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The fullwave frequency converter control is similar in design to
the mid frequency system. The significant difference is in the fact
that this system generates frequencies below that of the base frequency.
Instead of using IGBTs for pulsing the welding transformer, the
FWFC control utilizes SCRs for inputs 1 through 4. These SCRs are
simply used for directional control of current; passing current
either from 4 to 1, or from 2 to 3. There are six other SCRs which
are used for creating a DC charge across a bus. This is similar
to that of the MFDC system which uses diodes instead. The reason
for the use of SCRs in the FWFC system for producing the DC power
is that these SCRs are gated every half cycle for the duration of
the weld and can be turned off by the zero crossing of the three
phase power when gating current is no longer applied. This way,
the SCRs that are used for directional control at positions 1 through
4 will also turn off.
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