EQUIPMENT FOR RESISTANCE WELDING-Single phase vs three phase
The resistance welding machines can be connected either across 440 V mains (across two lines) or three phase mains. Among the three phase machines, frequency converter and rectifier systems are the two main categories. Fig. gives the line diagrams of single phase and three phase machines.
Single phase machines are simpler, less costly and cheaper than three phase equipments. Maintenance and installation are easier with single phase machines. The single phase machines having the same capacity as the three phase machines, possess the same performance capabilities. The power factor is lower (40 to 50 %) in the former case, but a high kVA demand is required for a brief period. Unless the loads are properly recalculated and properly redistributed and designed, heavy flickers (cyclic and non - cyclic voltage reductions) will results. Though, the total load is not much, fixed charges are high (because of trend by service, power factor, connected load). Because of the decreased frequency in the secondary, the power factor in the case of three phase machines is 85 % or more. They are not generally suitable for welding of thin metal or weld times of three cycles or less is required. Three phase transformers generally have not been applied to multiple electrode or multiple transformer machines since balancing is otherwise possible in those cases.