Jan 13, 2011

Variable Frequency Drive Fundamentals - VFD Basics Part 1

VFD stands for Variable Frequency Drive , also known as Inverter in Industry is now a days very popular in almost all of the industries due to its capabilities and energy savings functions. VFD is used to run the AC/Induction Motor in a controlled way or we can say to give us the power to run the motor as we want. In Simple words we can say that VFD is a system designed for controlling the speed of AC motor by controlling the supply frequency to the AC Motor.

Variable Frequency Drive Fundamentals - VFD Basics Part 1
Drive Classification :- Drives are always classified according to the type of motor they are running i.e AC or DC and not classified/named according to their Power supply type.
How the Speed of Motor is Controlled by supply Frequency ?
Well there is one well known formula :-
Ns = 120* F / P
where F is frequency , P is number of poles and Ns is the Speed . So you can see in the above formula that as the number of poles are fixed , as frequency changes so the speed.
So Controlling the frequency means controlling the speed , BUT controlling of frequency was very difficult until POWER ELECTRONICS was introduced . When Power Electronic was introduced , power semiconductors devices like IGBT , SCR made it possible. With VFD its possible to operate the Motor at its rated current and Torque , So VFD are the best Starter for Motor with advanced capabilities and function.
Construction/Structure of VFD :-
See the Below picture for the internal structure of VFD , at first stage there is Bridge Rectifier which converts the incoming AC supply to DC , followed by LPF (low pass filter ) which removes any spikes present and after that Inverter converts the DC to AC by using the PWM technique and finally power is delivered to the Motor.

Also if you note then there is feedback to inverter , i.e whenever load increases at the motor then feedback is given to inverter for more current and thus inverter delivers more current to overcome the load.

You can check out Automation Talk earlier tutorial on Multi - Speed settings in VFD.

We will Discuss in next tutorial about the PWM technique , Torque Characteristic of VFD and other functions supported by drive.
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  1. Good Article.
    Also give information how VFD saves Energy.

  2. Thanx ... You will soon a see a post on " How to save energy using VFD "


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