Tweco PCM-120 Machine Torch - Manual (Page 15)

Manual 0-2698 2-3 INTRODUCTION & DESCRIPTION
2.05 Introduction to Plasma
A. Plasma Gas Flow
Plasma is defined as a gas which has been heated to
an extremely high temperature and ionized so that it
becomes electrically conductive. The plasma arc cut-
ting and gouging process use this plasma to transfer
an electrical arc to the workpiece. The metal to be
cut or removed is melted by the heat of the arc and
then blown away.
While the goal of plasma arc cutting is separation of
the material, plasma arc gouging is used to remove
metals to a controlled depth and width.
In a plasma cutting torch a cool gas such as air or
nitrogen (N2) enters Zone B (refer to Figure 2-2),
where a pilot arc between the electrode and the torch
tip heats and ionizes the gas. The main cutting arc
then transfers to the workpiece through the column
of plasma gas in Zone C.
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A-00002
Workpiece
Power
Supply
+
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C
B
A
Figure 2-2 Theory of Operation
By forcing the plasma gas and electric arc through a
small orifice, the torch delivers a high concentration
of heat to a small area. The stiff, constricted plasma
arc is shown in Zone C (Figure 2-2). Direct current
(DC) straight polarity is used for plasma cutting, as
shown in the illustration.
B. Secondary Gas Flow
Zone A (Figure 2-2) is where secondary gas flows to
help cool the torch and and assist the high velocity
plasma gas in blowing the molten metal out of the
cut allowing for a fast, slag-free cut. Compressed air,
supplied by either a cylinder, plant air system or CO
2
is normally used as the secondary gas.
C. Coolant Flow
Liquid coolant is the primary means for cooling torch
parts. The coolant flows from the coolant recirculator,
through the coolant supply leads to the torch head
and returns through the return coolant leads.
D. Gas Distribution
The plasma gas flows into the torch through the
plasma (+) lead, through the gas distributor, around
the electrode, and out through the tip orifice.
The secondary gas flows into the torch through the
secondary gas hose, down around the outside of the
torch gas distributor, and out between the tip and
shield cup around the plasma arc.
E. Pilot Arc
When the torch is started a pilot arc is established
between the electrode and cutting tip. This pilot arc
creates a path for the main arc to transfer to the work.
F. CD (Capacitive Discharge)
Because direct current (DC) alone is not sufficient to
strike and maintain the pilot arc, high frequency is
also used. The CD jumps between the tip and elec-
trode with the DC following, then the CD turns off.
G. Main Cutting Arc
DC power is also used for the main cutting arc. The
negative output is connected to the torch electrode
through the torch lead. The positive output is con-
nected to the workpiece via the work cable and to
the torch through the plasma (+) lead.
H. Interlocks
Two pressure switches act as an interlock for the gas
supply. If supply pressure falls below minimum re-
quirements the pressure switches will open, shutting
off the power to the contactors, and the GAS indica-
tor, if supplied, will go out. When adequate gas sup-
ply pressure is available the pressure switches close,
allowing power to be resumed for cutting.
A flow switch acts as an interlock for the closed loop
coolant supply. If the flow switch does not activate,
then the Power Supply will not power on. The flow
switch will not activate if there is low coolant level,
or if the front end torch parts (tip or electrode) are
not installed.