DIFFERENT ARC WELDING MACHINE POWER SOURCE
Pulsed current finds increased use in TIG welding machine and MIG welding machine processes. Whereas in TIG welding machine it serves the purpose of controlling the weld pool size and cooling rate of the weld metal without any arc manipulation, in MIG welding machine it provides spray and controlled mode of metal transfer at lower welding current for a specific type and diameter of electrode used.
A typical pulsed arc welding machine power source normally consists of a 3-phase welding transformer cum rectifier unit provides background current and the single phase unit supplies the peak current. Both the transformer and rectifier units are mounted in a single housing with appropriate controls for individual adjustments background and peak currents.
Electrode size and feed rate are accounted for by the peak current setting. The peak current is set just above that provides spray mode of metal transfer for that electrode diameter and feed rate. The spray transfer occurs during the peak current duration while globular transfer does not take place due to the lack of time at the background current level. Thus, it provides the deposition rate between those for continuous spray transfer and globular transfer.
Transistorised Welding Power Source
Like a rectifier cell, a transistor is another solid-state device that is used in welding machine power sources. However, presently transistors are used only for such power sources which require accurate control of a number of variables.
A transistor is different from a SCR in that conduction through it is proportional to the control signal applied. Thus, when a small signal is applied there is a small conduction and for a large signal there is a large conduction. Also, a transistor can be turned off through a signal which is unlike a SCR wherein potential of the anode has to drop to a level lower than that of the cathode or the current flow has to stop for the SCR to stop functioning.
Transistors are used in welding machine power sources at a level between ‘off’ and ‘full on’ wherein they act as electronically controlled series resistance. Transistors can work satisfactorily only at low operating temperature which may necessitate cooling water supply to keep them within the desired temperature range.
Transistorised welding power sources have been developed for accurate control of welding parameters. The speed of operation and response of transistors are very high therefore such power sources are best suited to TIG welding machine and MIG welding machine processes. The latest power supply source is the outcome of developments in transistorised welding power source only. Such a power source can be adjusted to give any desired volt-ampere characteristic between constant current to constant voltage type. It is also possible to programme the control system to give the predetermined variable current and voltage during the actual welding operation. This feature makes it particularly attractive for pipe welding wherein the heat build-up demands higher welding speed as work progress wherein the heat build-up demands higher welding speed as work progress. Normally, such systems are of pulse current type for achieving maximum control over the mode of metal transfer and hence the quality of the weld.