Affinity laws explorer
For centrifugal pumps and fans, power scales with the cube of speed: run at 80% speed and the power falls to about half; at 60%, to about a fifth. Throttling with a damper or valve keeps the motor working hard and destroys the surplus instead. Drag the flow below and compare the two.
Reference example: 100 kW pump or fan running 4,000 h/yr at this flow, electricity £0.20/kWh. Ideal cube law — real drives add roughly 2–5% losses, which barely dents the saving.
Where the saving is real
The cube law pays wherever the flow genuinely varies: heating and cooling pumps that follow the weather, air-handling fans behind dampers, cooling tower fans. It does not pay on loads that always need full output, and a drive on a constant full-speed load only adds its own losses. Our Why VFDs lesson works the economics, and the applications lesson sorts real equipment into the loads that reward a drive and the ones that do not.