Tweco Max 300 w-TD-750 - Manual (Page 17)

Manual 0-2557 11 INTRODUCTION & DESCRIPTION
• Spare Parts Kits
Kits contain replacement front-end torch parts and
tools. Spare parts kits are available for air, nitro-
gen (N2), oxygen (O2), and argon/hydrogen (Ar/
H2) cutting and gouging.
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 Thermal Arc Plasma Cutting Torch a cool gas such
as air or nitrogen (N2) enters Zone B (refer to Figure
2-3), 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 col-
umn of plasma gas in Zone C.
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A-00002
Workpiece
Power
Supply
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C
B
A
Figure 2-3 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 2-3). Direct current
(DC) straight polarity is used for plasma cutting, as
shown in the illustration.
Zone A (Figure 2-3) 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 posi-
tive lead, through the gas distributor, around the elec-
trode, 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.
C. Pilot Arc
When the torch is started a pilot arc is established be-
tween the electrode and cutting tip. This pilot arc cre-
ates 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 frequency 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
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 a pilot wire in the positive lead.
F. 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 an interlock for the closed loop
coolant supply. If the flow switch does not activate
then the Power Base Module 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.