MESSER SolidWELD MIG 250GS Guide D'utilisation page 29

8.2 Inductance
Inductance is defined as the rise in current to above normal, in
any electrical circuit that results in the melting of the wire at some
point. Consider dip transfer (short ¬circuiting arc) in action. Before
the wire strikes the work-piece, there is no flow of current and the
OCV has maximum value. 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 heavier plate 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)
8.3 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.)
The 2T/4T function also allows for the shielding gas to continue
after the weld has finished and cooled. This eliminates the risk of
oxidation while the weld is still in its molten state. This is particularly
important when welding stainless steel materials and aluminium.
MIG 250GS Operating manual 029