The direct current component

The direct current component is a phenomenon that occurs during the TIG AC welding of aluminium. The cause of this is the large temperature difference between the tungsten electrode and the aluminium to be welded. The tungsten electrode reaches a temperature of 3000 degrees Celsius and the aluminium of about 658 degrees Celsius. This large temperature difference results in a large difference in electron emission capacity. When the tungsten electrode is charged negative, a lot of electrons will go to aluminium. When the aluminium workpiece is charged negative, given the much lower temperature, far fewer electrons will go towards the tungsten electrode. As a result, the negative half of the sinus becomes larger and the positive half smaller. That’s fine I hear you think, a small plus means little heat towards tungsten electrode and a lot of heat towards the aluminium. And that is absolutely true. However, there is a problem; we are talking about single-phase power sources where the height of the plus is responsible for the cleaning effect. This cleaning effect is very important to get a good weld and stable arc behaviour.

This is in a few words what a direct current component is, it doesn’t need much more attention because it no longer occurs with the current modern machines.

The balance control

The first single-phase AC/DC power sources, we are talking about the 60s and early 70s, had no balance control. That means no adjustable balance. To compensate the direct current component, a large capacitor was built into the power source, which charges during the negative half of the sinus wave and discharges again during the positive half. Sometimes a battery pack was used, connected in such a way that the DC component is obstructed. Both possibilities aimed to restore the balance between the positive and negative sinus halves. The welder could not influence this correction.

From the mid-1970s and onwards, more and more electronics were used in the single-phase power sources. This made more adjustable parameters available for the welder and one of them was the balance control. With these power sources it is possible to use a potentiometer to move the zero axis of the sinus up and down to determine the correct cleaning properties for the workpiece to be welded. It was now a real balance control.

In the early 80’s the chopper was introduced to the market. A chopper is a three phase AC/DC current source with, on the secondary side of the transformer, a relatively fast transistor that is able to shift a direct current between positive and negative. The now alternating current is a square wave and no longer affected by the direct current component! This meant that the balance control as on the single-phase power sources no longer made sense, because the height of the plus always remains the same with this square wave. It is the height of the plus that determines the cleaning properties, the width has no influence on that. The balance control on these machines will shift the vertical line, the zero passage. It is actually no longer a true balance control, but rather a parameter setting that determines how much heat is applied to the tungsten electrode and how much to the workpiece.

From the early 90’s the inverter was introduced at the AC/DC power sources. This made an enormous amount of parameters settings available to the welder. However, the balance control remained the same as that of the chopper. With the modern AC/DC machines we can not only adjust the zero passage but also, independently of each other, set the height of the sinus halves.

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