Arc burning stability of variable polarity TIG welding and control with constant duty cycle

The rapidity and stability of current commutation are the key factors that influence the arc burning stability of variable polarity TIG welding. Especially, arc extinguishing and burning instability occur easily when the variable polarity TIG welding is performed under low current value. The waveforms of voltage and current during the current commutation process were deeply analyzed. It was found that the phenomenon of current declination occurred easily after the current commutation process and even arc extinguished welding with low current when the current feedback control based on PID was applied in the primary inverter to obtain constant current output, which was closely relative to the term of di/dt within the PID control. In this paper, a new control method with constant duty cycle was proposed and adopted in the primary inverter during the current commutation process. The experimental results showed that this method improved the response speed of commutation current effectively and prevented the declination or overshoot of current after commutation, which enhanced the stability of current commutation process for the variable polarity TIG welding.