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Power Transfer - MFDC


Mid Frequency DC welding transformer

The MFDC power transformer is similar to the low frequency DC transformer. The major difference lies in the size of the magnetic iron core that transform primary current into secondary current. Since the MFDC transformer relies on the control to provided it with frequency pulses of 400 to 2,000 Hz instead of the base 50 or 60 Hz, the amount of iron in the core is reduced significantly.

This allows the manufacture of a MFDC power transformer that is the size of a toaster. This allows the transformer to be placed much closer to the welding tool in some cases hence providing further benefits. You will see in the MFDC Control section that the switching of the mid frequency pulses is preceded by a fullwave rectifier bridge that converts three phase power supply into a DC bus which is filtered by a capacitor within the control. It appears that the MFDC does not cause line disturbances as is the case with the low frequency DC transformer. In fact, there are advantages for the electrical power supply to install MFDC transformers for resistance welding.

MFDC Current Waveform

The output of the MFDC inverter will be squared waves of electrical current at a frequency of 400 to 2,000 Hz.  The conventional output frequency in used globally is 1,000 Hz.  The image to the left shows a comparison of a single AC wave (red line) as compared to that outputted by an MF inverter (blue line)

The output MFDC current from the welding transformer after the diodes remains constant.  The peak current and the RMS current are similar in intensity as compared to the low frequency AC or DC systems. 

You can see a comparison of a Low Frequency AC weld current pattern and an MFDC weld current pattern below in this graph showing two outputs equivalent to 40K amperes.

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