What's the cheapest way to deliver 50% flow?
This example represents a bank of cooling towers with three fans. Motor speed control is used to regulate the cooling effect as demand varies. The question is, what is the most energy-efficient combination of motors and speeds to achieve 50% flow? We start with one motor running at full speed and one at half speed: try different combinations (hint: the optimum combination uses only 13% of full power).
Fan 1		%
Fan 2		%			% flow with		% power
Fan 3		%
The reason for the dramatic result is that the power demand of the fan increases with the cube of speed. So halving the speed cuts the energy demand to one-eighth.