Protective Devices and Shutdown Circuits

E-4

THEORY OF OPERATION

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PROTECTIVE DEVICES AND SHUTDOWN CIRCUITS

THERMAL PROTECTION

Two thermostats protect the DC-655 from excessive operating temperatures. Excessive operating temper- atures may be caused by a lack of cooling air or oper- ating the machine beyond the duty cycle and output rating. If excessive operating temperatures should occur, a thermostat will open. This condition will be indicated by the illumination of the yellow Thermostatic Protection Light located on the case front. The fan should continue to run, but the welding output will be disabled until the machine cools and the Thermostatic Protection Light is out.

OVER CURRENT PROTECTION

If the average output current exceeds 900 amps for five to six seconds, the welding output will be dis- abled. The control board LED 4 shutdown light will turn on. The higher the current, the quicker the DC- 655 will shut down. If the average current exceeds 1200 amps, the shutdown will occur in 0.3 seconds. The over current shutdown circuit can be reset by opening the feeder gun trigger or switching the DC- 655 output terminal switch to the OFF position.

PEAK CURRENT PROTECTION

If the peak current exceeds 2500 amps, the welding output will be disabled and the control board LED 4 shutdown light will turn on. The peak current shut- down circuit can be reset by turning the DC-655 input switch off.

REMOTE CONTROL FAULT PROTECTION

The remote control leads from the 14-pin receptacles or terminal strip are protected against voltage faults to the electrode circuit or auxiliary voltage supplies. If a fault should occur, the welding output will be disabled and the control board LED 4 shutdown light will turn on. This fault protection shutdown circuit can be reset by opening the feeder gun trigger or switching the DC- 655 output terminal switch to the OFF position. On later production machines, LED 3 input shutdown light will turn on and the input contactor will open.

SHORTED OUTPUT RECTIFIER FAULT PROTECTION

If a short should occur across one of the SCRs, a potentially hazardous AC voltage could appear across the welding output terminals. If such a fault should occur, the input contactor will open and remove the input power to the primary of the main transformer. Also, the control board LED 3 input shutdown light will turn on. Welding and auxiliary output will not be pre- sent, but the pilot light and control board power will be present until the power switch is turned off.