Controls & Transformers
Questions and Answers
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.
SCR’s carry a large amount of current and generate a lot of heat. The larger units are water cooled. There are some definite recommendations that must be followed for plumbing the SCR contactor. Always use nonconductive low carbon hose.
Controls have a function available that is called retraction. This describes an action of the pneumatic cylinder or servo. Retraction can speed up the weld operation when repeating like welds near each other. We don’t want the cylinder to open fully just enough to allow part or gun movement to the next weld location. Then the weld repeats. Another example is to open enough to load the next part but not full open to reduce cylinder travel time.
Repeat or pulsing refers to repeating the weld and hold portion of a weld schedule. Normally there is a very short off period of one or two cycles with continued application of force between the two weld sequences. This gives you the ability to dissipate the heat energy a little further into the part away from the center of the weld nugget.
Squeeze is the first segment of the weld schedule. It’s function is to apply force on the work pieces to bring them together. This pressure under the face of the electrode in turn makes good electrical contact and controls and contains the resultant weld nugget during the weld and hold portion of the weld cycle. If current is applied prematurely before the proper force has been reached these functions will be compromised and expulsion and poor nuggets should be expected.
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