The SCR contactor is a component of the AC resistance welding control. The control initiates the time and frequency of the SCR’s activation and passing of current. This current of course goes to the transformer primary and is converted to low voltage high current in the secondary of the transformer to travel to the electrodes and make the spot weld. The SCR is the on off switch for this process. It functions on every half cycle of current as instructed by the control.
The SCR contactor is a solid state device with no moving parts. It is very fast and reliable. As with all electronics it is sensitive to heat and must be cooled by air or water depending upon its service rating. A temperature limit switch is recommended.
If one of the two SCR’s has failed as noted above, it usually fails in the on mode, The transformer will be under full power. The control will not see this. If activated the electrodes will get VERY HOT. The circuit breaker may trip?
Test voltage across the electrodes when open if they have voltage the SCR has failed.
Sometimes SCR’s don’t short out and trip the breaker. Occasionally they fire erratically. If only one SCR fires, this is called half cycling. The transformer is out of balance and will make a grunting sound, vibrate, current will spike and the breaker may trip.
Generally, a failed SCR leads to voltage when there should not be any at the electrode. The second less frequent events are strange erratic happenings, sounds and tripped breakers. If you are not sure what you are dealing with make a call to your local equipment supplier or distributor who likely has dealt with this situation before.
Reference: RWMA Resistance Welding Manual 4th Edition