Tweco 300 Maximizer - Manual (Page 17)

Manual 0-2554 2-3 INTRODUCTION
2.05 Introduction to Plasma
A. Plasma Gas Flow
Plasma is a gas which has been heated to an extremely
high temperature and ionized so that it becomes electri-
cally conductive. The plasma arc cutting and gouging
process uses 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 nitro-
gen (N2) enters Zone B (refer to Figure 2-2), where a pilot
arc between the electrode and the torch tip heats and ion-
izes 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 Typical Torch Head Detail
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 polar-
ity is used for plasma cutting, as shown in the illustra-
tion.
Zone A (Figure 2-2) is used as a secondary gas that cools
the torch. This gas assists 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.
B. Gas Distribution
The plasma gas flows into the torch through the positive
lead, through the gas distributor, around the electrode,
and out through the tip orifice.
The secondary gas flows into the torch through the sec-
ondary gas hose, down around the outside of the torch
gas distributor, and out between the tip and shield cup
around the plasma arc.
C. Pilot Arc
When the torch is started a pilot arc is established be-
tween the electrode and cutting tip. This pilot arc creates
a path for the main arc to transfer to the work.
D. High Frequency
Because direct current (DC) alone is not sufficient to strike
the pilot arc, high frequency is also used. The high fre-
quency jumps between the tip and electrode with the DC
following, then the high frequency turns off.
E. Main Cutting Arc
DC power is also used for the main cutting arc. The nega-
tive output is connected to the torch electrode through
the torch lead. The positive output is connected to the
workpiece via the work cable and to the torch through a
pilot wire in the positive lead.
F. Interlocks
Two pressure switches act as an interlock for the gas sup-
ply. If supply pressure falls below minimum requirements
the pressure switches will open, shutting off the power
to the contactors, and the GAS indicator, if supplied, will
go out. When adequate gas supply 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 cool-
ant 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 torch front
end parts (tip or electrode) are not installed.