AC Drives: Block Diagram of a Variable-Frequency Drive

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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 rectifier
  • the filter
  • 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.
variable-frequency drive block diagram
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.

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