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Medium Frequency Integrated DC Transformers



ISOMATIC Lab successfully developed and brought to market the MIT Line of Medium Frequency Integrated DC Transformers designed for integration in welding robots and machines. Medium-frequency DC transformers are a result of recent advances in resistance welding technology. They are powered by a transistor inverter, supplying usually square wave AC 500 V @ 1000 Hz which is converted by the MF transformer to a low alternating voltage at high current. This low voltage AC current is rectified to DC by a diode block built into the transformer and is then used for welding. The higher operating frequency of such a MFDC system allows efficient energy conversion using a smaller and lighter transformer. Other benefits with MFDC transformers used in portable and robotic welding guns are:

  • When welding with DC the reactance of the welding loop only limits the upslope and downslope of the welding current, whilst the peak welding current in limited only by the welding loops resistance. The net result is smaller transformer, especially when welding with large throat, high-impedance guns.
  • MFDC welding is less demanding on the plant power system too. The MF inverter draws balanced line current from all 3 phases and operates at high power factor (cos φ> 0,9).
  • With MFDC transformers the peak welding current almost equals its RMS value, which helps to obtain quality welds with no splash and constant strength even at lower pressure.
  • The high frequency permits much faster 1000 times per second - process control, which again results in better weld quality.


The only disadvantage of MFDC systems is the higher cost, but recent advances in the technology permitted ISOMATIC to overcome even this problem and introduce the MIT line - the smallest and most efficient MFDC transformers on the market.


  • 63,80, 100 and 120 KW @ 20% duty cycle in "H" size
  • 130, 250 KW @ 20% duty cycle in "J" size
  • Fully cast in epoxy resin
  • Fully protected rectifier assembly (IP 20)
  • Fast and convenient disassembly of the rectifier block.
  • Water cooled rectifier and transformers secondary.
  • Compact and lightweight

Technical specifications for the transformers and the fixing dimensions are given on the technical drawing.



All MIT transformers are designed in accordance with ISO 22829 and their primary and secondary terminals comply with this standart too. This simplifies the upgrading of existing guns for DC welding. A current sensing coil, with an output of 150mV/1kA is standart, as are 3 thermoswitches monitoring the primary, secondary and rectifier temperatures. These compact transformers are built into special machines, robot or manually operated welding guns, minimizing stray magnetic fields and acheving optimum electrical energy transfer. Thousands of ISOMATIC transformers are welding car bodies day and night at Ford, General Motors, DaimlerChrysler, Rover and other famous car manifacturers.

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