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•  אווירודינמיקה של טורבינות רוח בעלות ציר סיבוב אופקי, על קצה המזלג

• תכנון גנראטור (ציטוט מאחד הפוסטים ב-otherpower.com)

Magnetic circuits can be hard to analyze if high precision answers are needed.
These detailed calculations are best left to software tools. On the other hand,
for the simple geometries typical in our PM alternators, a good approximation to
the magnetic field in the gap is given by:
Bg = Br/(lg/lm + Ag/Am)
where:
Bg is the magnetic field in the gap,
Br is the residual magnetization of the magnet,
lg is the length of the gap,
lm is the length of the magnetic (along the direction of magnetization),
Ag is the area of the gap,
Am is the area of the magnet.
The above assumes that there is a non-saturated high-mu return path for the
flux. This is typically accomplished by using two iron rotors or a laminated
iron stator core.
In the above, Br is about 1.2 Telsa for NdFeB type-40 material and about 0.38
Tesla for Ceramic type-8 material.
In a simple radial PM generator, the area of the magnet is approximately equal
to the area of the gap, so the above reduces to:
Bg = Br/(1+lg/lm)
For the simple radial PM alternator, magnetic field fringing causes the area of
the gap to be a bit larger than the magnet area, but this is offset by the fact
that the coils usually are somewhat larger, on average, than the magnets. At
least they should be. Thus, the total flux seen by the coils is somewhat
independent of fringing as long as the fringing fields pass through the coils.
So, the above formula for the average magnetic field proves, from experiment, to
be quite close (within 10% in my experiments).
If two magnets are used on opposite sides of the gap (as in Hugh's alternator
design) then lm is twice the length of each magnet