Tweco Thermal Arc TA 500 - Manual (Page 17)

Manual No 0-0470 1-3 General Information
1.3 Plasma
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) enters in
Zone A. 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.
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A-00002
Workpiece
Power
Supply
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C
B
A
Plasma Torch
By forcing the plasma gas and electric arc through a small
orifice, 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.
The exclusive Dual-Flow design of cutting torches uses a
secondary gas. The secondary gas assists the high velocity
plasma in blowing the molten metal out of the cut, resulting in
fast clean, dross (slag)-free cuts. CO
2
, supplied from cylin-
ders, or water, from the plant water system, is normally used
as the secondary. Compressed air may be used as the sec-
ondary gas for some applications.
1.4 Theory of Operation
The main components of the TA500 cutting system are illus-
trated in the block diagram and their function is summarized
below.
Plasma and Secondary Gases
Plasma and secondary gases flow through the Control
Console to the cutting torch at pressures set at the ex-
ternal regulators. The pressure of each gas is indicated
on the gages mounted on the front of the Supply Con-
sole. Solenoid valves in the Supply Console turn the
gases on and off and receive their signals from the
Power Supply relay logic P.C. Board. The gas pressure
interlocks shut the system down if the plasma gas pres-
sure falls below 40 psi.
The plasma gas flows to the torch through the black
lead, around the electrode and out through the tip ori-
fice.
The secondary gas (or water) flows to the torch through
the yellow lead, through the outer insulator, inner tip
retainer and end piece and out around the plasma arc.
Pilot Arc/Arc Starter
When the START button is pressed the main contactor
is activated. After two seconds the pilot contactor
closes. The pilot bridge rectifier converts single-phase
AC power from the main transformer Arc Starter wind-
ing to DC for the pilot arc. At the same time the arc
starter circuit starts a series of high-voltage pulses.
These pulses jump the gap between the electrode and
the tip in the torch, which starts the pilot arc. The pilot
arc then runs for half a cycle (1/120 sec) when the volt-
age drops and the pilot goes out until another pulse
from the arc starter circuit starts it again. The arc starter
circuit provides 15 pulses per second which results in a
rapid-fire ‘pop pop pop’ pilot arc. This pilot arc makes
a path for transferring the main arc to the work.
Cutting Arc
The main bridge rectifier converts the 3-phase AC
power to DC for the main cutting arc. The negative
output is connected to the electrode through the green
torch lead. The positive output is connected to the
workpiece (through the work cable).