Lincoln Electric SQUARE WAVE TIG 355 Service Manual page 52

THEORY OF OPERATION
E-8 E-8
PROTECTIVE CIRCUITS
Protective circuits are designed into the Square Wave
TIG 355 to sense trouble and shut down the machine
before the trouble damages the internal machine com-
ponents. Both thermal protection and current overload
are included.
THERMAL PROTECTION
Thermostats protect the machine from excessive oper-
ating temperatures. Excessive operating temperatures
may be caused by a lack of cooling air or by operating
the machine beyond the duty cycle and output rating.
If excessive operating temperature should occur, caus-
ing the secondary thermostat to open, the input con-
tactor will shut the machine off. If the primary thermo-
stat opens, the machine will stay on (pilot light, meters
and fans running), but output current will be limited to
no more than 5 amps.
Once the machine cools sufficiently the thermostats
are self-resetting. If the thermostat shutdown is
caused by excessive output or duty cycle and the fan
is operating normally, the power may be left on, and the
reset should occur within a 15 minute period. If the fan
is not functioning properly or the air intake louvers are
obstructed, then the input power must be removed and
the fan problem or air obstruction corrected.
OVERLOAD PROTECTION
The machine is electronically protected from producing
excessively high output currents. Should the output
current exceed approximately 400 amps, an electronic
circuit will reduce and limit the output current. The
machine will continue to limit the output current until
the overload is removed.
SQUARE WAVE TIG 355