The block diagram of a typical variable-frequency
drive can be divided into three major sections:
the power-conversion section
the microprocessor control section (CPU) and the control section that
includes the external switches an signals to control the drive operations
the power section where AC voltage is converted to DC and then DC
is inverted back to 3-phase AC voltage
The block diagram below contains the power section of the VFD
as three separate sections to indicate the three main functions:
the switching section that uses regular transistors, darlington pair
transistors, or insulated gate bipolar transistors (IGBT) to invert
the DC voltage back to AC voltage with the proper frequency.
above: The VFD shown as three separate sections: rectifier (where AC is
converted to DC); the DC intermediate circuit which contains the capacitor and inductor for filtering. The third section is the DC-to-AC inverter where
DC is turned back to three-phase AC.