The RPM you get vs. the Voltage you need.

Several people have complained that the 400 rpm lower limit for forcing a charge into nominal 12 volt batteries is just not right for their application and asked for a fix, so here it is --- I think. Be warned: I haven't tried this, but I think it will work.

Before you jump into this though, consider the issue of "stiction." Stiction is the resistance to movement you get when you initially try to slide or roll something. In this case, when you try to start turning the alternator (ignoring back EMF effects and the like). The word comes from "static friction." The reason this is important is that static friction is always greater than rolling friction. This means that it will take more force to get your windmill turning from a dead stop than it will to keep it turning. So, it's a good idea to keep the thing turning even if you aren't generating any electricity. There is also the issue of the efficiency of the windmill blades as a function of their turning speed. Does the efficiency increase up to a point before air resistance kicks in noticeably? The point I'm trying to make is that having a dead zone where no generating is taking place at the lower end of the rpms is not necessarily a bad thing. In the present case, where the rpms have to get above 400 to charge a 12 volt battery the ideal situation would be to design the driving blades to be in that particular sweet spot during your prevailing conditions. That is, no charging at all until 400 rpm when the voltage climbs over the diode and starts to dump current into the battery. The battery load will then limit further increases in rpms and you are in the "sweet spot" for that system at a high enough rotational speed to have a little inertia and keep it rotating.

OK, I admit that I'm not smart enough to do that sort of designing either! I'm just hoping that you are. So what do we do now?

I would make my best guess as to the parameters I was dealing with and just build the thing; because as you will see below you will be able to adjust the output later to make everything match up. I would TRY for that sweet spot, but I'm experienced enough to know what a difficult trick that is. Then, after I got my machine cobbled together I would set the thing up and see what happens. I would put various loads on it during different wind conditions and generally play around until I had a better idea of what conditions generate the most power, not just voltage.

Once I had the power sweet spot identified I would measure the voltage it occurred at, through the diodes I was planning on using. We do know that 13.8 volts is a pretty good voltage to have for charging 12 volt (nominal) lead-acid batteries. The sweet spot voltage you measure will be --- something else, of course. Assume you measure a voltage of 10.35 volts at your sweet spot. The ratio of 10.35 to 13.8 is exactly 3 to 4, so three transformers with 3 to 4 winding ratios for our three phases will match things up nicely. Take a look at the circuit below.


On the left is the Alternator circuit you've already seen, followed by three transformers and a full-wave three phase rectifier circuit. Toroidal transformers are the only way to go for this application. They are much more efficient (up to 99.75%) and much more broadband (will work at a wider range of frequencies) than other types. The odds of you being lucky enough to buy some with the particular winding ratio you need are nil, but toroidals are easy to wind yourself and very good cores are available at reasonable prices. See Alpha Core's ( offerings for example. Just pick cores with power ratings at least equal to the maximum power you expect and put on two windings at the ratios you just determined. Wire as shown and you should be in business!

Identifying the Appropriate Alternator.


Modifying the Stator.

Modifying the Rotor.

Assembly and Voltage Regulation.

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