pressure drop represents energy loss. Sometimes it is economically practical to put in some device that recovers this otherwise lost energy. With a variable-speed drive on the pump or fan, the supply can be adjusted to match demand and no extra loss is introduced.

For example, when a fan is driven directly by a fixed-speed motor, the airflow is designed for the maximum demand of the system, and so will usually be higher than it needs to be. Airflow can be regulated using a damper but