the promotion of Strategy of Powering the Nation by “made in china 2025”,Welding production is moving in the direction of automation and intelligentization.Welding current is an important welding parameter.It not only influence the welding process directly,but it is also the important information about analyzing the quality of weld joints in intelligent production line.At present,the high-power invert DC spot welding power supply commonly detects the primary current to detect the welding current indirectly.The error of this method is large and varies with the transformer.It has effects on accuracy of current output of welder as well as consistency of welder production.As a result,it brings problems for centralized monitoring and quality analyzing of welding production line.Besides,the starting time of high-power resistance spot-weld power supply is too long and the surge of starting grid is violent in nowadays.To a large extent,these questions affect the construction and application of welding production line.This article aims at solving the above two problems,high capacitor charging of three phase semi-controlled rectifier and the detection of welding current based on Rogowski coil have been studied.According to the general scheme of high-power resistance spot welding power supply,the design and selection of thyristor module,filter capacitor,insulated gate bipolar transistor(IGBT),and mid frequency transformer were complete.The driven design of high power IGBT and the anti-interference design of the whole control system were completed.A test platform for high power resistance spot-weld power supply was set up.The control system of three phase semi-controlled rectifier bridge was designed.It concludes hardware circuit and control program.DSP controller was selected as the control core.Synchronous signaldetection circuit,thyristor drive circuit,voltage / current sampling circuit and other hardware circuits have been designed.The design and debugging of the control program have been completed which included main program,phase-shifting trigger program,soft starting program and alerting output program.Input current control test in start-up process of high current inverter spot-weld power supply was completed.Test resultindicated,compared to open loop current control system added with AC contactor and current limiting resistor,input current closed loop control system of three phase semi-controlled rectifier had the advantages of more mildly surge of starting grid and more efficient boot in in start-up process of high current inverter spot-weld power supply.Current detecting system base on rogowski coil has been designed.The theory of measurement and the principle on anti-interference of rogowski coil,and the error sources that commonly used conditioning circuits of rogowski coil have been relatively thoroughly analysed.For the characteristics of welding current,treatment method that based on symmetric compensation for Rogowski coil signal which controlled by analog switch was studied.A welding current detection system based on Rogowski coil has been developed.Under the condition of constant control method,when compared to the commonly used Holzer sensor primary current detection system,this welding current detection system with higher control accuracy and better linearity when feedback signal of this system was used for constant current control.