Tweco PAK 10 - Manual (Page 15)

Manual No. 0-0515 3 Specifications
1.3. PLASMA CUTTING
Plasma is a gas which has been heated to an extremely
high temperature and ionized so that the gas becomes
electrically conductive. The plasma cutting process uses
this plasma to transfer an electric arc to the workpiece.
The metal to be cut is melted by the heat of the arc and
then blown away.
In a plasma torch, a cool gas such as nitrogen (N2) en-
ters in Zone A, Figure 1-D. In Zone B a pilot arc be-
tween 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.
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A-00002
Workpiece
Power
Supply
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C
B
A
Figure 1-D Plasma Torch Operation
Plasma torches deliver a high concentration of heat to
a very small area. The stiff, constricted plasma arc is
shown in Zone C. Direct current straight polarity is
used for plasma cutting, as shown in the illustration.
Plasma cutting torches use a secondary gas, which as-
sists the high velocity plasma gas in blowing the mol-
ten metal out of the cut. This results in fast, clean, dross
(slag)-free cuts. In the PAK l0 system, the secondary
gas also cools the cutting torch. CO
2
or compressed air,
supplied by either a cylinder or plant air system, is nor-
mally used as the secondary gas.
1.4. THEORY OF OPERATION
The main components of the PAK 10 cutting system
are illustrated in the block diagram (Figure 1-E) and
their function is summarized below.
PLASMA AND SECONDARY GASES
Plasma and secondary gases flow through the PAK unit
to the cutting torch at pressures set at the external regu-
lators. The pressure of each gas is indicated on the front
panel gauges. Solenoid valves turn the gases on and
off. The gas pressure interlocks shut the system down
if the plasma gas pressure falls below 25 psi (1.7 bar) or
the secondary gas pressure drops below 30 psi (2.0 bar).
The plasma gas flows through the green/black lead,
around the electrode and gas distributor, and out
through the tip orifice.
The secondary gas flows through the red/yellow torch
lead, down the outside of the torch liner, through the
holes at the base of the liner and out around the plasma
arc.
PILOT ARC
When the torch is started, the pilot arc contactor closes
and an arc is established between the electrode and
cutting tip. The pilot arc makes a path for transferring
the main arc to the work.
HIGH FREQUENCY
Because direct current alone is not sufficient to strike
and maintain the pilot arc, high frequency is superim-
posed on the direct current.
CUTTING ARC
The main bridge rectifier converts the 3-phase AC
power to DC power for the pilot and main cutting arcs.
The negative output is connected to the torch electrode
through the torch lead. The positive output is connected
to the workpiece (through the work cable) and, through
a contactor and resistor to the torch tip.
CURRENT CONTROL
The desired cutting current is set on the CURRENT
ADJUST knob. A control circuit stabilizes cutting cur-
rent against fluctuations due to changes in line volt-
ages, material thickness, torch standoff and travel
speed. Changing the amount of saturating current in
the reactor changes the amount of AC power supplied
to the main bridge rectifier. The amount of saturating
current is controlled by a comparator which compares
the actual cutting current to the amperage selector po-
tentiometer setting.