Lincoln Electric 11519 Service Manual page 44

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THERMAL AND OVERLOAD
PROTECTION
The POWER MIG 255XT has built-in protective ther-
mostats that respond to excessive temperatures. One
is located on the output choke. The other thermostat is
located on the SCR heat sink assembly. They open the
wire feed and welder output circuits if the machine
exceeds the maximum safe operating temperature.
This can be caused by a frequent overload, or high
ambient temperature.
The thermostats are self-resetting once the machine
cools sufficiently. If the thermostat shutdown is caused
by excessive output or duty cycle and the fan is oper-
ating 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 air intake louvers are obstructed,
then the input power must be removed and the fan
problem or air obstruction be corrected.
THEORY OF OPERATION
OVERCURRENT PROTECTION
The machine will automatically reduce the output if the
load on the machine exceeds 300 to 320 amperes.
This protects the welding power SCR's from excessive
short circuit currents and from exceeding their temper-
ature rating before the thermostats can react.
WIRE FEED OVERLOAD PROTECTION
The POWER MIG 255XT has solid state overload pro-
tection of the wire drive motor. If the motor becomes
overloaded, the protection circuitry turns off the wire
feed speed and gas solenoid. Check for proper size tip
liner, and drive rolls, for any obstructions or bends in
the gun cable, and any other factors that would impede
the wire feeding. To resume welding, simply pull the
trigger. There is no circuit breaker to reset, as the pro-
tection is done with reliable solid state electronics.
Power MIG® 255XT
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