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                                    How does a VFD works? 
The electronic circuit in a VFD unit is discretely divided into three main stages, viz. an input
converter (bridge rectifier stage), a DC Bus (filter stage), and an output inverter (using
microcontrollers and IGBTs).

Let's look at how each stage works.
•  Input Converter: This stage consists of high power diodes arranged in a regular bridge
   configuration. The AC mains applied here is rectified and converted into DC. But this DC is not
   free from the residual AC components and harmonics. It requires further filtering.
•  DC Bus: Here the rectified DC is stripped and filtered from the left-over harmonics and AC
    residues using inductors and capacitors. This stage helps in making the output to the motors
    totally ripple free and ideal for AC motors.
•  Inverter: As the name indicates, this stage converts the DC from the DC bus back to AC, but in
    a very special way that forms the heart or rather the brain of the circuit. It consists of sophisticated
    microcontroller ICs designed and programmed especially to change the output frequency along
    with the voltage proportionately and also create a three-phase output from a single phase input.
    This stage particularly makes VFDs very unique and most ideal for controlling AC motor speeds.
•  Output: The command from the above stage (microcontroller ICs) are sent to the output IGBTs 
    (Insulated Gate Bipolar Transistors) which switch the voltage received from DC bus into narrow 
    chopped steps (quite similar to theprinciple used in Dimmer Switches). To do this the ICs employ 
     PWM technology and convert the DC into quasi-sinusoidal waves. The longer the switching time 
     of these waves, the higher is the voltage at the output to the motor and vice versa. This procedure
     is actually responsible for two important functions – to change the output voltage without any
     wastage of electricity and, very crucially, to change its frequency simultaneously at a particular
     given rate to keep the torque and the magnetic flux constant.

 Briefly, a VFD has the following important features:
 •  Over-current protection, especially useful while controlling motors with high inertia.
 •  Constant torque ensures wider range of speed control, enabling an energy efficient control
    throughout the range.
 •  The VFD acts as a barrier in between all input voltage disturbances like harmonics, ripples, sags,
    surges, etc., and obstructs them from entering the motor.

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