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III. GTAW Equipment
Safety First
Even though the majority of welding done is in the direct current
mode, welding power is most often obtained from the local
power company out of an AC wall socket.
Light welding, (low output requirements of about 200 amps
or less) can often be done with single-phase welding
machines. Duty cycles are often in the 60% or less range.
These types of welding machines are especially suited for
shops and garages where only single-phase power is available.
Some of these smaller single-phase machines may be capable
of using 115 volt AC primary power. Other machines may use
230 volt or higher primary power.
Larger DC TIG welding machines used for heavy plate, structural
fabrication and high production welding generally need three-
phase AC input power. Most industrial locations are supplied
with three-phase power since it provides the most efficient
use of the electrical distribution system and it is required by
many electric motors and other industrial electrical equipment.
These welding machines often have capacities of over 200 amps,
and often have 100% duty cycles.
Figures 3.2 through 3.7 show some different types of
welding machines and controllers.
Figure 3.1
GTAW power source plugged into wall connection. Primary
connection to the commercial power.
Notice the fuse box on the wall, where primary power to the
machine must be shut off if work needs to be done on any
part of the welding equipment. Also, the primary power at the
fuse box should be shut off when the machine is idle for long
periods of time.
Figure 3.2
An inverter-based welding machine which has the capability of
modifying the frequency of the AC arc. This machine has multiprocess
capability including GTAW, SMAW, and pulsing capability.
Caution should always be taken when installing any welding
equipment. Should a welding machine be improperly connected,
a dangerous situation could exist. Improper connections
could lead to an electrically “hot” welding machine case,
which could result in a severe shock to anyone touching it.
Primary wiring should only be done by an electrically qualified
person who is absolutely sure of the electrical codes in a
given area. Before any primary power is connected to welding
equipment, the equipment’s operation manual should be
read, and the instructions strictly followed.
Figure 3.3
An electronically controlled AC/DC power source. Features
include wave balance control to selectively unbalance the wave to optimize
welding characteristics.
Selecting a Power Source
With the many types of welding machines available, certain
considerations must be made in order to fit the right machine
to the job.
Rated output of the welding machine is an important consid-
eration. The ranges of voltage and amperage needed for a
particular process must be determined. Then, a welding machine
can be selected to meet these output needs. Remember, the
output must be within a proper duty cycle range.
Figure 3.4
An AC/DC machine which was specifically designed for GTAW.
It includes many built-in components that make it adaptable to a wide
variety of applications.
19
The Constant Current Power Source
Arc welding power sources are classified in terms of their out-
put characteristics with regard to voltage and amperage. They
can be constant current (CC), constant voltage (CV) or both.
A constant current machine, the kind used in GTAW welding,
maintains close to a constant current flow in the weld circuit
no matter how much the voltage (arc length) varies.
Processes like GTAW and Shielded Metal Arc Welding (SMAW)
require the welder to maintain the arc length not the equipment.
Figure 3.5
An AC/DC machine of the type commonly used for Stick
electrode (SMAW) welding. With the addition of other components, it will
meet the requirements of many GTAW applications.
A constant voltage power source maintains voltage at close to
a preset value no matter how much current is being used in the
process. This is the type of power source that is used in Gas
Metal Arc Welding (GMAW) or Metal Inert Gas (MIG) welding.
Processes like GMAW and Flux Cored Arc Welding (FCAW)
require the equipment to maintain a specific arc length.
You’ll notice that in both cases we say these machines maintain
current and voltage values close to preset values respectively.
They will vary slightly due to the fact that no power source is
perfectly efficient.
The relationship between voltage and current output is best
represented by plotting these values on a graph.
40
5
4
6
3
7
2
8
1
9
30
0
10
ADJUSTING VOLTS
Figure 3.6
A multiprocess engine-driven welding generator capable of
AC and DC GTAW welding when fitted with an optional high-frequency
arc starter.
20
10
0
10
20
AMPS
30
40
50
Figure 3.8
Volt-amp curve of a perfect battery.
Figure 3.8 shows the volt-amp curve of a perfectly efficient
battery. This would be considered a CV power source
because no matter how much current is produced, the
voltage remains constant at twelve volts.
Figure 3.7
An advanced power source with a built-in programmer that
enables the operator to program the entire welding sequence. This is
recommended for automatic welding or whenever repeatability is required.
In order to best understand the arc welding power source and
its requirements, it is best to start at the arc and work back to
the wall receptacle. The GTAW process requires the welder to
maintain the arc length. Any variation in arc length will affect
the voltage. The longer the arc the higher the voltage, and the
shorter the arc the lower the voltage. The welder will have dif-
ficulty maintaining the arc length, the voltage will change, as
the arc is moved across the part being welded. This change
in voltage (arc length) causes the output current (amperage)
to fluctuate. This output current should be kept as constant
as possible with the TIG process. The amperage creates the
heat that melts the metal and allows for consistent welding.
