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.
©2002-2004 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!
Questions or comments? Click here to send us an email at our new email address.!
This page last updated 11/03/2004
