I am trying to repair my aquarium pump. It is an axial-flow (rotary) propeller pump. It accidentally drew air when I was refilling the aquarium after moving it.

After that it didn't pump anymore, but made an unhealthy noise.

As I had to work with (MUCH MUCH) bigger rotary pumps some time ago, I know that those pumps have a bearing that is lubricated using the water flowing through the pump (and not by some lube). When big pumps draw air, the heat melts the bearing. Fortunately, this didn't happen with my (small) pump.

I know that because I opened it.

Drawing of pump

Looking at the drawing, how is it possible that the magnetic cylinder, to which the propeller is attached, can rotate around its own axis (resulting in the propeller turning) and its tilt direction (resulting in a circular movement of the propellers center)

Do you have any idea how this is used to pump the water (I know how a rotary pump works, but am unsure here because of the complete open top. Is a circular movement of the propellers center necessary?) ?

And: What do you expect to be in the surrounding "black box"? I guess there must be some sort of electromagnet.

BUT: Even when the pump is turned off, there is some magnetic force on the cylinder (resistance against pulling it outside of the surounding box).

Any hints on this?

I am trying to understand how the pump is working, to then be able to locate the problem and fix it.

  • 2
    The mysterious black box has wire windings in it. When you feed it with current, an induced magnetic field is created. Since you're using alternating current, the field flips 60 times a second, and this forces the rotor to spin. Commented Apr 9, 2013 at 17:42
  • Do you have an idea how the wire windings must/could be placed to get this sort of rotation? I do understand physics concerning the Lorentz force. What really puzzles me is why there is a resistant force even when no power is applied. Is this self-induction? I have made some physics experiments, and haven't come accross such a strong force (with such small magnets). Commented Apr 9, 2013 at 21:16
  • The windings are often around ferrous cores to amplify the generated magnetism. The magnets in the armature are attracted to these cores even though there is no current flowing. I'm pretty sure the tilt does not help move water, it is to allow for less than perfect balancing to reduce manufacturing cost.
    – bcworkz
    Commented Apr 9, 2013 at 21:48
  • @DanielOertwig Yes, it's likely self induction as well as interaction with the core as suggested by bcworkz Commented Apr 10, 2013 at 13:05
  • @bcworkz That makes sense to me (now). The gap, when filled with water, seems to serve as a buffer. Commented Apr 11, 2013 at 20:26

1 Answer 1


actually you have a centrifugal pump with magnetic coupling

but your problem seem a bad priming job, ensure the entire pump does not have any air in it. if you are confident in the cleanliness of the water put the other end in your mouth and suck it up. then move the pump around to get all the air bubbles out.

  • Yes, you are right. The (huge) gap and observing the tilt (when running dry) mislead me. Actually air was the problem: The pump has an air vent, but its not placed very well. Commented Apr 11, 2013 at 20:29

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