From http://www.pond-doctor.co.uk/longpondpumps.htm
I think the ehiem pumps work as the synchronous ones below

Submersible pond pumps can be divided into two groups, depending on how the
motor causes the impeller to rotate.
a.. Synchronous. This involves a permanent magnet that is attached to the
impeller. The motor causes an outer electromagnetic field to drive the inner
permanent magnet that is mounted to the impeller in a circular motion,
causing the impeller to rotate and pump water. A useful way of determining
whether your pump is driven by a synchronous motor is to test whether the
impeller is fixed to a permanent magnet. If it is, then it is highly likely
that the pump is synchronous. They are called synchronous motors because
they use an inner and outer magnet ring with an equal number or size of
magnets in each ring, with the inner impeller rotating in unison with the
outer magnetic field.
b.. Asynchronous. This kind of pump only uses a permanent magnet in the
motor, with the impeller that is attached to an iron core with either
aluminium or copper bars covered with a corrosion-resistant material. The
rotating magnetic field induced by the electromagnetic motor makes the
impeller itself into an electro magnet that follows the magnetic field and
spins accordingly. They are called asynchronous because the impeller will
spin at a different speed to the inducted magnetic field.
These magnetic drive pumps do not employ a moving process shaft seal (that
would be found in say an external pump preventing the pond water from
entering the electrics). Consequently, these sealless submersible pumps are
made safe by a stationary physical barrier (the plastic casing and resin)
that lies between the impeller and the motor, preventing any water ingress.

a.. The impeller. The impeller end design must not be underestimated as a
highly influential feature that will affect a pump's performance. Their
size, proximity to each other and shape (straight or curved) will all
combine to produce different pump performances. The solids-handling pumps
will have larger, widely spaced impeller vanes, whereas high-pressure pumps
will tend to have closely spaced vanes.
b.. The volute. Sometimes also called the diffuser, this encases the
impeller and will also affect a pond's performance by how it marries with an
impeller. The tighter the fit, the higher the pressure that pump will be
able to produce, but the more likely it will be to clog. The volute also
determines the diameter of the discharge pipe, which in turn will set the
diameter of pipework used around your pond.
wrote in message
oups.com...

Richard wrote:
My experience is not with the Eheim pumps, but their canister filters.

The ceramic rod through the impellor assembly should be one piece. it
fits
(on my filters anyway) into a cylindrical rubber "bearing" at each
end -
this holds the shaft steady and centered. The shaft does not turn.

The coil in the pump head makes the magnet spin on the shaft. As the
impellor can only turn about 180 degrees on the magnet it turns with it
and
hence pumps the water.

.


Richard - thanks for the reply.

"The shaft does not turn" - the impeller is attached to the impeller
shaft. iI the impeller shaft does not turn, what causes the impeller to
rotate? - MIke