DC and AC welding have been used for years but recently inverter technology has become a very large factor in the resistance welding industry. So let’s look at frequency and where it is used in resistance welding technology.
Many inverters in industry operate in the 1000 Hz range which we refer to as mid frequency or midfreq. This is the frequency of the output current of the transformer.
Reference: RWMA - Resistancw Welding Manual 4th Edition