Power Transfer - Low Frequency to AC

Low Frequency AC (50/60Hz) Welding Transformer

The low frequency AC welding transformer is by far the most widely used power transfer system. 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. 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 workpiece. The total impedance (inductive and resistive) must be accounted for determining the required voltage.

Single Phase AC Current Waveform

The welding current appears as above on single phase AC welders. The graph shows two cycles of the time period. Notice that between pulses of alternating polarity that there is time when current is zero. This is sometimes referred as "inter-cycle cooling" period.

You can see a comparison of a Low Frequency AC weld current pattern and an MFDC weld current pattern in the comparison page in the controls section.

Resistance Welding Presentations Sections