Tweco PWH-M 2A - Manual (Page 15)

Specifications 1-3 Manual 0-2005
1.3. THERMAL ARC PLASMA
Plasma is a gas which has been heated to an extremely
high temperature and ionized so that the gas becomes
electrically conductive. The welding process uses this
plasma to transfer an electric arc to the workpiece. The
metal to be welded is melted by the heat of the arc.
In a plasma torch, a cool gas such as Argon enters in
Zone A, Figure 1-C. In Zone B a pilot arc between the
electrode and the front of the torch heats and ionizes
the gas. An arc transfers to the workpiece through a
column of plasma gas in Zone C.
POWER
SUPPLY
Workpiece
A
B
C
Figure 1-C Thermal Arc Plasma
By forcing the plasma gas and electric arc through a
small orifice, Thermal Arc torches deliver a high con-
centration of heat to a very small area. The stiff, con-
stricted plasma arc is shown in Zone C. Direct current
straight polarity is used for most plasma welding, as
shown in the illustration.
The Dual-Flow design of Thermal Arc welding torches
uses a shield gas, shown by the small arrows. The shield
gas surrounds the plasma arc and channels it to the
workpiece, thus improving weld characteristics and
shielding the weld puddle from atmospheric contami-
nation.
Coolant from the coolant recirculator flows through the
liquid-cooled power leads to the torch head and back.
The plasma arc is infinitely variable from soft to stiff.
Most applications can best be accomplished with a
softer arc, i.e. lower plasma gas flow. Full penetration
or Key-hole welds require a stiff arc, i.e. high plasma
gas flow.
1.4. THEORY OF OPERATION
TORCH CONNECTIONS
The plasma gas flows to the torch through the black
lead, around the electrode and gas distributor and out
through the tip orifice.
The shield gas flows through the yellow torch lead,
around the torch front end and out through the shield
cup.
The torch coolant and negative (-) power flow to the
front end of the torch through the green (color coded)
lead.
The coolant return and positive (+) power for the pilot
arc flow through the red (color coded) lead.
PILOT ARC
When the torch is started, an arc is established between
the electrode and welding tip. This pilot arc appears as
a small “flame” at the front of the torch. This serves to
illuminate the workpiece and assists in starting the main
(welding) arc. It can also be left on while welding at
low current levels to stabilize the welding arc.
HIGH FREQUENCY
A high voltage, high frequency current is superimposed
on the direct current to establish the pilot arc.
WELDING ARC
The power supply provides the direct current (DC) for
welding. The negative output is connected to the torch
electrode through the negative liquid cooled lead. The
positive output is connected to the workpiece through
the work cable. The electrically charged plasma gas
serves to close this electrical circuit and thus becomes
the welding arc.