Lincoln Electric SVM144-B service manual Thermal and Overload Protection, Overcurrent Protection

E-6 THEORY OF OPERATION E-6

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THERMAL AND OVERLOAD PROTECTION

The POWER MIG 255 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.

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.

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POWER MIG 255