Classic DC welding is resistance pressure welding with a three-phase transformer with secondary rectifier. This requires special three-phase controls. Today, these machines are found less and less, as they are often and increasingly replaced by medium-frequency systems. They are still used today for very high welding currents above 200 kA. The basic construction of such DC machines consists of three single-phase transformers, which are connected via one thyristor set per phase (the most common circuit types are open delta connection and star connection).
The rectifier sets are then connected on the secondary side. By using all three phases, a symmetrical grid load is one of the great advantages of direct current technology. Due to the inductance of the secondary circuit and the ohmic load (secondary cable and weld metal), there is a time constant which ensures that the welding current does not have any zero crossings. A constant direct current is possible, which ensures faster and more even energy input and thus welding. This results in lower welding currents, shorter welding times and longer electrode life.
In addition, the adjustment range for various welding tasks is significantly larger than for AC applications. With many advantages, one unfortunately also finds some disadvantages. In addition to the higher investment costs due to the three-phase control, the 3 thyristor sets and the 3-phase transformer with secondary rectifier, the possibility of current control is limited. Due to the commutation and the linking of the 3 phases, the regulation is only very slow and tends to oscillate. These points then speak again in favour of medium-frequency technology. In addition to the classic three-phase technology described for DC welding, there are also transistorised DC sources for applications in small parts welding in the low power range up to approx. 9 kA with the smallest duty cycle. These are also being displaced more and more by medium-frequency systems.