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.