Lincoln Electric POWER MIG SVM170-A Service Manual page 38

E-6
THERMAL AND OVERLOAD
PROTECTION
The POWER MIG 255C 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 has solid state overload protection
of the wire drive motor. If the motor becomes over-
loaded, 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 protection is
done with reliable solid state electronics.
POWER MIG 255C
E-6