On Thu, 31 Dec 2009, Harry Veeder wrote:
billb wrote:
On Wed, 30 Dec 2009, Craig Haynie wrote:
> By moving a magnetic field across a conductor, don't we get induction,
> and hence, electricity?
Not in a toroid inductor with unsaturated core. The ring-shape core will shield
the inductor against fields coming from nearby magnets.
For no induction to happen, wouldn't this also require that the magnetic
field (of the permanent magnet) be entirely uniform as experienced by the
torriod?
Nope. Look at it this way: if b-fields cannot leak out, then there is
zero coupling to external magnets ...and external fields cannot induce
any current in the coil. Or start out by imagining an air-core toroid.
Air-core toroids are self-shielding, and external fields cannot induce any
current in an air-core toroid, even if the fields are nonuniform. Now if
you add a ferrite core, and keep it far from saturating: same effect, and
the toroid inductor still self-shields against external fields.
But if you bring a magnet too close, so the core is overloaded, then it
cannot perform its self-shielding trick anymore.
In a fluxgate magnetometer, the toroid coil brings the ferrite very close
to saturation, then the added field from the Earth is enough to saturate
it. Then two other coils can "explore" the directional saturation effect
and determine the X and Y components of the Earth's field.
If the PM magnets on the Steorn rotor are designed to be far enough away
that they don't saturate the ferrite, then the toroid will self-sheild,
and there will be no current induced in the toroid. But if the drive
pulse *plus* the PM magnets *does* saturate the ferrite, then the toroid
will behave very differently and will strongly interact with the rotor
magnets when the drive pulse is there (when the two fields saturate the
ferrite and ruin the self-shielding effect.)
Fully saturated ferrite is non-magnetic! At least, for small AC signals
it is. Ideally if you stick a fully-saturated core into a simple
cylindrical coil, the inductance of the simple coil will not rise. A
fully-saturated ferrite is magnetically like a hunk of air.
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William J. Beaty SCIENCE HOBBYIST website
billb at amasci com http://amasci.com
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Seattle, WA 206-762-3818 unusual phenomena, tesla coils, weird sci
