Tweco 1000 Merlin(Dec2004) - Manual (Page 18)

INTRODUCTION 2-4 Manual 0-2708
2.05 Theory Of Operation
A. Plasma Arc Cutting and Gouging
Plasma is a gas which is heated to an extremely high tem-
perature and ionized so that it becomes electrically con-
ductive. The plasma arc cutting process uses this plasma
gas to transfer an electric arc to a workpiece. The metal
to be cut is melted by the intense heat of the arc and then
blown away by the flow of gas.
With a simple change of torch parts, the system can also
be used for plasma arc gouging. Plasma arc gouging uses
the same process to remove material to a controlled depth
and width.
B. Input and Output Power
The unit converts AC input power to DC power for the
main cutting arc. The negative output is connected to
the torch electrode through the negative torch lead, and
the positive output connects to the workpiece through
the work cable.
C. Pilot Arc
When the torch is activated there is a 2 second (service
adjustable) gas pre-flow, followed by an uninterrupted
pulsed DC pilot arc established between the electrode and
tip. The pilot arc is initiated by a momentary high fre-
quency pulse from the Power Supply or the Optional
Remote Arc Starter. The pilot creates a path for the main
arc to transfer to the work. When the main arc is estab-
lished, the pilot arc shuts off. The pilot automatically re-
starts when the main arc stops, as long as the torch re-
mains activated.
D. Main Cutting Arc
The power supply converts the AC input power to DC
power for the main cutting arc. The negative output is
connected to the torch electrode through the negative
torch lead. The positive output is connected to the work-
piece via the work cable and ring lug connection.
E. RF Shielding
All machine torch systems are shielded to minimize ra-
dio frequency (RF) interference which results from the
high frequency arc initiation. These shielded systems are
designed with features such as a wire for establishing an
earth ground and shielded torch and control leads. The
CNC control signals are filtered on an internal PC Board.
F. Interlocks
The system has several built-in interlocks to provide safe
and efficient operation. When an interlock shuts down
the system, the fault condition must be remedied and the
system recycled using the applicable control device.
1. Parts-In-Place (PIP) Interlock
The Power Supply has a built-in parts-in-place in-
terlock that prevents accidental torch starting
when torch parts are not properly installed. A flow
switch on the coolant return lead detects reduced
coolant flow caused by improper torch assembly.
If not satisfied, the switch interrupts power to the
tip and electrode.
2. Gas Pressure Interlock
A pressure switch acts as an interlock for the
plasma gas supply. If the plasma gas supply pres-
sure falls below minimum requirements the pres-
sure switch will open, shutting off the power to
the contactors, and the GAS indicator will go out.
When adequate plasma supply pressure is avail-
able the pressure switch will close, allowing power
to be resumed for cutting.
NOTE
There is no gas pressure interlock for secondary
gas.