The greatest cost savings are often found in applications
involving centrifugal pumps, because the energy savings made by
slowing down a pump are determined by the Cube Law: reducing speed
by 10% could reduce energy consumption by 25%, and 20% less speed
could mean a 50% reduction.
You might think this sounds unlikely - surely a 10% reduction in
speed should equate to a 10% reduction in energy usage? A little
bit of background into how a pump works makes this more
understandable.
A pump speeds up water flow: the faster it works, the faster the
water travels, both into and out of the pump. As the water flow
rate increases, so does the dynamic head of pressure that the pump
has to overcome. Inside the pump is a set of vanes (called an
"impellor") which accelerate the water from the centre of the
impellor to the circumference, increasing both the flow rate and
pressure. Therefore, slowing down a pump not only slows down the
impellor, but also reduces the pressure against which it has to
work. The combination of these factors means that energy-saving is
related to the cube of the reduction in input speed (saving =
reduction x reduction x reduction).