Converting a Denso Automotive Alternator for

Wind Turbine Battery Charging

William B. Cushman, Ph.D.

assembled rotor

There is a wealth of energy freely available from the wind, if it can be harvested, and many clever people are finding out that it can be. At first it might seem that the problem is simple; just go to your neighborhood You-Pull-It junk yard and get a nice used automotive alternator, put it on a pole with some blades and away you go. But, there are serious technical difficulties with this simple approach. The automotive alternator was designed to rotate at a high engine RPM, not the low RPMs that can conveniently be produced from a wind turbine. The automotive alternator almost always uses a field winding to control it's output, and the current used by this field winding will typically consume about 48 watts when the field is fully energized. Which means that the power you get from the system as a whole will be whatever is directly produced minus the 48 watts you are putting into the field. This would not be so bad if the direct output of the alternator was always greater than the power requirement of the field winding, as would be true in an automobile, but it is rarely true when the power source is a wind turbine. A wind turbine often simply does not produce the necessary torque to rotate an alternator at a high enough rate to overcome the losses inherent in a field winding, and when this situation prevails the net result is to discharge the battery rather than charge it. Even worse, an automotive alternator usually has a voltage regulator built in that is designed assuming effectively unlimited torque to turn it. So, if the battery is low the regulator will attempt to quickly charge it by energizing the field winding at maximum current. In an automobile there is enough torque available to do exactly that. But, because a wind turbine rarely has sufficient torque with which to generate high currents, the load that the alternator presents to the driving source (the wind turbine blades) from sending excessive current to the field winding can bring the wind turbine to a nearly complete halt . . . which effectively eliminates any current production whatsoever. And this situation will occur while the voltage regulator continues to drain power from the batteries at the maximum rate possible - - - in order to overcome the losses it senses. In this situation the discharge rate on the battery will be almost exactly the power drain of the field winding, about 48 watts. Thus, overall, an unmodified automotive alternator is an extremely poor choice for a wind turbine generator.

On the other hand, I can pick up a Denso alternator (my current favorite, pun intended) at my local You-Pull-It for $15. This gives me a nice aluminum case to work with, with some good bearings in a machine that is, in fact, designed to be turned and produce electricity. As an important additional plus, the shape of the rotor pretty much eliminates "cogging" from a strong magnetic field. This alternator also has a very sturdy mount, and the driving pulley is mounted on a threaded shaft that looks as though it is just begging to be fitted with a propeller blade. When I initially began to investigate the possibilities of using an automotive alternator for a wind turbine the Denso alternator looked like a darn good place to start, and as it turned out, it was. The modifications shown in the linked pages below allow a skilled craftsman to produce an alternator that will start charging a 12 volt battery directly from the diodes at about 400 RPM.. This alternator uses a single permanent rare earth magnet (NdFeB) for it's field, and this magnet produces a flux density that is much stronger than any field that it would be possible to produce from the original electromagnetic field windings. But most importantly, the field from this permanent magnet requires no power whatsoever, at any time, under any condition of operation. The 48 watts you would be spending to produce a field in this alternator without a permanent magnet can be used to charge your batteries instead. Moreover, there is no possible way for this alternator to discharge it's associated battery if everything is working properly. The increased field strength thus obtained also greatly enhances the ability of the machine to develop significant charging current at the lower RPMs that are convenient for a wind turbine alternator.

There are some serious issues to be addressed in attempting this project, however. The craftsman attempting it will need to do (or have done) some machining, he will need some special tools, and the magnet used will easily take the ends off of most of his fingers if he is not very, very, careful with it. This magnet has a pull strength in excess of 150 pounds! I cannot emphasize strongly enough that the magnet used for this conversion is strong enough to be just plain dangerous. It is strong enough that it can stop the movement of a mechanical watch on the wrist of the person holding it in his hand. Literally, I am not kidding about that. I learned that fact the old-fashioned hard way. And don't even think about getting close to any computer equipment or magnetic storage media of any sort with this magnet, it will very likely wipe your media absolutely clean. On the other hand, if you ignore this warning and find that you have wiped your hard disk completely . . . well, I told you not to get close. Wiping out your hard disk is not all bad, however. A clean hard disk that has finally shed all of those Windows virae and associated problems is a great opportunity to load up a free Linux distro and start over with a real operating system. But I digress.

The magnet used for the modification described here was specifically designed for this application and this specific alternator, and I had to buy 100 of them to get the price down to a reasonable level and amortise the tooling costs. So I'm selling the extras for $50 each, if you think you would like to give this a try. Plus shipping.

Please study the following pages carefully before jumping into this, however. A project like this isn't for everyone, and does require a fairly high skill level to be successful.

The following pages give the essential details needed to modify a Denso alternator for use as a wind turbine generator. Please note that just because you have a nice alternator you still need something to turn it! The Denso alternator is capable of producing ROUGHLY 1000 watts ... if you can provide the torque to turn it. At a low wind speed of, say 3.7 m/s, which will give you roughly 25 watts per meter squared of turbine, that would mean you need 40 square meters of swept area. If this was in the form of a circular "propeller" of some sort that would be a diameter of 8 meters! Of course if your average wind speed is only 3.7 m/s, then you really shouldn't be thinking about wind turbines. Most people will have much more wind than this, and the power increases as the square of the wind velocity.

Please be sure to work all of this out before starting. A most useful place to start is a recent article that was published in Low-tech Magazine, which can be read here: Be sure to read the comments at the bottom!

Identifying the Appropriate Alternator.


Modifying the Stator.

Modifying the Rotor.

Assembly and Voltage Regulation.

Wind Turbine Blade Construction

Resources and Services.

Purchase our Special Magnet.

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