Vector Controlled AC Drive

refer any site of ABB ,Siemens or even a search in Google will give lot of references.One I have is a manual of Sicme Motori Italy which gives a lot of info

Assuming you already understand how VFDs work in the first place, then this should help a little.

http://eng-tips.com/faqs.cfm?fid=1062

If you don't already know how VFDs work, you need more help than what you can get in forums such as this.

VFD, Veriable Frequency Drive, controls the frequency of a given load to a large load demand. This gives the unit the full load in current and voltage to start a given motor, but controls the actual frequency of the output voltage to run the given unit.

Your load will be a 480V ac, 3-phase, 150HP drive motor for a large industrial HVAC unit turning a 12' diameter squirell cage. If you take this motor and hit the start button, the motor will start running at full speed instantly as the voltage is given. Thus, creating a full load in start-up current draw, thus it will either burn the belts off the pullies, or if the cage is generating more friction then usual, will over current the disconnect and blow the fuses.

Now the usage of a VFD allows for full voltage to pass to the motor, and allows for full load current to be passed to the motor, but controlls the output frequency, keeping the motor from actually slamming the mains on start-up, thus giving the motor a soft start to bring the cage into action at a very slow pace, not creating such a current draw on the load center. Once this unit is up and running at full speed, the VFD will actually control the output to set the motor at the most optimum speed for power usage and conservation. Hope this helped, Maximo