Theory of operation, Output board and torch – Lincoln Electric PRO-CUT 55 SVM140-A User Manual

Page 32

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OUTPUT BOARD AND TORCH

The output board contains an Insulated Gate Bipolar
Transistor (IGBT) which, upon receiving a pilot signal from
the control board, either enables or disables the current in
the pilot winding. The cutting and pilot rectifier diodes are
also incorporated in the output board. There are two
diodes for each of the secondary windings.

There are two P.C. board mounted current sensors. One
sensor regulates pilot and cutting current. The other sen-
sor indicates to the control board when and how much
current transfers to the workpiece.

The output board also includes the trigger circuitry, the gas
solenoid driver, the electrode solenoid driver and the torch
parts-in-place circuitry.

The output choke, which is in series with both the pilot cir-
cuit and the cutting circuit, provides current filtering to
enhance arc stability.

The PCT 80 torch uses a patented touch start mechanism
that provides superior starting performance over other
touch start systems. The torch head consists of 3 major
parts: torch body, insulator and piston. The insulator pro-
vides an electrical barrier between the piston and torch
body. The piston provides a path for electrical current to
the electrode. The piston also drives the electrode to the
nozzle for arc initiation. The torch body contains the main
torch components: the trigger, pilot arc, cutting arc, and air
flow systems are included.

A copper nozzle with a patented internal swirl is used to
focus the arc. A small, precise hole in the end of the noz-
zle constricts the arc and increases the current density.
As the air enters the torch head, it is directed between
the electrode and nozzle for maximum electrode cooling.
A portion of the cooling air exits the chamber through
vents in the side of the nozzle. A swirl texture located
inside the bottom of the nozzle increases the plenum air
swirl strength, and improves arc start reliability and parts-
in-place verification.

Plasma arc initiation occurs as follows: First, in the idle
state, a spring inside the torch head pushes the piston
and electrode forward to make continuity with the nozzle.
When the trigger is pulled, air flow begins and creates
enough back force on the electrode to overcome the
force of the spring. However, the solenoid valve allows
enough forward force on the piston to maintain continu-
ity between the consumables. After this continuity has
been verified by the Pro-Cut’s parts-in-place circuit, out-
put current is established and regulated. Once the cur-
rent stabilizes, the solenoid valve turns off, removing the
forward force on the piston. The back pressure drives
the piston and electrode away from the nozzle, creating
the plasma arc. The air stream forces the arc out the ori-
fice of the nozzle. This appears as a pilot arc, which can
then be transferred for cutting.

THEORY OF OPERATION

E-5

PRO-CUT 55