Lincoln Electric IM677 RED-D-ARC LN-25 - Manual (Page 20)

LN-25
B-9 B-9
OPERATION
Do not use LN-25 models below Code 9200 with
any TIG or Square Wave welding power sources.
Do not use LN-25 models equipped with internal
contactors with non-Lincoln TIG or Square Wave
welding power sources. Damage to the LN-25 circuit
can occur as a result of the high output inductance
typically associated with these power sources. TIG
high frequency power should never be applied to
the LN-25.
While welding with a continuously fed electrode,
weld current variations are continuously taking place.
There are many causes for these variations, but
predominantly they occur due to changes in
electrical stick out (operator hand movements,
nozzle tip contact variations, etc.) and the dynamic
metal transfer characteristics of the process or
procedure being used (short arc, globular transfer,
etc.).
When using a CV power source, these current
variations have essentially no effect on the welding
arc stability since CV power sources can provide a
wide range of weld current levels with virtually no
change in average arc voltage. These “flat slope” CV
power sources, therefore, provide the arc power
required to produce the best welding characteristics
and ease of operation for most open arc, constant
wire feed speed, welding processes.
CC (formerly VV) power sources, on the other hand,
permit the arc voltage to decrease with increases in
welding current. The output characteristics of these
power sources can range from a “drooping slope,”
which provide minor arc voltage changes with weld
current variations, to “steep slope” which provide
broad voltage changes with only minor current
variations. The steeper the slope the more difficult it
becomes to maintain arc voltage stability with a
constant wire speed open arc process.
To aid in stabilizing the arc voltage when welding on
CC power sources the LN-25 is provided with a VV
(CC) wire feed mode. This arc voltage sensing feed
mode decreases (or increases) the wire speed when
the arc voltage decreases (or increases).
However ,
if the electrode shorts to the work, the arc voltage
will essentially drop to zero and the short circuit
current supplied by the CC power source may not
provide enough power to re-establish the arc
(especially with steeper slope machines). Under this
condition the LN-25 feeder will stop, as if the trigger
was released, and not restart until the short has
been opened.
Arc shorting is more difficult to avoid when using
a CC power source, and requires a more refined
operator technique than when using a CV power
source. Improved performance can be obtained
o n C C p o w e r s o u r c e a p p l i c a t i o n s w i t h i n t h e
following guidelines:
1. Flatter slope power sources with higher arc
force (short circuit current) will improve
performance over steep slope machines. The
steeper the slope, the more critical it is to hold
proper electrode stick out to maintain arc
stability. Welding techniques, such as weaving,
will be more difficult to control. Use the highest
output current tap or setting capable of providing
the voltage adjustment required for the
procedure.
2. Use spray or non-shorting small ball type
transfer processes at higher procedure voltage
levels. Generally, open arc processes with
procedure voltage levels over 22 volts perform
satisfactorily. Arc stability at procedure voltage
levels below 22 volts may be more difficult to
control, with the general exception of fine (.023—
.035) solid steel electrodes with Argon-rich
shielding gas or NR-152 and NR-211 Innershield.
CAUTION
AUTOMATIC PROTECTION
SHUTDOWN
The LN-25 control provides automatic electronic
protection circuits which shut down the LN-25 for
excessively high power source open circuit voltage
or excessive motor overload.
OVERVOLTAGE SHUTDOWN
If the power source open circuit voltage exceeds
about 110-120 volts, the LN-25 will not operate until
the power source voltage drops below about 102-
110 volts.
MOTOR OVERLOAD SHUTDOWN
If excessive motor overload occurs (due to prolonged
excessive feeding force, jammed drive rolls or
shorted motor leads), the LN-25 will shut down within
a few seconds after the overload occurs.
The shutdown will reset automatically when the gun
trigger is released, but will reoccur if the overload
situation is not remedied.