3. Testing the Genny - The Amperage Test

Note: For best results, read these posts in order. See menu on the right.

Test #2 - The Amperage Test

The second test is completely different than the first. This one actually causes electricity to flow from the wind generator into a battery. How much electricity is flowing (measured in Amps) is what we determine here. The first test measured voltage. This one measures voltage and amperage. Therefore there are 2 meters to observe.

The other fundamental difference between the tests is that this is not an "open circuit" test. And because electricity is flowing, there is more potential for danger. Specifically, this test simulates the work performed by the actual, finished wind generator. The faster it spins, the harder it's working, the hotter it gets, etc.

There are valid results from this test, but the best part is from vanity's perspective. Since this test mimics the finished generator's abilities, it is a chance for you to see your project actually working. A little positive feedback never hurt anyone, right?

I was very happy with the results. Here they are:

RPM	"Resting" Voltage	Amperage	Charging Battery Voltage
203	11.98	0.54	12.05
353	11.00	5.48	12.05
602	11.05	14.11	12.41
702	11.06	17.12	12.40

This time, the RPM and Amperage numbers don't have a fixed relationship. Generally, they are supposed to follow a rising parabolic curve. In the first test, the faster it spun, the higher the voltage climbed. In this test, as RPM increases, the amperage climbs. And as the RPM increases, the amperage climbs at a faster rate. At some point, that curve "levels off."

Compare 200 and 300 RPM. That's only a 50% increase in speed, but represents a 1000% (10x) increase in power production! Near the top, the curve isn't so extreme - twice the RPM equals approximately twice the power. If we tested higher speeds, we'd eventually reach a point where large increases in RPM would represent very small increases in power.

For this test, 702 RPM was about as fast as we wanted to go. My cheap Harbor Freight multimeter is only rated for 10A. We were feeding nearly twice that amount through it. I've fried these meters before - it's pretty stinky. Luckily we didn't fry this one.

Remember that we figured cutin at 200 RPM. As a confirmation, this test shows 540mA flowing into the battery at just over that speed!

Notice that in each line, the Charging Voltage is higher than the Resting Voltage. This is because a battery's voltage rises temporarily while it is being charged. At periods of rest, when no electricity is flowing, the voltage will settle to a lower number. Naturally, this means when you charge a 12v battery, it is not sufficient to stop once the charging voltage reaches exactly 12 volts. Once you stop, it will settle to a lower number.

The Resting Voltage is displayed to check the consistency of our results. It shows that we are putting varying amounts of current into a (relatively) similar battery. If we charged our batteries for too long during the test, the Resting Voltage would rise, meaning the Charging Voltage numbers would also rise, and we'd have skewed results.

Using the Charging Voltage, we can determine Wattage. Power (Watts) is measured by multiplying volts and amps. Doing the math, we come up with these numbers:

203 RPM - 6.5 Watts
353 RPM - 66 Watts
602 RPM - 175 Watts
702 RPM - 212 Watts

How many watts are produced at specific RPMs is information we will use to determine the size of our blades.

Coming up Next - Natural Voltage Regulation and "Stall"
(and I promise we'll tie this all in to cutting blades soon, too.)

2. Testing the Genny - The VOC Test

Note: For best results, read these posts in order. See menu on the right.

I brought the body of my newest wind generator project to a guy I know who runs a machine shop. He hooked it into his lathe, and we ran a few tests.

There are two tests that we performed. I'll explain the tests, as well as why they're important, in individual posts.

Test #1 - OC (Open Circuit) Voltage Test.

An VOC (Volts, Open Circuit) test measures how many volts the wind generator produces at a specific RPM. The two numbers (voltage and RPM) have a fixed relationship to each other. In other words, the results of this test gives you a ratio that you use in other parts of your project. Specifically, these results are used to determine how large the blades should be on your finished wind generator.

Here are the results from my test of this generator:

153 RPM - 10.1 Volts
303 RPM - 20.7 Volts
403 RPM - 27.8 Volts
652 RPM - 45.4 Volts

So, what exactly does a person do with these numbers? I'll explain.

The main purpose of this wind generator is to produce electricity that will be used to charge batteries. In order to charge a battery, you have to feed it voltage that is higher than the current voltage of the battery. A wind generator will produce electricity from the moment it begins spinning. But that electricity is only useful to you if it is charging your batteries. The speed at which a generator starts producing useful electricity is called "Cut-in" (sometimes written "cutin.") This test uses only a voltmeter. There is no electricity flowing through a circuit, so it is called "Open Circuit."

My system is a 12 volt bank of batteries. 12 volt batteries actually operate closer to 13 volts. So the wind generator has to produce at least 13 volts to be doing anything useful. Using the information from the VOC test, we have to determine what speed represents 13 Volts.

Remember, the numbers represent a ratio. They vary slightly within a certain margin of error. None of this is rocket science. Close is usually good enough. Let's examine the first and last pair of numbers:

10.1 volts at 153 RPM = 1 volt for every 15.15 RPM. That means 13 Volts equals about 197 RPM.

45.4 volts at 652 RPM = 1 volt for every 14.36 RPM. That means 13 Volts equals about 186 RPM.

For this project, I've used 200 RPM as the measured cutin.

Next Post: Cutin Speed leads to Cuttin' Blades.

yuck yuck. Oh, sweet alliteration.