For AC welding it is important to operate at a relatively high percent heat. This gives you the maximum efficiency and minimum off time during each weld cycle of the sine wave. So your weld schedule should be designed to optimize this situation as opposed to a high tap and a low setting where a large percentage of the time (each cycle) there is no current flow and the weld is actually cooling down.
Yes, it is a series weld. The weld guns are all arranged on one side of the part with the lower side being a fixed platen shunt die. Current flows through one wire joint into the lower shunt die to the next wire joint through that joint and back to the next gun to the transformer. This completes the circuit and the projection weld is complete. A series of many welds happen sequentially and the machine indexes to the next group.
Series welds do have shunting currents as shown in the sketch below. The transformer is attached to weld guns both on the same side of the part and the current must flow through the shunting die or part to return to the transformer. Some amount of current will flow across the top sheet unless this distance is large which would increase the resistance and retard this flow. Any current flow in the top sheet is wasted current "shunting current" that does not contribute to the weld nugget. This type of tooling arrangement is used when access to the underside of the part is restricted.
In North America, Europe and many other parts of the world AC power is frequently converted into DC at 1000 Hz. This is referred to as MFDC which stands for MID FREQUENCY DIRECT CURRENT. In some parts of the world AC is converted to DC single phase and others to DC at various frequencies higher and lower than 1000Hz. One such MFDC example is DC at 600 Hz.
The general rule of thumb is to turn the water off when you turn the power off and in turn the water should be turned back on when the machine is powered up again.
See Article: “When Should the Cooling Water be Turned On?”
If the chiller remains on during a machine off period, it may be fine provided that there are no water leaks and condensation does not accumulate anywhere on the machine. This can be critical in the SCR, the electronics of the controls and the transformer. Condensation on or in any of these components can lead to a short and failure of the SCR, Transformer or Control.
So the question might be how cold is the chilled water? What is the ambient humidity? How long will the machine be idle? Are there hose leaks that normally are dried up by the equipment operation? If any of these answers would indicate water accumulation then shut the chiller down and protect the equipment.
If the water was left on during the down time and water did not accumulate on any surfaces today the gamble paid off and you should be able to turn the machine back on and operate again.
Remember the recommended method to protect the SCR’s, Control and Transformers is:
References: AWS Standard J1.2M/J1.2:2016 "Guide to Installation and Maintenance of Resistance Welding Machines"
RWMA Manual Fourth Edition
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