80
5
4
6
3
7
2
8
9
1
60
0
10
ADJUSTING AMPS
40
20
0
50
100
AMPS
150
200
250
Figure 3.9
Volt-amp curve of a perfect CC power source.
20
A perfectly efficient power source of the CC variety as seen in
Figure 3.9 would exhibit a volt-amp curve where a constant
current of 100 amps is output no matter what the voltage.
The true constant current power sources are an advantage in
that what current is set is what is delivered to the welding arc.
These electronically controlled power sources are desired
over the older-style power sources and find applications in
manual through automatic welding. The current settings are
very accurate and welds are very repeatable. The electronically
controlled and inverter-type power sources have special
circuits that maintain their output very consistently. This is
accomplished with a closed loop feedback circuit. This circuit
compares the output current going to the arc against what
has been set on the machine. It acts much like a car with the
cruise control activated — if going up and down a hill the
speed is maintained. If the welder raises and lowers the arc,
the amperage is maintained. Figure 3.13 shows a block dia-
gram of this closed loop feedback sense circuit. This feature
is also helpful for line voltage compensation. By law the power
company must supply a consistent voltage. However they are
allowed a range, which can be as much as plus or minus 10%
of the nominal voltage. If the primary voltage to a non-com-
pensated GTAW power source changed up to 10%, the power
going into the arc can fluctuate from 10 – 20%. With the line
voltage compensated machine, a plus or minus fluctuation of
up to 10% on the primary will only have a plus or minus
2% change in the arc, thus a very consistent weld. Most
electronically-controlled power sources can also be used to
provide pulsed welding current. Due to their fast response
time and great control over the current level setting, two different
heat levels pose no difficulty for these type power sources.
These machines can also be remotely controlled and these
controls can be very small and compact. They are small
enough to be mounted directly on the torch or built into the
torch handle. Limitations of this design can make them more
complex to operate, and are relatively expensive in comparison
to simpler control designs.
80
CC
40
50
100
150
AMPS
Figures 3.10
CC volt-amp curve.
25
CV
20
15
10
5
0
100
AMPS
200
Figures 3.11
CV volt-amp curve.
The volt-amp curve shown in Figure 3.10 is indicative of
those seen in GTAW power sources, and the volt-amp curve
seen in Figure 3.11 represents the output of a constant voltage
or GMAW power source. The sloping line on the constant current
graph represents the output of a magnetic amplifier power
source. Because of this sloping characteristic, these power
sources are often referred to as droopers.
Squarewave Silicon-Controlled
Rectifier (SCR) Power Sources
These type power sources were introduced to the welding
industry in the mid 70s. They have now virtually replaced all
the AC sine wave power sources for the GTAW process. The
block diagram shown in Figure 3.14 is a representative of this
type of control. These type power sources use the large bulky
50 or 60 Hz transformer. They are typically very similar in size
and weight to the older style mechanically or magnetically
controlled power sources. They do have simple wave shaping
technology and possess closed loop feedback for consistent
weld output.
Figure 3.12 is an example of a basic DC power source for TIG
welding. The single-phase high voltage, low amperage is
applied to the main transformer. The transformer transforms
this high voltage to low voltage and at the same time transforms
the low amperage to high amperage for welding. It does not
affect the frequency, 60Hz in and 60Hz out. This low voltage
high amperage is now rectified from AC to DC in the rectifier.
This produces a fairly rough DC unlike the power provided by
a battery. The filter is used to smooth and stabilize the output
for a more consistent arc. The filtered DC is now supplied to
the TIG torch. These line frequency type power sources tend
to be large and very heavy. Their arc performance is slow and
sluggish and won’t allow them to be used for advanced wave
shaping or pulsing.
21
VOLTAGE
TRANSFORMATION
CONTROL
CIRCUIT
AND ISOLATION
RECTIFIER
FILTER
WELDING
AC PRIMAR
Y
OUTPUT
POWER
POWER
(50/60 Hz)
AC
AC
DC
Figure 3.12
A conventional line frequency power source block diagram.
VOLTAGE
TRANSFORMATION
AND ISOLATION
CONTROL/
CONDITIONING
FILTER
ELEMENTS
WELDING
OUTPUT
POWER
AC PRIMAR
Y
POWER
(50/60 Hz)
AC
DC
CONTROL
CIRCUIT
SENSE
CIRCUIT
Figure 3.13
The closed loop feedback keeps the output consistent when the arc voltage is varied and to compensate for primary line voltage fluctuations.
