Resistors in the welding circuit
As the name suggests, a decisive factor in resistance pressure welding is the electrical resistance. Since there is a resistor at the welding point, a current is set up by applying the voltage. With the start signal to the welding controller, voltage is supplied to the welding transformer via the power unit. On the secondary side, the welding tool, the welding current flows due to this electrical resistance. The important thing is to convert the energy at the welding point. The total resistance is then composed of various partial resistances.
The greatest resistance and thus the place of greatest energy conversion and heating is then between the workpieces to be welded (sheets in spot welding). In the field of welding, a distinction is made between two groups of resistors, the material resistors and the contact or transition resistors. These are then the resistances in the upper and lower electrode (material resistances), the resistances in the upper and lower sheet (material resistances) and the contact resistances at the contact between upper and lower electrode to the sheet and the contact between the sheets. The latter must be the one with the greatest energy conversion for welding to occur.
In addition to the effect of the large resistance between the sheets, it also plays a role in the melting process that the heat generated cannot dissipate at this point. Since the electrodes are usually cooled, the heat can be dissipated better there.
Further resistances are, of course, still effective in the entire secondary circuit of the welding device, in the supply lines, the electrode arms and other contact points. To improve the energy balance, these should be small in order to keep the efficiency high and the losses low.