Electric Pumps (not variable speed)

[Reef_Hobbyist]

My observation is that the impeller is designed so that it does not completely push out every drop of water that comes into the pump from the input side. It pushes as much as it can and then only draws in what it pushes out. That is why the water drains backwards through the pump when the power is off and the impeller does not spin backwards. The impeller is actually quite narrow compared to the width of the chamber.

So that explains why we can safely throttle back a valve on the output side of a pump (to reduce flow) without creating more burden on the pump motor. It just keeps spinning at full speed like always but pushes less water (out a smaller opening) so it draws in less. If the impeller was tight in the chamber, it would strain the motor as it tries to push the water through a smaller opening.

That's my theory.

If I guessed all that right so far, then why does the pump draw less watts when I throttle back the valve on the output side? Why does it use less electricity? Its not changing speeds or slowing down because its an electric motor.

I could understand the other way around if the motor draws more watts struggling to push the water through a smaller opening but that is not what is happening.

That is a mystery to me. Any thoughts?

 

[legionofdoon]

Maybe it's the reverse of the Bernoulli principal. If velocity increases at pressure decreases the maybe as pressure increases velocity decreases. Or it could be something related to the magic of electromagnetism.

 

[ca1ore]

Different pumps have different impeller designs. Some with very little space between the blades and the volute, others with quite a lot; some designed for higher flow, others to handle greater pressures. It is also incorrect that the impeller does not backspin with back siphon - it certainly does on my pump. As I understand it, more back pressure means less torque requirement and lower power draw.

 

[legionofdoon]

As I understand it, more back pressure means less torque requirement and lower power draw. Like I said reverse Bernoulli.. pressure increases velocity needs decrease hence less power demand. Going back to what you said about the impeller not moving when the power is off.. I'm not sure if you meant that it does or doesn't move when the power off. If you meant that the impeller does not move that is understandable because being a fixed magnet surrounded by a "eletro magnet," ( which only has a magnetic field when supplied by electric energy) the fixed magnet "sees" the "dead" magnet as metal to "fix" to without being alternately attracted and repelled from. I'm not a technical writer so hopefully you can get what I'm saying.

 

[legionofdoon]

The water drains backwards because water seeks the lowest point if you are pushing water up and then cut power the water due to siphon pressure will seek to drain out of the input til either the siphon breaks or the pressure equalizes. A siphon doesn't really "suck" the atmospheric pressure pushes the water down to the point of equilibrium. I know it's counter intuitive but that's how it works.

 

[legionofdoon]

There are many books and papers dealing with hydraulics, to the point that you could be a Phd in fluid dynamics and still have not read a significant amount of them.

 

[legionofdoon]

Imagine you have a pipe that is 2 inches, and another that is 1 inch. The pipe that is 2 inches will flow way more water at a lower "pressure" than the 1 inch. In that example volume replaces velocity.