Implementation Of High-power AC-square-waved Welding Source Based On Digital Control

High-productive welding has become the trend of welding technology, with the rapid development of industrial manufacturing, especially in wind towers, ship building, steel, plate and other occasions. So how to further improve the welding efficiency has become a focus in the welding industry. Submerged arc welding is considerably efficient, and multi-wire & multi-arc submerged arc welding is the most effective way in terms of improving productivity compared with other traditional welding approaches. Multi-wire & multi-arc welding system consists of more than one AC power to overcome the interference between multiple arcs. Moreover,9% nickel steel used in the construction of LNG storage tanks by National Energy Strategy Reserve Center (NESRC) is quite sensitive to the heat input, which makes it impossible to use the average DC arc welding machines because they cannot effectively get the heat-input under control. Therefore, the research towards phase-changeable and heat-input controllable digital AC inverter submerged arc welding power supply is of great strategic significance.An AC square-wave power supply with single-inverter structure and the ad hoc full digital control system were presented in this thesis base on the Waveform Control Theory and Welding Technology. Meanwhile, with regards to the technical problem of arc-extinguish caused by the AC current zero-transient, this thesis proposed an effective solution. The design and implementation of the mains was detailed and the working process of the first-inverter circuits was briefly introduced in this thesis. Moreover, the parameters of the power electronic devices were calculated, and the junction temperature of power semiconductor devices was estimated and the thermal analysis was thoroughly conducted.This thesis has deeply researched the factors causing the phenomenon of arc-extinguishing when AC current goes through zero, designed an effective second-inverter main circuit topology, detailed the working procedure of the above mentioned topology and proposed a commutation strategy based on sufficient calculation, which finally realized reliable performance of the welding system. The control system of welding power is based on an MCU & FPGA architecture, where the MCU is in charge of welding parameter storage, transaction coordination, and other unreal-time operations while the FPGA which is of high-speed parallel computing ability, is responsible for the inverter control. A DPRAM is used for the communication between the MCU and FPGA. Current mode double-loop control was used to improve the system response and a multi-parameter PI controller and a high-speed digital PWM generator module were constructed within the FPGA, which improved the computing speed and control accuracy.A prototype welding was made and welding experiments were conducted. Results show that the first inverter circuit can work in soft-switch mode both under heavy and light load; second inverter can successfully trigger arc when current is crossing zero; the waveforms output and the welding technologies are in good agreement; the temperature ranged are with the requirements; the power output is of high precise, and the whole system stable performs well, and has meet all the design requirements.