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magnet strength, homemade generator, and waterwheels

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Posted by Anthony Tomah on March 19, 2007, 6:17 pm
Hi all,

I'm a newbie to the topic of home alternative energy but it is interesting
to me none-the-less.  Bear with me here as I use ignorant language.

Question # 1 concerns magnets in the motor itself.  Does the strength of the
magnet help determine the energy output?  Some magnets certainly have
stronger pulling power than others.  The magnets in the side of a chip clip
are not as strong as the magnets I pulled out of a hard drive recently.
Question # 2 concerns generator motor rpm.  The motor on my large tablesaw
requires a certain amount of watts to turn at roughly 3600rpm.  Would it
produce the same amount of energy if rotated in the opposite direction at
3600 rpm?
Question # 3 concerns waterwheels.  Is there a calculator on the web or a
formula that will tell me how large a waterwheel sprocket has to be to
rotate a motor connected via chain or belt at, say, 3600 rpm?  And, if there
are ways to do this and produce enough power, would it be feasible to design
a waterfall with a waterwheel to generate power, which in turn, powers the
pump to feed the water AND, and this is a big AND, produce excess power?

Thanks so much.


Posted by mg on March 19, 2007, 11:32 pm

Years ago someone once told me that there is only one university in
the United States that offers a Bachelor's degree in electromagnetism.
I'm not sure if that's true or not. However, I can tell you that most
electronic/electrical engineers have very little understanding of
magnetism. If they want to build something they merely cookbook it in
some way or use a computer program, etc. So, any answers I can give
you are just knee jerk guesses.

Generators work by converting mechanical energy into electrical
energy. So, obviously you are not going to be able to devise any sort
of a generator that puts out a lot of electrical power without
inputting a lot of mechanical energy.

There's a bicycle generator, for example, at:
According to the website an average rider can produce 150 to 200
watts, or enough to light 1 or 2, 100-W light bulbs until he stops
pedaling. An electric table saw might take about 1400 watts, or more
for example. So, that would be the rough equivalent of 7 men pedaling
7 generator bicycles.

The voltage and current (e.g. power) produced by a generator is
related to the strength of the magnetic field. If you take a look at
the formulas at:
http://www.spots.ab.ca/~belfroy/polarmagnetics/GeneratorNLL.html  ,
for example, you will see the relationship.

All things being equal, I don't think the direction would make any

I doubt if it would be feasible for you to do that. My guess is that
your waterfall wouldn't produce enough power. If you wanted to take an
indepth look at the problem, though, you would probably want to start
with the formula:


Where W = watts and B = magnetic field intensity and w = rotor spin
speed and T = mechanical torque, etc.

Posted by mg on March 19, 2007, 11:48 pm
Whoops. I forgot. To look for waterwheel examples enter the following
in Google:

Also, look at the "Pedley Wheel" at:

It has a diameter of 3-3.5m (about 11 feet) and has a water flow of
70-200 litres/second (19-53 gallons/sec) and puts out a minimum of
1500 watts.

Posted by Solar Flare on March 20, 2007, 12:35 am
 Consider this. What happens if your magnets lose all their magnetic

Posted by Anthony Matonak on March 20, 2007, 3:34 am
 Anthony Tomah wrote:

I'm no expert but I can point out the blatantly obvious.

I assume you're talking about using a permanent magnet motor spun
by some outside force to generate electricity. As the other fellow
said, all the electricity you get out comes from the mechanical
energy you put in (minus some losses).

Stronger magnets just mean a physically smaller generator for the
same power.

Assuming the motor can operate as a generator, most permanent magnet
motors can, then you generally get about the same number of watts out
as the motor consumes at any particular RPM. It's never quite the same
because there are losses, after all.

One trick you can do is take two identical PM motors and wire them
together in parallel. Spin one and it'll act as a generator which
will spin the other. The one acting as a motor won't spin quite as
fast as the one you're moving as a generator. Works best with high
voltages since wire losses are dependent mostly on current (amps).

Gearing is pretty simple stuff. Any bicycle mechanic can give you all
the details but it generally comes down to counting teeth. If you hook
two sprocket wheels together with a chain and one has double the teeth
of the other then it'll spin half as fast. Just figure the multiple
that you need and get the appropriate sized sprockets, gears, wheels
or whatever.

For instance, say your waterwheel spins at 36 RPM and you want to
turn an auto alternator designed to run at 3600 RPM. You'll need
a small sprocket wheel on the alternator and a larger one with 100
times as many teeth on the wheel. Say the small one has 36 teeth,
the large one would need 3600 teeth.

No. You're talking about an overunity device. In fact, one of the first
ones ever proposed thousands of years ago, the self pumping water wheel.
You lose a bit of the energy every time it's transformed. You don't get
enough electricity to pump enough water to power the wheel.


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