In some welding operations it is beneficial to apply a period of low heat to preheat the part prior to application of full heat. One option is to use upslope described in a previous article. Preheating is also used frequently. In preheating current is applied at some constant level lower than the final welding current.
Controls also have the ability to cool a part down slowly. One option is downslope described in another article. Post heat is also a means to accomplish this goal. In post heat the current is reduced at the end of the weld cycle to a level less than the weld current for a period of time before initiating the hold and off portion of the weld sequence.
Constant current is a very useful feature of today’s controls. If this feature is present and you ask for 10,000 amps. The control will deliver that amperage each weld. It won’t matter if you are near the edge of the part or putting half of the file cabinet into the throat of the AC machine as you move across the part. The control with constant current will deliver 10,000 amps to the weld. It has a feedback coil that tells it the amperage that is flowing and it makes corrections to deliver the proper amount during the weld.
To convert incoming air pressure to the force at the electrodes is a very simple calculation. To do so you must know the line pressure and the diameter of the air cylinder.
SCR contactors are subject to heating due to the high currents that they sometimes have to carry. They are normally air or water cooled but can still over heat if driven too hard or not cooled correctly. Heat is one of SCR’s main failure modes. SCR’s fail in the “ON” shorted mode. This makes caution very important. Over temp monitors on the contactors is recommended by the SCR manufacturers.
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