In message <kbur84+3bni@eGroups.com>, Roger L <rogerlov@ix.netcom.com>
writes
>
>The discussion of the Gemini Motor makes a good sounding board for some
>magnetic discussion. I've followed some good electrical discussion on
>this forum, but there's been little on magnetic side.
>
>Looking at the Gemini site, part of Gemini electric motor prototype
>seems to work around the idea of incorporating extra magnets into the
>magnetic field circuit.
>
> On the face of it, the idea makes sense. Common sense says that
>adding magnets to a magnetic circuit might will work in much the same
>way as adding batteries to an electric circuit. And we already know
>from experience that adding more batteries increases the ability to do
>work.
>
>But magnetics doesn't work quite the same way. It turns out that adding
>batteries works to advantage for a couple of reasons that aren't true
>of magnets. When a battery is added, the battery's own internal
>resistance needs to be low - and it's conductivity high - compared with
>the rest of the circuit. While that happy situation is true for charged
>batteries, the analogous situation is not true in magnetic circuits. In
>fact, all of the magnetic material we have available so far has a
>rather poor ability to serve as a conductor of magnetic energy or
>"flux". The materials are said to have "low permeability".
>
>Low permeability in a magnetic circuit has a double impact because of
>another difference from electrical circuits. Not only is the total
>magnetic "flux" reduced in sort of the same way that resistance reduces
>current....but the flow path itself can change to become less efficient.
>
>Looking at electrical circuits, electrical current is mostly
>predictable in the direction that it goes. It prefers to flow only
>along conductive surfaces and is blocked by insulators. And if the flow
>of current is resisted, the result is simply that less current flows
>and some heat is generated. Restricting the path made electrical paths
>are restricted which makes calculations and predictions simple. Even
>before computers, a guy with a pencil, paper, and a slide rule could do
>a pretty good job designing electrical circuits.
>
>The flow of magnetic energy - often called "flux" - is way different.
>Magnetic flow is NOT constrained by a conductive or permeable path.
>When the permeability of some portion of a magnetic circuit is
>lessened, the magnetic energy flows in alternate paths and loses energy
>by doing so. Magnetic energy can leave the designed path, flow out
>through space, and then re-enter the circuit at many different places
>with different values. Calculations of magnetic circuits in the days
>before computers was a miserable task - and technically formidable. But
>that's in the past. Now that there is software to help with the math,
>we are free to model magnetic circuits even more simply than electrical ones.
>
>This magnetic modeling software has already led to better motors, and
>the field is yet young. There are now a free programs available that is
>fantastically powerful. Today the self-taught magnetic amateur has the
>ability to design magnetic motor circuits far more sophisticated than
>anyone in the world could do only a few years ago.
>
>Consider this: Big motors are expensive, but all of the components of
>electric motors are commonly available online at reasonable prices. A
>nice feature of electric motors is that prototyping is so easy. An
>electric motor isn't an explosive high temperature, high pressure, high
>dollar engine full of monkey motion and custom hardened parts. Every
>part in an electric motor can be obtained online and assembled by hand.
>The only movement is rotation about a single axis. Basically there is a
>with bearings, a shell with mounts, and the guts which consist of an
>electrical path interacting with a magnetic path. Instead of welding
>and machining, we have bolted, glued, and hand-wound parts. Using the
>free magnetic software completely eliminates the need for expensive
>custom shapes. The software also predicts quite accurately what the
>results will be even before the motor is built.
>
>There is lots that can be done. Magnetic design is wide open for
>innovation and could make for a pretty good career, too.
>
>But mostly it would be neat to see more folks design their own motors.
> Enjoy!
> Roger Loving
>
In PM-BLDC and (especially) switched reluctance motors, the real issue
is the magnetic losses due to the magnitude of the delta-B in the
stators and rotors. In PM-BLDC the rotor is not too much of a problem
as the very high surface flux density on Neodymium-iron-Boron magnets
means that the change of flux (delta-B) in the rotor backing material
can be quite low, so you can get away with solid cast iron if you have
to. For the stators on both types, and also for the rotors on
switched-reluctance motors, you need thin laminations of electrical
steel, either stamped or (for small quantities) cut by NC water-jet
machines.
I would be interested in knowing what magnetic simulation software there
is for the design of salient-pole BLDC motors of the PM or
switched-reluctance type. There is free software for general ac and dc
magnetic simulation, but something specifically designed for motors
would be useful, provided the cost is not too high and it's easy to use!
My interest is for a low speed rim-drive thruster to propel a narrowboat
on a stretch of a preserved canal in the UK (see
www.cromfordcanal.info). The boat has to operate on a part of the canal
with many wildlife preservation orders, so pure electrical propulsion,
with a thruster that produces minimum tip-scour from the prop on the
silt bed of the canal is desirable.
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. Hence my interest in the simulation software.
Anyone else here have any working knowledge of rim-drive ring thrusters?
Thanks,
--
Chris Morriss
