Gingoog Electric Trading (G.E.T.) ™

March 6, 2009

What is a VFD? Part 4

Filed under: GET to know VFD's — Ralf @ 1:43 AM

What is a VFD and how does it work? Part 4

The following article is a bit more compressed, just a quick overview, made with just a few short words to gain an initial idea what VFD’s are all about.

This is our chance to take a little breather because right after this we will go to the last part of this series and dig one more time a little deeper into this issue.


Variable Frequency Drives or VFD’s are electronic devices used to control the speed of an Alternating Current Motor (AC Motor).  Variable Frequency Drives are also commonly known as Adjustable Frequency Drives, Adjustable Speed Drives, AC Drives, Frequency Inverters, Frequency Converters often just only Drives or else simply VLT.

Variable Frequency Drives have a wide range of application use that include, but are not limited to:

  • Variable Air Volume Systems
  • Circulating Pumps for Hot Water Heating Systems
  • Chilled Water Circulating Pumps
  • Geothermal Heat Pump Systems
  • Injection-molding Equipment
  • Air Compressors
  • Conveyors
  • Chillers
  • Cooling Towers

Variable Frequency Drives operate as load controls within these applications that may accomplish up to a 50% reduction in energy costs.  In general, an electric motor will turn at a rate proportional to the frequency of the alternating current (AC) applied to it.  The majority of Variable Frequency Drives in the market today contain electronic circuitry that converts a 60Hz Line power into direct current.  The VFD converts this line power into a pulsed output voltage that duplicates varying alternating current to a desired frequency.


Advances in technology over the past decade have allowed for Variable Frequency Drives to become a very cost efficient way to reduce energy costs and increase system efficiencies.  More and more companies within a wide range of industries are finding more ways to apply VFD’s to their applications.

An AC Variable Frequency Drive (VFD) is commonly referred to as an “Inverter”. This is because of the way a VFD works.


The following details the inner workings of a VFD:

1.) Alternating Current (AC) power is applied to the input of the VFD and feeds a bridge rectifier.

2.) The rectifier converts the Alternating Current (AC) voltage into Direct Current (DC) voltage.

3.) The Direct Current (DC) voltage then feeds the Direct Current (DC) bus capacitors on the VFD where it is stored for use by a transistor or Insulated-Gate Bipolar Transistor (IGBT).

4.) Direct Current (DC) from the capacitors feed the input of the transistor(s).

5.) The transistor(s) then continuously turns on and off at the appropriate frequency to build a new sine wave for use by the motor connected to the output of the VFD.


The process above is often referred to as inversion because it changes from one form to another then back again.

The voltage frequency, as distributed in the Philippines, is 60 cycles per second and the unit of measurement is Hertz (Hz). The output frequency and voltage of an AC Variable Frequency Drive is variable and controlled by the speed at which the output transistor is continuously turned on and off.

The variable speed is controlled digitally in modern VFD’s and changed by the operator through programming, an operator interface, or by changing an analog input to the VFD that is programmed as “speed reference input”.

Ralf Wabersich

Gingoog Electric Trading (G.E.T.)

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