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Plasma Cutters
How a Plasma Cutter Works
Lincoln Electric
What is plasma?
To properly explain how a plasma cutter
works, we must begin by answering the basic question “What is plasma?
In its simplest terms, plasma is the fourth state of
matter.
We commonly think of matter having three states: a
solid, a liquid, and a gas.
Matter changes from one state to the other through the
introduction of energy, such as heat.
For example, water will change from a solid (ice) to
its liquid state when a certain amount of heat is applied.
If the heat levels are increased, it will change again
from a liquid to a gas (steam).
Now, if the heat levels increase again, the gases that
make up the steam will become ionized and electrically conductive, becoming
plasma.
A plasma cutter will use this electrically conductive
gas to transfer energy from a power supply to any conductive material,
resulting in a cleaner, faster cutting process than with oxyfuel.
The plasma arc formation begins when a gas such as
oxygen, nitrogen, argon, or even shop air is forced through a small nozzle
orifice inside the torch.
An electric arc generated from the external power
supply is then introduced to this high pressured gas flow, resulting in what is
commonly referred to as a “plasma jet”.
The plasma jet immediately reaches temperatures up to
40,000° F, quickly piercing through the work piece and blowing away the molten material.
Plasma system components
Power supply -- The plasma power supply
converts single or three phase AC line voltage into a smooth, constant DC
voltage ranging from 200 to 400VDC.
This
DC voltage is responsible for maintaining the plasma arc throughout the cut. It
also regulates the current output required based on the material type and
thickness being processed.
Arc Starting Console – The ASC circuit
produces an AC voltage of approximately 5,000 VAC at 2 MHz which produces the
spark inside of the plasma torch to create the plasma arc.
Plasma torch – The function of the
plasma torch is to provide proper alignment and cooling of the consumables.
The
main consumable parts required for plasma arc generation are the electrode,
swirl ring, and nozzle.
An
additional shielding cap may be used to further improve cut quality, and all
the parts are held together by inner and outer retaining caps.
The vast majority of plasma cutting systems today can
be grouped into either conventional or precision categories.
Conventional Plasma Cutters
The approximate amperage of this type of plasma arc is
12-20K amps per square inch. All handheld systems utilize conventional plasma,
and it is still used in some mechanized applications where the part tolerances
are more forgiving.
Precision Plasma Cutting Systems
Precision plasma systems (high current density) are
designed and engineered to produce the sharpest, highest quality cuts that are
achievable with plasma.
The torch and consumable designs are more complex, and
additional pieces are included to further constrict and shape the arc.
A precision plasma arc is approximately 40-50K amps
per square inch.
Multiple gases such as oxygen, high purity air,
nitrogen, and a hydrogen/argon/nitrogen mixture are used as the plasma gas for
optimum results on a multitude of conductive materials.
Handheld Operation
When the trigger is squeezed, the power supply
produces a DC current that flows through this connection, and also initiates
the plasma gas flow.
Once the plasma gas (compressed air) builds up enough
pressure, the electrode and nozzle are forced apart, which causes an electrical
spark that converts the air into a plasma jet.
The DC current flow then switches from electrode to
nozzle, to a path between the electrode and work piece. This current and
airflow continues until the trigger is released.
Precision plasma operation
Inside a precision plasma torch, the electrode and
nozzle do not touch, but are isolated from one another by a swirl ring which
has small vent holes that transform the preflow/plasma gas into a swirling
vortex.
When a start command is issued to the power supply, it
generates up to 400VDC of open circuit voltage and initiates the preflow gas
through a hose lead set to the torch.
The nozzle is temporarily connected to the positive
potential of the power supply through a pilot arc circuit, and the electrode is
at a negative.
How Plasma Works - Step 1
Next, a high frequency spark is generated from the Arc
Starting Console which causes the plasma gas to become ionized and electrically
conductive, resulting in a current path from electrode to nozzle, and a pilot
arc of plasma is created.How Plasma Works - Step 2
Once the pilot arc makes contact to the work piece
(which is connected to earth ground through the slats of the cutting table),
the current path shifts from electrode to work piece, and the high frequency
turns off and the pilot arc circuit is opened.How Plasma Works - Step 3
The power supply then ramps up the DC current to the
cutting amperage selected by the operator and replaces the preflow gas with the
optimum plasma gas for the material being cut. A secondary shielding gas is
also used which flows outside of the nozzle through a shield cap.How Plasma Works - Step 4
The shape of the shield cap and the diameter of its orifice forces the shield gas to further constrict the plasma arc, resulting in a cleaner cut with very low bevel angles and smaller kerf.
Lincoln Electric was founded in 1895 and
is headquartered in Cleveland, Ohio. With more than 40 manufacturing
locations, including operations and joint ventures in 20 countries and a
worldwide network of distributors and sales offices covering more than 160
countries, Lincoln Electric has a global work force of more than
10,000.
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