We have a coating and laminating machine with VFD and for different materials we need to adjust the P & I values on the drives. So far the supplier has not defined the clearly the P & I settings on the drive and its effect.

Can this be explained in brief or suggest any web site where I can get some information.

This VFD is used for driving the winder/unwinder ?

Which part of the machine is it driving?

Is there any analog feedback coming from the machine to VFD.?

If you have some sort of arrangement to have the feedback (speed) given to VFD then you don't have to change PI values everytime.

try this site:
http://www.embedded.com/2000/0010/0010feat3.htm

That's a fairly good article.

The quickest example I like to give that most people understand is that the "Cruise Control" on your car is a PI controller. You set the Set Point (SP) as the speed you want to maintain, the Error (ER) is the speed droop you encounter by gravity (and friction, but ignore that for this demo), the Process Variable (PV) is the engine torque required to maintain that speed depending on if you are going up hill, flat or down hill, and the Output (OP) is the throttle control to create more or less torque at the wheels. So in that scenario, if you have ever rested your foot on the throttle with the Cruise Control engaged, this is what you feel;

As you start going up hill, the car starts to slow down so the ER increases. The throttle then gets pulled down by the controller to increase the engine output. How far down it gets pulled is PROPORTIONAL (P) to the change in the ER; thus the OP response is proportional to the the steepness of the hill. If you are going along very fast and you hit a very steep hill quickly, the pedal gets pulled down very rapidly, but if you are going up a very gently sloping hill, the pedal gets pulled down very slowly. The rate at which the pedal gets pulled down then is INTEGRAL (I) to the rate of change of the ER.