HFBP-200 Medium Frequency AC Spot Welding Machine Different materials require different welding conditions. High-strength steel, galvanized sheet, aluminum, copper wire and electrical contacts do not respond to the same current, pressure and welding time. The HFBP-200 is designed as a medium frequency AC resistance welding machine for production environments where the welding process needs controlled electrical output, stable electrode pressure and repeatable welding results.