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THERMAL PROTECTION
Two normally closed (NC) thermostats protect the
machine from excessive operating temperatures.
These thermostats are wired in series and are con-
nected to the Weld Control board. One of the ther-
mostats is located on the Chopper Assembly board,
and one is on the secondary of the Main Transformer.
Excessive temperatures may be caused by a lack of
cooling air or by operating the machine beyond its duty
cycle or output rating. If excessive operating tempera-
tures should occur, the thermostats will prevent output
from the machine. The yellow thermal light, located on
the front of the machine, will be illuminated. The ther-
mostats are self resetting once the machine cools suf-
ficiently. If the thermostat shutdown was caused by
excessive output or duty cycle and the fan is operating
normally, the power switch may be left on and the reset
should occur within a 15-minute period. If the fan is not
turning or the intake air louvers are obstructed, the
power must be removed from the machine and the fan
condition or air obstruction corrected. The cooling fan
runs only when necessary. The F.A.N. (fan as needed)
system is controlled by the Weld Control Board via
solid state relay.
OVER CURRENT PROTECTION
If the average current exceeds 300 to 320 amps, the
peak current will be limited to 100 amps until the sys-
tem is re-triggered or the average current decreases to
under 50 amps.
WIRE FEEDER OVERLOAD PRO-
TECTION
The Power MIG 300 wire drive motor has its own solid
state overload protection. If the motor becomes over-
loaded, the protection circuitry turns off the wire feed
and the gas solenoid. Overload may result from
improperly sized tip liner and drive rolls, obstructions or
bends in the gun cable, and any other factors that
would impede the wire feeding. To resume welding,
simply pull the gun trigger. There is no circuit breaker
to reset, as the protection is provided by reliable solid
state electronics.
THEORY OF OPERATION
E-8 E-8
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POWER MIG 300