On 02/01/13 02:22, Chris Morriss wrote:
> A switched reluctance motor having no permanent magnets is desirable,
> as a PM motor always has a strong magnetic field in the gap between
> the stator and rotor, even when not running. The canal silt has a lot
> of ferrous material in it which will quickly get trapped in this field
> on the sort of rim-drive motor I envisage (outer stator, ring rotor,
> with a 4-blade prop fitting inside the ring). An S-R motor has no
> residual field, but has the problem that the gap between the salient
> poles of the rotor and stator has to be narrower than with a PM type
> to ensure a high efficiency.
While I can't comment on the SR motor magnetism, but don't assume that
there is no field because the current is off.
I drive heritage trams (street cars) that almost universally have
series-DC motors. In theory no power, no field current, no magnetism -
yet if the 'reverser' set and the power is off, the tram car will not
roll at any speed backwards - as the motors become opposing generators.
One type I just learned to drive uses this effect as it's main service
brake!
So with no power at all, there is enough residual magnetism left in
the iron to start the motor-generator effect once the circuit is closed
and the effect is reliable enough that some vehicles rely on it for
their main service brake!. (By self exciting the motors, the braking
effect is available even if the main power supply is lost).
While your residual magnetism will be quite low, if magnetic rubbish/mud
in the water is a concern, it's going to clog you up anyway, the
residual fields might not be strong, but if you have a steady current of
muddy water passing through your machine, it could quickly add up.
--
Matthew Geier,
Systems Administrator, Australian Centre for Field Robotics
+61 2 9351 8149
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