Inductance; 2T Trigger Latch Selection - Linde BOC Smootharc Advance MIG 425R Operating Manual

7.1.1 Inductance

Inductance in welding is the control of current surges during short circuits.
The wire touches the weld pool 50 to 150 times per second. When the wire
strikes the work-piece it creates what is known as a "dead short". This short-
circuit causes the current to rise rapidly and burn off the wire violently. This
rapid melt flings about metal globules as it takes place. These globules settle
on the work-piece and welding torch in the form of "spatter".
To control the rise rate of short-circuiting current, a choke (or inductor) is
fitted in series with the welding power cable. This inductor "chokes" the rate
of rise of current and ensures a smooth arc condition.
Inductance controls the rate of rise of short-circuiting current. It becomes
evident that inductance must be a function of time and current, since it
controls the time taken for the current to rise to the pre-set value. Hence an
increase in inductance results in less frequent short-circuiting. Fewer short-
circuits imply that the arc is present for a longer duration of time. Increased
"arc-on" time means increased heat.
Therefore, on a thicker material where more heat is required to ensure
good fusion, more inductance would contribute immensely. Conversely on
thinner sections less inductance would lead to a decrease in "arc-on" time,
and consequently a "cooler" arc which enables thin materials to be welded
with ease.
Summarising, inductance has the following effects:
→ reduced spatter
→ heat control
→ has no effect when spray-arc welding, because there is no more change
in current, the inductance coil plays no further part. A certain amount
of inductance with spray-transfer will provide softer starts (the initial
short-circuit)
BOC Smootharc Advance III MIG 425R & 555R Operating manual
7.1.2 4T/2T Trigger Latch Selection
On all MIG machines there is no current or wire feed until the trigger on the
torch is depressed. If a welder is doing a lot of welding then he/she has to
hold the trigger down for long periods of time and may cause discomfort.
This is can be similar to repetitive strain injury (RSI) that has become a very
popular topic for compensation by office workers.
On all machines a special function called 2T and 4T is available. Also referred
to as trigger latching, this special feature allows the operator to relax the
trigger after first depressing it and the gas shielding to start before the
welding commences. This feature is of particular importance as it ensures that
the weld will have adequate gas shielding to eliminate the risk of oxidisation
(contaminants) causing a defective weld. (Remember, a defective weld may
not be detected by a visual inspection.)
Please note that on the Advance III range different features are available for
the 2T and 4T trigger selections.
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