Controls & Transformers
Questions and Answers
Steppers are a means of changing the weld schedule to match the increase in the face size of the electrode as it mushrooms/wears during use. All spot-welding electrodes mushroom as they are used. As this face increases in size the original weld schedule is no longer sufficient to maintain a constant weld nugget size. The current density has decreased due to the increased electrode face area. To counteract this one can either dress the face back to original size or increase the weld schedule to match the face growth.
Weld controls like most other components on a resistance welder conduct large currents. This current can generate heat. This heat must be dissipated for the equipment to operate efficiently. Yes, weld controls must be cooled. Lower amperage units can be air cooled. Higher rated units are water cooled.
In many manufacturing facilities the power in the plant is not sufficient to maintain constant power throughout the workday. A resistance weld schedule can be developed and during the course of the day the operators notice that by mid-morning their welds are getting smaller. They need more power. Then when second shift comes in they have too much power and have to turn it back again. In some cases this can be traced back to a lack of power in the plant and a voltage drop due to the large usage during the main productive hours of the plant.
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.
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