In message <50E3B17F.40705@acfr.usyd.edu.au>, Matthew Geier
<matthew@acfr.usyd.edu.au> writes
>
> 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.
>
>
Yes, I think you are probably right that even the small amount of
residual magnetism left on the un-energised rotor/stator poles of a
switched-reluctance motor will attract enough of the ferrous silt to
clog up the gap between them over time.
I see that all of the commercially available rim-drive ring thrusters
(of which all of them below a few tens of kilowatts appear to be PM-BLDC
types) have effective seals between the stator and the rotor to prevent
water ingress. This is one thing that prevents a knowledgeable
enthusiast with the right workshop facilities from making up a prototype
motor of this type. There's lots on unsealed home-made PM-BLDC motors
around, though normally of the axial-flux type which are much easier to
make. There's a self-published book available on building such a motor
to replace the engine on a 125cc Yamaha motor bike.
I am surprised that no entrepreneurial Chinese companies aren't
marketing such a thruster. It would serve well for auxiliary power on
(say) a 9-metre catamaran as well as for inland waterway propulsion in
environmentally sensitive areas. The Chinese have collared the market
for the control units to drive three-phase PM-BLDC motors after all.
As well as being quiet and efficient, such a thruster could also produce
useful amounts of electrical power on a vessel under sail, and as a
result of the very low mechanical friction, might provide a useful
amount of battery charge when at anchor in a tidal stream.
--
Chris Morriss
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