tailieunhanh - Advanced Welding Processes Episode 1 Part 9

Tham khảo tài liệu 'advanced welding processes episode 1 part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 154 Advanced Welding Processes by selection of the voltage is the maximum level to which the current can rise when the output of the power source is short circuited for example during dip transfer. With a constant-voltage characteristic power source this level may be very high and result in explosive rupture of the short circuit with high levels of spatter. In order to limit this current it is usual to incorporate inductance in the output of the power source this reduces the rate of rise of current and limits the maximum value reached before the short circuit ruptures. If the inductance is too high the current will not reach a sufficiently high level to cause detachment and irregular operation will result. In the dip transfer mode the voltage and inductance should therefore be adjusted to optimize the short-circuit current and obtain stable spatter-free detachment. Figure Switch output power supply control. Alternative control techniques If a constant-current output power source is used the heating effect current of the arc does not vary with small changes in stand-off and voltage. Furthermore an inherent self-adjustment mechanism operates with resistive wires since if current and wire feed speed are fixed a unique value of I the electrical stick-out is defined by equation . If I decreases momentarily due to some process disturbance the second term in the equation decreases and the melting Gas Metal Arc Welding 155 rate decreases thereby restoring the original extension length. Unfortunately this self-regulation does not occur with high-conductivity materials such as aluminium. Constant-current power sources may be used for GMAW of high-conductivity materials by using a variable-speed wire feed unit which responds to arc length changes by adjusting the wire feed speed. Electronic AVC arc voltage control systems may be used with more modern power sources to cope with this situation. Some of the developments in this area are described in section . .