All fan and pump applications share the characteristic of fluid flow. The rate and amount of flow through a pipe, duct, damper or valve, depends upon an imbalance of pressure across the pumping device. This imbalance is called pressure difference.
There are certain physical laws that describe these pressure differences and flows; they are called the Affinity Laws. In short, what these laws state is this:
First, the rate of flow in a system is directly proportional to the speed. In other words, if the pump runs 25% faster, then the rate of flow will be 25% greater.
Secondly, the torque of the pump will vary as the square of the change in speed. If the speed increases by 25%, the torque would increase by 56%.
Lastly, and of the most interest to our examples, is the law that states: horsepower is proportional to the change in speed cubed. Considering a 50% reduction in required speed, the laws yield an 87.5% decrease in horsepower requirement to meet the different speed requirement.
It is this law that makes variable speed drives economical in applications where varying flow rates exist.
Top of Motors | Index | FAQs
