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.
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.
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.
©2002 by FORCEFIELD
Homemade Permanent Magnet Alternators
Homemade Volvo Brake Disc PM alternator, 800 watts, $150!
Induction Motor Conversion Alternators
Armature converted with permanent magnets
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
Brushless DC PM Servo Motors
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.
The inside layout of our tiny Brushless PM DC Motor looks just like the Wood 103's alternator!
mailto:info@otherpower.com
This page last updated 2/19/2002
