Tweco 400 MST Arc Master - Manual (Page 23)

400MST
3
OPERATOR CONTROLS
3 – 4
4.3 HOT START
This parameter operates in STICK mode and
improves the start characteristics for stick elec-
trodes. e.g. low hydrogen electrodes. It sets the
peak start current on top of the WELD current.
e.g. HOT START current = 150 amps when Weld
Current = 100 amps & HOT START = 50A
4.4 WELD (A)
This parameter sets the STICK & Lift TIG weld cur-
rent.
4.5 ARC CONTROL
This parameter operates in STICK mode only and
is used to adjust percentage increase in welding
current and is proportional to arc length (arc volt-
age). This control provides an adjustable amount of
arc control (or dig). This feature can be particularly
beneficial in providing the operator with the ability
to compensate for variability in joint fit up in certain
situations with particular electrodes, eg cellulose
and hydrogen controlled electrodes. In all welding
processes, the amount of penetration obtained is
dependent on the welding current; ie the greater
the penetration, the greater the current.
Table 3-3: Weld Parameter Descriptions
In general, having the ARC CONTROL set at 100%
(maximum) allows greater penetration control to be
achieved. With the ARC CONTROL set at 0%
(minimum) the Power Source has a constant cur-
rent characteristic. In other words, varying the arc
length does not significantly effect the welding cur-
rent. When the ARC CONTROL set to 100%, it is
possible to control the welding current by varying
the arc length. This is very useful for controlling
penetration on root runs and side wall wash on ver-
tical up fillet welds.
i) Root runs
During root runs the weld pool forms a "keyhole"
shape. If too much weld current is used, the hole
blows out and the weld collapses. If too little
weld current is used, the hole closes up and
penetration is lost. The size of the hole also
determines the arc length; ie as the hole gets
bigger, the arc gets longer.
If arc force is used, the increase in the arc
length causes the weld current to decrease until
the hole starts to close up but if the hole closes
up to much then the arc length decreases which
causes the weld current to increase. Too little or
too much arc force makes this process unstable.
The operator must adjust the arc force until a
happy medium is reached.
ii) Vertical up welding
When welding vertical up with arc force on, the
operator can control the amount of current by
changing arc length, ie voltage. Weld metal is
deposited by "digging" the electrode into the
side of the base metal joint and then increasing
the arc length with a flicking motion, to allow the
weld pool to freeze, before digging the electrode
into the other side of the base metal joint.
Without arc force, increasing the arc length does
not decrease the weld current sufficiently and
the operator has to manually decrease the cur-
rent via a remote current control to freeze the
weld pool. This welding current reduction also
reduces the penetration.
The arc force allows the weld pool to freeze during
the "flick" phase without decreasing the amount of
weld current available during the "dig" phase thus
maximizing penetration.
Arc Force
Position
Current Increase
when Arc Voltage is
less than 18V
Effect on Welding
Performance
Minimum
(0)
0A
Soft arc, Low spatter,
Low penetration
Medium
(20%)
32A
Normal arc, Improved
fusion characteristics,
Normal penetration
Maximum
(100%)
160A
Hard arc, Deep
penetration