Development Of The Variable Polarity Plasma Arc Welding Power Source Based On DSP Control Chip

Plasma arc has the advantage of big energy density and good welding quality, but cathode atomizing oxide film need to be removed in the process of aluminium and aluminium alloy welding, the traditional DC or pulse plasma arc welding power source can not meet this requirement, new type of variable polarity plasma arc welding equipment need to be developed. DSP as the representative of the high integration digital chip has high processing speed, good stability, high precision, able to make variable polarity plasma arc welding power source control system anti-interference stronger, interrupt processing speed faster.This article has developed a set of variable polarity plasma arc welding power source based on DSP device. No-load voltage is 75V. Rated output variable polarity current is 200A, positive and negative half wave current amplitude and conduction time can be adjusted independently. It can provid different cathode atomization to membrane depending on aluminum and aluminum alloy surface oxidation film with stable and reliable current passing zero beam and ignition characteristics.Main circuit has the full bridge inverter structure with safety performance and good controllability. Control system includes the hardware and software of DSP control circuit and inverter drive circuit. The control core of DSP control circuit is TMS320F28035, implements the power source external characteristic as well as the timing precision, digital control. Former level inverter drive is designed as switch tube amplifier circuit. Frequency of driving signal is 20kHz, dead zone time is 5μs, voltage rising time is 229ns, decreasing time is 150ns, and ues the TLP250 optical coupling isolation chip guarantee anti-jamming performance.After level inverter drive circuit adopts M57962 chip which has the positive and negative bias, over voltage, over current detection of the main features, make the IGBT work more safety and reliable.The auxiliary equipment mainly includes the high frequency arc circuit, arc switching device, resistance capacity absorbing circuit and the overheat protection circuit. The neck-in of Tungsten electrode is 5mm, the ignition rate of small arc between tungsten electrode and the nozzle is 100%; within the neck-in, the switching rate from nozzle to workpiece is 100% when the distance between the tungsten electrode and workpiece is not more than 8 mm, realizing the switching from small arc to main arc. Resistance capacity absorbing and overheat protection circuit mainly protect the whole machine working safely.Finally, test and record the output characteristic of the welding source. The largest welding current reaches 200A; current amplitude ranges from 40A to 200A; current frequency ranges from 0 to 100Hz; half wave pulse duty-rate ranges from 10% to 90%. The output current of power supply is continuous, stable, safe and reliable, realizing the digital and capable control.