4. Natural Voltage Regulation and Stall

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Between the two tests, there is one glaring difference. The relationship between voltage and RPM that we determined in the first test all but disappears in the second one. In the first test, 652 RPM produced 45.4 volts. In the second test, 602 RPM produced 12.41 volts. And at 702 RPM, we measured 12.40 volts!

This demonstrates a phenomenon known as voltage regulation. I call it "natural" regulation because there are no gimmicks involved. No fancy circuits or expensive devices. The 45 volts is regulated naturally and automatically to 12.41 by the battery itself!

Voltage regulation is an important concept for a wind / solar builder. When you understand that the battery regulates, or limits, the voltage in your system, you realize that your generator's output voltage doesn't have to match your battery's voltage. In other words, if your generator produces 10, 15, or even 100 volts, you can still hook it up to a 12 volt battery. The battery, in general, doesn't mind. Generally speaking it "converts" the extra voltage into amperage. Notice on the second test, the Charging Voltage doesn't rise, but amperage does. We're producing power now, which is wonderful - but there is a catch.

Regulation comes at the cost of presenting a load to your generator. In other words, the more your battery is regulating the voltage, the harder your generator is working. If you make your generator work too hard, it spu-spu-sputters to a halt. Those of us who learned to drive a vehicle with a manual transmission know what happens when you let the clutch out too fast and present too much of a load to your transmission. Your engine stops immediately.

With wind generators, stall doesn't necessarily mean that your blades (which are the engine of the generator) grind to a halt. What they actually do is something much harder to see with your eyes or instruments.

Imagine pedaling an exercise bike. This bike is configured so that the faster you pedal, the harder it becomes to turn the wheel. You can pedal slowly all day long with nearly zero resistance. But as soon as you really try to pedal fast, the bike seems to "push back" at your feet. As you continue to accelerate, you will reach a point where you know you can move your legs faster, if you could only muster up the strength to pedal harder. Get it? At that point, you've stalled.

Back up on your wind generator tower, a system that is "in stall" is spinning as fast as it possibly can, but there just isn't enough wind (strength) to push it any harder against the resistance. This is a bad place to be, because you are not making good use of the wind. And if you're not putting the wind to good use, what's the point of building a wind generator?

Remember, the more your batteries have to regulate the voltage, the easier it is to stall. That is why people design their systems around all kinds of different "bus" or "system" voltages. Twelve, 24, and 48 are the most common.

So if my generator produces over 40 volts, wouldn't a 48 volt system be less likely to stall? Yes! But remember the concept of cutin. If I had a 48v system, I wouldn't produce any useful electricity until my generator was spinning over 700 RPM. That's pretty fast! At lower speeds, it wouldn't stall, but it also wouldn't be doing anything useful.

My generator actually would probably make a good machine for a 24v system. I chose to stick with 12v because of the wide adundance of inexpensive 12v electronic items. Inverters are cheap for 12v, and any Wal-Mart, auto parts store, or RV dealership has everything from fans to coffee makers to televisions that run straight off a 12v battery.

Also, because a 12v system starts being useful at a lower RPM, I am better prepareed to take advantage of the low winds where I live. I have chosen to compromise between the possibility of stalling at high speeds and squeezing every last watt out of slower winds.

Next Up . . . Taking this information and actually DOING something with it.

Warning - Get out your pocket protectors and slide rules. We's gon' be doin' some math.