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.