Some continuous process lines do change speeds for different products or on startup. In these cases inertia compensation may become very important. If not used, the web may become very slack (ie. Web drooping to the floor) while the line speed is increased. This will normally happen at the winder and with a large diameter roll on the spindle.
If this is a problem, there are two solutions. Add inertia compensation based on the torque required to accelerate the winding roll at its present diameter. Alternately slow the acceleration rate enough that inertia compensation is not an issue.