VOLTAGE
TRANSFORMATION
AND ISOLATION
CONTROL/
CONDITIONING
FILTER
ELEMENTS
AC PRIMAR
Y
POWER
(50/60 Hz)
WELDING
OUTPUT
POWER
AC
DC
Figure 3.14
Block diagram of an SCR controlled power source, utilizes a line frequency weld transformer.
INVERTER SECTION
INPUT
RECTIFIER
POWER
SWITCHES
TRANSFORMER
ISOLATION
OUTPUT
FILTER
RECTIFIER
FILTER
WELDING
OUTPUT
POWER
AC PRIMARY
POWER
(50/60 Hz)
50/60 Hz AC
DC
25 kHz AC
DC
CONTROL
SENSE
CIRCUIT
CIRCUIT
Figure 3.15
An inverter power source block diagram.
22
The Inverter Power Source
Inverter power sources were first conceived in the 1940s, but
weren’t successfully marketed until the 1970s.
tasks nearly anywhere in the world. Most of these machines
are welder generators that along with welding output produce
AC/DC current for the operation of lights and power tools.
Engine driven welding power sources are usually referred to
as rotating power sources of which there are two basic types.
The ALTERNATOR, which produces alternating current,
and the GENERATOR, which produces direct current. Most
manufacturers produce machines that provide both AC and
DC from the same unit.
Instead of operating at a common input power frequency of
50 or 60 Hz, inverters boost the frequency as much as 1000
times that of input frequency. This allows for a drastic reduction
in the number of transformer coil turns and reduced core area
resulting in a machine much smaller and lighter in weight
than a conventional transformer rectifier power source.
Another major advantage of this type of machine is its primary
power requirements. Some inverters can be used on either
three-phase or single-phase input power, and either 50 or 60 Hz.
This is due to the fact that incoming primary power is recti-
fied and converted to the extent that it is not a critical factor.
Some inverters due to their unique circuitry, are multiprocess
machines capable of GTAW, GMAW, SMAW, FCAW (Flux
Cored) and Carbon Arc Gouging. Although these inverters are
capable of accomplishing these multi-processes, some are
specifically designed for and specialized for the TIG process.
Figure 3.15 is a block diagram of an inverter type power
source. Machines of this type can run on single or three-
phase power, which will be covered later in this section. The
first thing the inverter does is rectify the high voltage low
amperage AC into DC. It is then filtered and fed to the inverter’s
high-speed switching devices. Just like a light switch they
turn the power on and off. They can switch at a very fast rate,
up to 50,000 times per second. This high voltage, low amperage
fast DC switching looks like AC to the transformer, which is
many times smaller than a 60 Hz transformer. The transformer
steps the voltage down and increases the amperage for welding.
This low voltage high amperage is filtered for improved DC
arc welding performance or converted to AC through the
electronic polarity control. This AC or DC power is then
provided to the TIG torch. This AC is fully adjustable as
described in the section on Advanced Squarewave AC.
The DC is extremely smooth and very capable of being pulsed
or sequenced.
Figure 3.16
Maintenance welding on agricultural equipment with an
engine driven power source.
Duty Cycle
As mentioned earlier in this section, duty cycle is of prime
importance in the selection of a welding machine. The duty
cycle of a welding power source is the actual operating time
it may be used at its rated load without exceeding the
temperature limits of the insulation in the component parts.
The duty cycle is based on a ten minute time period in the
United States. However, in some parts of the world, Europe
for example, the duty cycle is based on a five minute time
period. Simply stated, if a power source is rated at a 50%
duty cycle and it is operated at its rated output for five minutes,
it must be allowed to cool for five minutes before operating
again. The duty cycle is not accumulative. For example, a
power source with a 50% duty cycle cannot be operated for
thirty minutes then allowed to cool for 30 minutes. This violates
the ten minute rule. Also a machine rated at 50% should not
be operated at maximum for five minutes and then shut off.
The cooling fan must be allowed to operate and cool the internal
components, otherwise the machine might incur damage.
The Engine-Driven Power Source
Some of the first electric arc welding power sources invented
were the motor generator type that produced welding current
by means of a rotor moving inside a stator. This is the same
principle of current generation by means of moving a conductor
through a magnetic field. The movement in these machines
was provided by an electric motor.
The concept is still being put to good use by modern power
sources that replace the electric motor with gasoline or diesel
engines. The most important feature of these electro-mechanical
devices is that they free the welder from dependence on com-
mercial power, and allow them the mobility to accomplish
A power source with a 100% duty cycle may be operated at
or below its rated output continuously. However if the machine
is operated above its rated output for a period of time, it no
longer has a 100% duty cycle.
23
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