Plasma Torch Height Control The Secret to Success

Plasma Torch Height
Control The Secret to
Success
Linde Canada
O
ne of the most important components on an
automated plasma cutting machine is unfortunately
one of the least understood. Maintaining the
correct torch to work piece distance with an
automatic voltage control (AVC) torch height system
is critical in achieving good cut quality throughout
all stages of the cut.
“
There are many parameters that affect the
characteristics, and ultimately the voltage of a plasma
arc. Manufacturers have done extensive research to
develop cut charts with specific parameter settings
for various amperage processes, material types and
thicknesses to optimize the performance of their
products. However, when “out of the box” settings do
not seem to work properly, an understanding of the
basic process can help operators diagnose and fine
tune the cutting parameters to achieve the desired
results.
The primary reason for
controlling torch height is to
minimize the bevel angle of
the cut edge.
”
Let’s start by reviewing how an automatic voltage
control torch height system works. Similar to stick
(SMAW) and tig (GTAW) welding, plasma arc
cutting (PAC) is a constant current (CC) process. The
process amperage across the plasma arc is constant,
but the voltage can vary. By definition, voltage is the
difference in electrical potential between two points
(in our case the torch and the work piece) and it is
the force that drives the current across the resistance
of the arc. In plasma cutting, changing the distance
between torch and work piece means changing the
arc length and arc resistance. A shorter arc has less
resistance and the voltage will drop. A longer arc has
more resistance and the voltage will rise.
Arc voltage therefore can be used to control the
distance of the electrode to the work piece. The
function of the AVC torch height system is to
monitor the arc voltage, and either raises the torch if
the arc voltage is too low or lowers the torch if the arc
voltage is too high in order to maintain a target arc
voltage set point.
8
Sequence of Operation
When starting a cut, the torch height control will
perform an initial height sense cycle to locate the
surface of the work piece. The torch will be lowered
until plate contact is sensed either by electrical
continuity on touch, or motor stall force. It will retract
to a pierce height which is usually higher than the
cut height. After the arc is started and the material is
pierced, motion begins and the torch is then lowered
to the correct cut height. Some thick material
piercing methods actually raise the torch after the
pierce to clear the slag puddle at the start of motion.
Only after motion builds up to programmed cut speed
is when automatic voltage control is enabled. Once
enabled, the arc voltage control will determine the
torch height.
Pierce Height vs Cut Height
Plasma torch consumables are most susceptible to
damage during the piercing procedure. Starting the
cut at the correct pierce height will help to optimize
your consumable life by ensuring arc transfer to the
work piece as well as reducing the chance of molten
material from being blown back into the torch,
damaging the nozzle and shield.
If the torch is too far away from the work piece, the
pilot arc will not transfer to the plate, and a miss-fire
will occur. This excessive pilot arcing will damage the
nozzle.
If the torch is too close, or even touching the plate,
double arcing can occur where the nozzle remains in
the arc circuit to the work piece, again resulting in
damaged consumables.
Effect of Torch Height on Bevel Angle
The primary reason for controlling the torch height
is to minimize the bevel angle of the cut edge. As the
diagram (courtesy of Hypertherm) illustrates, if the
torch is too far above the plate, a positive cut edge
bevel will be produced. Too close to the plate will
result in a negative bevel angle.
Keep in mind that other factors can also contribute
to excessive bevel. Torch angle to plate, cut direction,
speed, and consumable conditions can all influence
the bevel angle of the cut.
Effect of Speed on Height
When the plasma arc is established and the torch is in
motion, the arc will operate at a certain power level as
determined by the cutting parameter settings.
If travel speed slows down, the material removal path
(kerf) widens on the work piece, offering less area for
the arc to attach. This causes the arc to also widen and
stretch as it searches for more material to
maintain transfer. This increases the resistance of the
arc, resulting in an increased arc voltage, which causes
the automatic height control to lower the torch.
If the speed continues to slow down, the torch will
eventually crash into the work piece. Due to the speed
reductions that occur when changing directions when
corner cutting, the common practice is to disable the
torch height control to maintain the correct height.
Conversely, when speed increases, the kerf narrows
offering more area to support transfer which decreases
the resistance of the arc and the arc voltage will drop
causing the height control to raise the torch to
compensate. If speed continues to increase, the torch
will continue to rise, stretching the arc until the
plasma flow can no longer support transfer and it
“snaps off ”, loosing the cut.
Effect of Gas Supply on Height
Plasma arc cutting is made possible by the ability of
super-heated, ionized gas (plasma) to be electrically
conductive. Fluctuations in pressure or volume of the
ionized gas path will affect the resistance and resulting
voltage of the arc. Precise control of gas pressures and
flow rates are necessary to maintain the stability of the
arc.
9
If a reduction in gas pressure or flow rate occurs
during a cut, the arc resistance will increase and arc
voltage will rise causing the height control to lower the
torch. Excessive gas loss will cause the height
control to lower the torch until it eventually crashes into the
work piece.
If gas pressure is increased, the arc resistance will
lower and arc voltage will drop causing the height control
will raise the torch. If there is enough of an increase, the
torch can rise until the arc is eventually snapped off, or is
blown out by excessive gas pressure.
Along with maintaining correct gas pressures and flow
rates, gas purity affects arc stability. Following equipment
manufacturers’ specifications for gas purity requirements is
critical. Higher than recommended moisture and
contaminant content will alter the arc characteristics to
adversely affect torch height control as well as negatively
impact consumable life and cut quality.
Effect of Consumable Wear on Height
In a plasma torch, the arc attaches to an insert (emitter)
at the center of the electrode. As the electrode wears, the
emitter is worn away creating a pit in the electrode. The pit
will effectively lengthen the arc, and thus increase the arc
voltage. The torch height control will then lower the torch
to compensate. Therefore, as the electrode wears, the target
arc voltage setting should be increased to maintain the
correct height required for best cut quality.
Some torch height control systems will automatically
compensate for this electrode wear by adjusting arc voltage
targets through arc voltage sampling and encoder
positioning.
Summary
All too often, technicians are called out to a customer site to
diagnose a problem of torch crashing, or lost cut sense
errors.
One customer was experiencing random torch crashing. We
found that the plasma machine was sharing the compressed
air supply pipeline with the painting department. The
machine would work fine until they started painting. This
created a drop in air pipeline pressure which would in turn
cause the torch to crash. Another customer was
experiencing cut loss errors. It was discovered that the
oxygen delivery pressure to the machine had been increased
beyond the manufacturers recommended settings, thus
causing the torch to raise and lose the cut after piercing.
When it comes to supply gas pressure, more is not
necessarily better. Today’s high tech plasma systems are
designed to operate within a certain window of
specifications.
By understanding the plasma cutting process and how these
basic process variables can affect the torch height control,
you can diagnose most common problems, reduce down
time, and maintain productivity with your plasma cutting
machine.
By Mike Trupp
10