Para
Español, traducción de Julio Andrade.
Vehicle Alternators
The biggest problem with using car alternators for wind power is that
they are designed to rotate at
too high a speed to be practical in wind power applications
without significant modifications. Even a small, seemingly fast windmill
might do most of its work at 600 rpm, not nearly fast enough for a car
or truck alternator. This means that gearing up with pulleys or other
methods is needed, so lots of power is lost to friction--a big problem
with wind or water power, but
not a problem with a gasoline engine. Check out how useful car
alternators can be for building a small gas-powered charger HERE.
A standard car or truck alternator is electromagnetic--
meaning that some of the electricity produced by the unit must
be used internally and sent to the armature through brushes and slip
rings to make the magnetic field.
Alternators that use electricity to generate the field current are less
efficient and more complicated. They are quite easy to regulate,
however, since the magnetic flux inside can be changed by adjusting the
field power.
Also, the brushes and slip rings wear out, requiring more maintenance.
Car and truck alternators can also be rewound to produce power at lower
speeds. This is done by replacing the existing stator windings with more
turns of smaller gauge wire.
This project is not for the faint of heart, but check our PRODUCTS page
for the inexpensive booklet Alternator Secrets by Thomas
Lindsay if you are interested. The booklet is invaluable for any
alternator experimentor! Also, some alternator/electric motor
shops may have the knowledge to do this for you.
Homemade Permanent Magnet
Alternators
Hugh
Piggott in Scotland was the pioneer in building permanent magnet
alternators from scratch. Much of our inspiration came from his designs.
Thanks Hugh!
Our experiments have consistantly shown that homemade PM alternators are
the most powerful and cost-effective solution for building a wind
generator. Their low-rpm performance is excellent, and at high speeds
they can really crank out the amps thanks to their efficiency. Our more
recent PM alternators have been based on Volvo disc brake assemblies,
which are very sturdy and have thrust bearings built into the unit. Our
larger units are "Disc" or "Axial" designs...a flat plate of magnets
rotating next to a flat plate of coils. Our smaller PM alternators are
"Radial" designs, where the magnets are fastened to the outside radius
of the armature. Since all alternators produce AC, the output must be
converted to DC with bridge rectifiers for battery charging.
Our designs to date have been single phase for ease of construction.
Three-phase alternators have some advantages (they are somewhat more
efficient, and make better use of available space), but they are
somewhat more difficult to build.
With a 7 ft diameter prop, our Volvo brake designs can put more than 60
amps into a 12 volt battery in a 30-mph breeze--that's about 700 watts.
We've seen the Volvo design peak at over 100 amps during high winds!
This gives these homebrew designs a big advantage over similar-sized
converted induction motors, which become inefficent quickly and top out
at 20-25 amps output with a 7 ft. diameter prop.
Check out all of our PM alternator projects on our EXPERIMENTS
page!
Induction Motor Conversion
Alternators
A normal AC induction motor
can be converted into a permanent magnet alternator at very low
cost. Our experiments have shown that these conversions produce
significant
power at very low speeds, but become inefficient quickly at higher power
levels.
An induction motor has a center core with no wires in it, just
alternating
plates of aluminum and steel (it will look smooth from the outside).
If you rout a groove in this center core to accept permanent magnets,
the unit becomes a permanent magnet alternator! We sell super-powerful
neodymium magnets that
are shaped and polarized perfectly for this application--check our
products
page.
In practice, our wind generators made with these do quite well until
they reach 10-20 amps of output. At this point, they become inefficient
quickly--it takes a large increase in windspeed to make only slightly
more power, and the rest is wasted as heat inside the unit. The
induction motors are wound with wire that's simply too thin for
generating large amounts of power. In our tests, DanB's PM induction
motor conversion windmill peaks at around 25 amps in 30 mph winds, with a
7-foot diameter prop. By comparison, a 7-foot prop on an efficient PM
alternator made from scratch gives peaks of 50-60 amps in similar winds!
Converted motors also have the tendancy to "cog" when starting...you
can feel the resistance when you turn the shaft. This affects low-speed
startup somewhat.
If the lesser output in high winds is acceptable to you, these units can
make for a pretty easy wind generator project. Look for AC induction
motors of the lowest rpm rating possible. 3-phase motors will perform
better than single phase. Since alternators produce alternating current
(AC), the power must be converted to DC with bridge rectifiers.
Tips
and photos--converting an AC induction motor into a permanent magnet
alternator.
DC Generators
Generators make DC current, and batteries need DC for charging.
Generators were used in automobiles until around
1970, when alternators became more practical (due to the availability of
cheap, small diodes). Even old car generators
must spin too fast to be practical for wind power, but there
have been many good plans for modifying them. Check out our PRODUCTS page
for the LeJay Manual , which contains many useful, though
involved, plans for doing this. Generators are fairly complex compared
to alternators. They must have
brushes, and complex commutators. Brushes require maintenance,
and commutators can wear out. For most purposes, alternators are
more practical today, although generators do have certain advantages at
times.
Certain low rpm DC motors can be purchased as surplus and
work very well as 12 volt low rpm generators. These are from old
mainframe computer
tape drives, and are sometimes available in local and mail-order
electronics stores, and on Ebay. Check out Our
tape drive motor page HERE. They don't make a whole lot of
power...you can expect only 100-200 watts of output...but these motors
are almost a science project in a box! Slap on a frame and a 3-4 ft
prop, and you have a small working wind generator.
Surplus tape drive motors can make a quick and easy generator for
small windmills
A brushless DC permanent magnet motor is really just a permanent magnet
alternator! A special driver circuit provides AC power that is in phase
with the rotation. If you are able to find a large one of these surplus,
it's possible you might have an excellent start for a wind power
project. They are used in robotics and precision control applications,
and some use Nd-Fe-B magnets for high torque in a small space. As with
surplus tape drive motors, we would not trust the bearings to stand up
in a wind power application...add more bearings so you don't ruin the
motor's original front bearing.
We have not yet been able to locate any
of these surplus for experimentation. If you have tried this, or have
more information on sources, please Email us! However,
we do have a small version...our Homemade anemometer
uses a small surplus brushless DC PM motor, which is available for
cheap on our Products
pages.
The inside layout of our tiny Brushless PM DC Motor looks
just like the Wood
103's alternator!
Induction Motors as Alternators
It's possible to make a 3-phase induction motor produce electricity,
either 3-phase or single phase. This requires a controller and
capacitor. The generator must run at a fairly constant
speed. For this reason, this type of generator is more suitable for
constant-speed hydro
power installations than for wind, where speed varies--though it can be
done. We have not experimented
with this technique yet, since we don't have a suitable hydropower
source.
For more information, check out the book Motors as Generators for
Micro-Hydro Power by Nigel Smith.
©2002-2004 by FORCEFIELD
This page last updated 11/03/2004
