Inductive losses – Error during resistance welding

A factor influencing the welding quality in resistance welding with AC machines is the change in secondary impedance. This results in losses in the secondary circuit due to the ohmic and inductive resistance of the machine. The inductive resistance (secondary impedance) depends on the window area of the welding gun or machine (simplified: throat * arm spacing), frequency of the secondary current, as well as mass and geometry of magnetisable components, fixtures or workpieces in the secondary circuit. By increasing the inductance, i.e. immersing the workpiece, the achievable secondary current is reduced.

Ideally, the design of the components and any auxiliary structures should be such that the iron content in the welding window is as small or constant as possible. Devices in the immediate vicinity of the secondary circuit should be made of non-magnetisable materials. With variable iron content, i.e. with variable secondary impedance, the inductive losses in AC systems can be compensated by controls with constant current regulation.

With DC and medium-frequency systems, the inductive losses do not occur, of course, due to the DC characteristic of the welding current. Since the inductive resistance and thus the inductive losses are frequency-dependent, they are not present with direct current (frequency = 0). If variable iron content in the secondary circuit cannot be avoided, medium-frequency systems should be provided. As described above, the losses can be compensated for by constant current control, even with AC machines, but as the name suggests, they are still losses. And such should be avoided on principle for energy and environmental policy reasons.