Pond pump speed control

In uk.d-i-y HTH wrote:
Ian Stirling wrote:

In uk.d-i-y Tim Downie wrote:

Is is possible to use a speed controller (like a light dimmer) with a small
pond pump?

I have a Bermuda 3000 (3000l/hr) submersible pump that's really a bit more
powerful than I need. The motor has a solid rotor that appears to be made
of a ferrite like material.

I know that speed controllers can cause overheating problems with some
motors but given that it's underwater with water bathing the rotor, I don't
imagine this would be a problem in this case.


In this case, simple speed controllers won't actually work, it'll stay at
a constant speed (3000RPM) until it stops working, and hums loudly.

I am not sure about the following and it would apply only to mag drive
pumps.

The pump works because the magnet is forced to follow the rotating
magnetic field. When the pump is "dimmed" the field becomes weak and
will be unable to force the magnet to stay ahead of it. As the rotating
field passes the magnet it will slightly demagnetize the magnet. In
time it will weaken the magnet to the point that the impeller will not
work at all.

The case is similar to when the pump is just started, or the impeller
is stalled.
Magnets in general only demagnetise if you exceed the maximum field.
They generally do the first time it does, not over time.
So, probably not.