Variable Polarity Plasma Arc Welding Power Development

This paper makes an attempt to introduce the characteristics of VPPAW (variable polarity plasma arc welding) and current research situation in welding field, and the design scheme of VPPAW, including main circuit, control system and control software, is introduced, in terms of its technological demands of welding aluminum alloy and the trend of inverter power source.The build-up configuration and control principle, as well as main circuit, control circuit, safeguard circuit, wire feeding circuit and control software of the VPPAW source, are presented. Furthermore, a specific analysis is made to the experiment results.Full-bridge dual inverter configuration is adopted by main circuit which is supplied by single power source. IGBT (insulated gate bipolar transistor) is comparatively chosen as a switch component to perform energy transfer and power conversion. The main circuit is made up of input rectifier & filter circuit, first inverter circuit, high-frequency transformer, second inverter circuit, rectifier & filter circuit and output reactor. The design kernel and parameters are discussed at length in this paper where the realization and control of second inverter are given emphasis to.High performance 16-bit micro-processor 80C196KC is chosen as the core of control system who adopts closed-loop and minus-feedback control. The control variable is obtained by micro-processor system after real-time sampling, and current parameters are converted into digital signals. In order to adjust output current, the variable is used to control the pulse width of PWM (pulse width modulation) chip SG3525, before constant current output characteristics are acquired, during which PI algorithm is adopted.The inverter circuit is composed of first and second inverter circuit. The PWM mode is applied by first inverter circuit to control output current. The digital signal out of micro-processor is firstly converted into analog signal through MAX530, and then is modulated by SG3525 into two pulses not overlapped each other. At last, the pulses are used to trigger former IGBTs after they are magnified by M57959L, which can achieve pulsed DC output. Software programming is utilized by second inverter circuit to attain PWM pulse signal. Similarly, two pulses from HSO of 80C196KC are used to trigger latter IGBTs after they are magnified by M57959L, which can achieve AC square wave to meet requirements of welding technics.Reliability and anti-jamming design are also involved in both main circuit and control circuit of the welding machine. Various noises probably appearing in the whole system, as well as their mechanism, are analyzed systematically. Subsequently, in orderto reduce their bad interference to great extent, some hardware and software measures are adopted during the design processes of schematic chart, PCB board and compiling program code.Made up of HD7279 and LED, the display and keystroke circuit is designed to facilitate welding operation. In addition, the saieguard circuit is composed of over-current, over-heat, over-voltage and low-voltage circuit.Through online debugging, test data and results are attained and analyzed. Moreover, some unexpected problems are probed and corresponding improvement is made during the tests. The experiment results indicate that the VPPAW power source gains asymmetrical square-wave whose current frequency, amplitude, DCEN/DCEP ratio can be adjusted separately. Therefore, VPPAW power source runs stably, with high reliability.