Many VFD articles are typically full of hype or misinformation. There is no way to make a 100 HP motor pull only 12.5 HP load, and still maintain the head or pressure required. Authors show the formula for the Affinity Law. However, like all VFD salespeople, they fail to use any part of it except the part that reduces the HP by the cube of the speed. The most important part of the Affinity Law is that the head is reduced by the square of the speed. This means that the pump can only be slowed by 10% to 15% at the most, or it is no longer getting water to the surface of the well, or it is no longer producing enough pressure to operate the sprinklers.
Here are a couple of accurate curves that show the relationship between VFD and throttling. Note the line for TDH required, and the minimum speed possible to produce this head. Then you can see that in reality, there is very little if any difference in power consumed by the VFD system and a throttling valve system.
Thanks
Cary
