Medium Frequency Direct Current machines use an inverter, powered from a 3 phase supply in which the current is rectified using diodes and then chopped by means of IGBT's into high frequency alternating current. This alternating current is then transformed by the welding transformer and again rectified. DC current is thus obtained at the secondary side in to the parts to be welded.

The MFDC welding machine load is evenly distributed across all three phases. This allow equal load to be drawn from each line, thus reducing peak draw on any one line. The price of MFDC machines is higher than the conventional Alternating current (AC) machines but users will get advantages such as three phase balanced loading, weight reduction in transformer, about 20 to 25% less power requirement as compared to AC machines, better control over time and uninterrupted energy input into the parts to be welded. 

Medium frequency is an ideal machine for spot welding of Aluminum, Galvanized sheets (GI), Electro galvanized sheets (EGI), Stainless steel and of course low carbon steel