Power source

The plasma arc is generally operated with a DC, drooping characteristic power source. Because its unique operating features are derived from the special torch arrangement and separate plasma and shielding gas flow, a plasma control console is often added on to a standard TIG power source. Purpose-built plasma systems also are available. The plasma arc isn’t readily stabilized with wave AC. Arc reignition is difficult when there’s an extended electrode to workpiece distance and therefore the plasma is constricted, Moreover, excessive heating of the electrode during the positive half-cycle causes balling of the tip which may disturb arc stability.

Special-purpose switched DC power sources are available. By balancing the waveform to scale back the duration of electrode positive polarity, the electrode is kept sufficiently cool to take care of a pointed tip and achieve arc stability.

Arc starting

Although the arc is initiated using HF, it’s first formed between the electrode and plasma nozzle. This ‘pilot’ arc is held within the body of the torch until required for welding then it’s transferred to the workpiece. The pilot arc system ensures reliable arc starting and, because the pilot arc is maintained between welds, it obviates the necessity for HF which can cause electrical interference.


The electrode used for the plasma process is tungsten-2%thoria and therefore the plasma nozzle is copper. The electrode tip diameter isn’t as critical as for TIG and will be maintained at around 30-60 degrees. The plasma nozzle bore diameter is critical and too small a bore diameter for the present level and plasma gas flow will cause excessive nozzle erosion or maybe melting. it’s prudent to use the most important bore diameter for the operating current level.

Note: overlarge a bore diameter, may give problems with arc stability and maintaining a keyhole.

Plasma and shielding gases

The normal combination of gases is argon for the plasma gas, with argon plus 2 to five hydrogen for the shielding gas. Helium is often used for plasma gas but because it’s hotter this reduces the present rating of the nozzle. Helium’s lower mass also can make the keyhole mode harder.

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