Tweco 15XC Merlin - Manual (Page 21)

Manual 0-2251 2-7 INTRODUCTION & DESCRIPTION
2.5 THEORY OF OPERATION (continued)
Plasma Gas Flow
(continued)
By forcing the plasma gas and arc through a consrticted
orifice, the torch delivers a high concentration of heat to a
small area. The stiff, constricted plasma arc is shown in
Zone B (Figure 2-D). Direct current (DC) straight polarity
is used for plasma cutting, as shown in the illustration.
The torch also uses a secondary gas (or water) which
assists the high velocity plasma gas in blowing molten
metal from the area of the cut to create a fast, slag-free cut.
The secondary flow (Zone C, Figure 2-D) also cools the
torch and minimizes heat input to the workpiece. The
secondary flows into the torch through the secondary lead,
down around the outside of the torch liner, and out be-
tween the tip and shield cup around the plasma arc.
Compressed air, supplied by either a cylinder or plant air
system, nitrogen, CO
2 , or water can be used as the secon-
dary. An exception to this is oxygen plasma operation,
which requires no secondary.
The torch is liquid-cooled by an internal closed cooling
system. De-ionized coolant is distributed from a reservoir
in the power supply through the coolant supply lead. At
the torch, the coolant is circulated around the torch tip and
electrode (Zone D, Figure 2-D), where the extra cooling
effect helps to increase parts life. The coolant then flows
back to the power supply through the return lead.
When the torch is started, a DC pilot arc is established
between the electrode and cutting tip after a two-second
pre-flow delay. The pilot arc is initiated by a momentary
high frequency pulse. The pilot arc creates a path for the
main cutting arc to transfer to the work. When the main
arc is established, the pilot arc shuts off. The pilot auto-
matically restarts when the main arc stops, as long as the
torch remains activated.
The PAK 15XC accepts 50 or 60 Hz three-phase line input.
An internal changeover switches input line voltages in
three ranges, for 200/220/230V, 380/415/460V, or 500/
575V operation. The power supply 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. The positive output is connected to
the workpiece via the work cable and clamp connection.
All machine torch systems are shielded to minimize radio
frequency (RF) interference which results from the high
frequency arc initiation. These shielded systems are de-
signed with features such as a wire for establishing an
earth ground and shielded torch and control leads.
Secondary Flow
Coolant Flow
Pilot Arc
Main Cutting Arc
RF Shielding