Frequency converter systems
Especially in the field of high welding currents and for applications in the aerospace industry, frequency converter systems (also known as low frequency systems) have been and are still used. Controls for frequency converter systems are three-phase controls in which the power thyristors are controlled via a special logic so that a welding voltage with a frequency of less than 50 Hz is produced at the welding transformer. This frequency is variable, usually between 6 and 30 Hz. This makes use of the advantages of the direct current characteristic and at the same time eliminates the disadvantages of lens shift at high welding currents due to the Peltier effect.
Special transformers with 3 primary and one secondary winding are required for such installations. In some process variants, a three-phase intermediate transformer is used in addition to the single-phase welding transformer, which is tuned to the low frequencies. The main areas of application for these frequency converter machines are welding tasks with high currents for aluminium and aluminium alloys in the aircraft industry and in other areas where a very high amount of heat has to be applied in a short time. The machines are available as spot, projection and also as seaming units.
The Harms & Wende frequency converter control MPS7054 in various versions was successfully used worldwide for this purpose. In many applications outside the aircraft industry, where only frequency converter technology has been approved connection technology to date, more and more medium-frequency systems are being used in the high-current range.