Head produced by a pump drops off by the square of the speed. Pumps that must lift a certain distance or produce a specific pressure (or TDH) have a minimum speed that will accomplish this task. The speed of a correctly sized pump can only be reduced a small amount if a static head or constant pressure is required. Horse power reductions by the cube of the speed can be insignificant compared to a full speed pump simply pushed to the left of it's curve. Before you can use the Affinity Law the following steps must be taken to determine the minimum speed possible from your pump. The affinity law can then be used to determine the minimum horsepower possible from your pump when using a VFD.