Most hot upsetting/butt welding applications are AC. AC controls are relatively inexpensive compared with mid frequency (MFDC) controls. If you have AC equipment and power is not an issue it would be logical to hot upset with AC. AC controls with SCR’s are very robust and will operate for many years.
Hot upsetting can require a lot of power. This could be a large requirement on the plant electrical system. If the plant does not have the power without a large investment in new power, MFDC controls would reduce this power requirement substantially and could become economically advantageous over AC. MFDC also offers very fine control of the actual weld cycle which can be helpful. MFDC controls can result in higher quality levels and more good parts.
If you were considering the need for DC equipment MFDC may be a better choice because it delivers the DC current, reduces the total plant power load and reduces the size of the butt welder transformers.
Until recently most flash welders have used AC controls. In the last few years MFDC controls have been designed and sized to perform successfully in the environment of flash welding where electromagnetic pulses and high voltage spikes are present. The advantages are most apparent in large parts where the MFDC power savings and reduction in the transformers size is maximized. The advantage of MFDC can be realized for all size applications when you consider users are realizing improvements in quality and more good parts.
If your flash welding application needs DC then the equipment size reduction along with the lower line draw of MFDC make it the preferred choice over traditional DC equipment.
These process and quality improvements along with the electrical cost reductions may make MFDC the best choice for flash welding.
Reference: RWMA - Resistance Welding Manual Section 5