heritage@... wrote:
>
> If the rotor were double from 6 or 8 to 12 or 18 poles
> would this decrease the input rpm by a percentaged
> hopefully in half, the goal is too drive the alternator
> direct not with any belts or gears if so.

Although Hugh Piggott is much more qualified to answer,
I've been playing around with homebuilt alternators for
the last few months and have done this type of experiment.
Hugh will correct me if I'm wrong, I'm sure!

I will assume you are talking about a PM alternator in this discussion.

The short answer is, yes, increasing the poles/coils increases the power
as the cube of the number of poles/coils, while decreasing RPM reduces
power as the square of the RPM. The real answer is, this is a
complicated subject.

First of all, the math says that doubling the number of magnetic poles
doubles the voltage and also doubles the current, and so produces four
times as much power. If you also double the number of coils, you will
double voltage again but you will leave current unchanged since you have also
doubled resistance. Therefore, *all other things being equal*, doubling
the poles and coils would result in four times as much voltage and twice
as much current, or 8 times as much power. So, power increases with the
cube of the alternator "size" if we say the "size" is just some number
of coils of fixed wire gauge/turns and some number of magnets. Note that
you get the same effect by doubling the magnetic pole surface area and the
coil area. So, you don't really need to double the physical number of
poles, you could just double the size of the magnets and scale up the
coils accordingly (but now you've got more wire again and more resistance).
You could also use magnets that are twice as strong
in their intrinsic field, for example by using rare earth magnets and
again that is more or less the same as doubling the number of magnets.

Both voltage and current are linear with RPM, so
power varies as the square of RPM (subject to the other effects
previously mentioned). So, if you want to decrease RPM by a factor
of 2 yet maintain the same power output, you must increase the
number of poles/coils by a factor of the cube root of 4, which is
about a factor of 1.56. I think I did that correctly... but we won't
know for sure until Hugh logs on...

Now, there are many subtle effects that blur this pretty picture. If
you do indeed keep doubling the number of magnets the frequency will
increase. At some point you will start increasing the core losses due
to eddy currents in the cores. At some point you will even start to cause
RF interference that will make your neighbors mad at you. There are other
effects, such as parasitic losses from nearby coils. And, of course, you
may not be able to keep the rotor/stator assemblies stable if they get
too big. In addition, at some point you will need to increase the wire
thickness because all that extra power you are producing is heating them
up too much, so you cannot count on continuing this game indefinitely
without making all kinds of engineering adjustments. You will also have
too much voltage at some point and will have to reduce the number of coil
windings while increasing coil thickness.

I recently made another subtle mistake that Hugh diagnosed for me: my
coils were positioned so that the back-magnetic field caused by the
current flowing through the coils canceled out much
of the PM field at relatively low RPM, making the alternator fall far short
of its design power.

Some more effects. I recently redesigned the little PicoTurbine alternator
to produce more voltage by increasing the number of turns per coil while
decreasing the thickness of wire. I actually got proportionately more voltage
than the math said I should. This was probably because the thinner wire was
able to be wound with more precision on my winding apparatus and thus there
was less phasing loss, and also the coils were flatter so the average
magnetic field was a bit higher in the coils. The real world is just a lot
more complicated than the pure world of math.

Hugh discusses some of this in "Wind Power Workshop". Also, there is a
very brief overview of the math in the technical notes section of my free
"PicoTurbine Educational Windmill Plans" available for download from my
website. Most college physics books give the formulas involved but don't
go into the more complex cases.

--
Steve Pendergrast
Renewable Energy Homebrew Projects and Books
http://www.picoturbine.com