Power Transfer - Low Frequency to DC

Low Frequency (50/60 Hz) to DC Welding Transformer

The low frequency DC welding transformer is used to eliminate the effects of the inductive impedance of a welding circuit. The power frequency on the primary of the transformer is 50/60 Hz with a nominal primary voltage of 220V, 380V, 480V, or 600V. The power to the transformer is controlled by inverse parallel SCRs in the welding control. The secondary voltage varies from 3 volts upwards to 30 volts depending on the turns ratio between the primary and secondary windings. There are diode packs hooked up on the secondary of the transformer. The transformer above represents a center tapped fullwave bridge. Since current on the secondary does not alternate between positive and negative impulses, the inductive impedance becomes negligible. In determining the size of transformer necessary for a specific welding task, one first determines the required current output for the weld, then determines the voltage required to push the current through the resistance of the tools and the workpieces. Only tool and workpieces resistance must be accounted for determining the required voltage since the inductive impedance is only sustained during the first cycle or so.

The low frequency DC transformer is known however to cause severe line disturbances since the SCRs do not isolate the commutations from the welding bus.

Single Phase DC Current Waveform

The welding current appears as shown on single phase DC welders. The graph shows two cycles of the time period. Notice that between pulses of similar polarity that there is time when current is zero. This is sometimes referred as "inter-cycle cooling" period. Since the DC welding current pulses remain in the same polarity, there is reduced induction hence this power transfer is used for very large loop welding systems.

Resistance Welding Presentations Sections