The power limit of a ventilator is shown by the specific characteristic curves in a graph. This power limit supplies the information how much pressure is generated at what volumetric flow or vice versa.
The practical range of action should be designed in such way that the working point is within the stable range of this characteristic curve.

Above this range, most of the characteristic curves drop off suddenly and the flow conditions get very unfavorable. Underneath the stable range of action, the dynamic pressure should fall below so that the ventilator reaches a certain minimum output limit.
If, as shown in the above example, the theoretically calculated quantity and pressure is in the field underneath the haracteristic curve, the working point of the ventilator can be read from a parabolic curve. The practically generated quantity is now directly at the generated intersection point.