Automatic Protection Shutdown; Overvoltage Shutdown; Motor Overload Shutdown - Lincoln Electric LN-25 Service Manual

B-10
CAUTION
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
stickout (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 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.
OPERATION
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 stickout 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.
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.
LN-25
B-10