Abstract: A frequency converter of the series-capacitor type is made substantially insensitive to changes in output load by providing the same with means for limiting the voltage across the transformer and load capacitor or capacitors thereof to a pre-determined value.

Abstract: An a.c. operated arc welding apparatus comprises a controlled frequency converter which substantially increases the frequency of the alternating current and the output of which is connected to the welding electrodes via a transformer and a rectifier. The connection between the rectifier and the welding electrodes includes a shunt capacitance having an impedance low enough to attenuate the alternating current components of the welding current to a level acceptable from the acoustic aspect, the said connection further comprising means for preventing the capacitor from adversely affecting the rectifier and frequency converter and also comprising a choke arranged between the capacitor and the electrodes to prevent the capacitor from adversely affecting the welding properties.