AboutDan Fink Expertise Though my experience is mostly in the fields of electricity, magnetism, and physics, I have a broad science background. My career is in the field of alternative power sources -- solar, wind, water and battery power. But any questions about electricity, magnetism, energy conservation, power generation, electric motors, and even general physics are very welcome--especially from kids. They ask the best questions of all! I pride myself in answering science questions accurately, with ideas for SAFE, easy experiments that kids can perform by themselves--and that let them prove the answers to their own satisfaction. I think science should be fun, and available to everyone, regardless of age.
Experience I have volunteered in our local public schools for 5 years. I currently make presentations at our schools about electricity and magnetism, with a focus on solar, wind, water and other alternative power sources. I try to demonstrate at our schools how easy it is to make electricity, with simple devices using spinning magnets and coils of wire--powered by wind, water, bicycles, gerbils...etc. And of course solar panels! I am the webmaster of Otherpower.com, an alternative energy website. I have lived 10 miles from the nearest power pole for 11 years--I make all my own electricity from scratch with sun, wind and water.
Publications Otherpower.com
Wondermagnet.com
Co author of the book "Homebrew Wind Power" ISBN 978-0-9819201-0-8
My articles appear regularly in such magazines as Home Power and Back Home Education/Credentials BA Technical Journalism
I hit the button too soon. I realize that when you are not measuring the current then there is no closed circuit and thus no magnetic field to act against the permanent magnets in the motor to slow the turbine down. One a bulb is hooked up the turbine slows down a lot. If the bulb is of a higher resistance then what happens. Is there any point to measure current of a wind turbine with no load hooked up to it? My daughter was measuring volts and amps with a mulitmeter and then tried to find a bulb to match the higest volts and amps or Watts rating from the wind turbine. But then when a real bulb was hooked up, the wind turbine slowed right down so that not nearly the same volts and amps were powering the light bulb. So perhaps all current measurements should be done under load conditions?
Thanks,
Ray Van Raamsdonk for Raeann.
Thanks,
Ray
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Followup To
Question -
Hi Dan,
Thanks again for your answer to my motor question from before.
As you know, I am helping my daughter in grade 5 with a wind power project. We are using a variable speed fan to generate some wind to drive a wind turbine that uses a small DC motor to generate electricity. One thing I was wondering was how can you measure the wind speed of a fan. I am wondering how the wind speed from the fan varies as a distance from the fan so that we can relate the output power generated by the fan to the formula P = Area x Speed x Speed x Speed x (some factor). My daughter would try various blade type to relate to the area factor and we were thinking to vary the distance from the fan in order to get a Speed change. I don't know if the distance is proportional to speed but it might not be that simple. I understand that there is fan turbulance so that it is not easy to measure fan speed but I found a few locations from the fan where a suspended ping pong ball would get blown at a fairly constant angle and stay fairly stable. So those would be useful locations for measuring wind power. So we are seeing if the wind power formula will work on our small scale demo by keeping the speed fixed and varying the blade type and then by keeping the blade type fixed and varying the speed (by varying the distance from the fan and also by trying the three speed settings - but we don't know what speeds these are.) I know that as the wind increases the angle from the vertical that a standard ping pong ball gets blown changes. There are tables on the net of how this relates to wind speed but I haven't found anyone who has figured out the formula that relates the angle to wind speed. The table seems to be empirically derived maybe by using another wind anemometer or I can think of using the car method where you drive a car at differnt speeds and hold the home made ping pong ball anemometer out the window and measure the angle for speeds of 5, 10 , 15, 20 km/s. So would you have any idea on how the wind speed varies as a function of distance from a fan in an ideal case (forget turbulance ) and how to figure out how wind speed relates to the angle a ping pong ball would swing at when subjected to wind?
Thank you very much.
Ray Van Raamsdonk
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Followup To
Question -
I am helping my daughter to do a wind power project. So far we have been experimenting with a Radio Shack motor 273-23 which says 8,300 RPM, 0.98 Amp, 1.5-3 volts 15g torque. So far we have 7 inch propellers on them and use one to drive the other until we find a bigger fan. When one motor directly drives the other, then if we use a 3 volt battery as input then 2 volts are generated. If we use 6 volts as input, then 4 volts are generated. When we use wind power then we get 0.035 volts generated and 0.333 milliamps. My daughter was wondering if there is any way to light up an LED with this kind of motor? We may also try a video rewinder motor as a generator because it seems to light up an LED when you spin the thing by hand. This motor says RF-520C-17410 ,12V, 5,500 RPM. We also wondered that if 12 volts spins a motor at 3,000 RPM, then if you spin the motor at 3,000 RPM using another motor say, then shoul it produce 12 volts or is there a big loss somewhere? My daughter would like to build a little house and then have the LED go on when the wind generator is spinning. ANy kind of advice is welcome.
Thank you very much.
Ray Van Raamsdonk
Answer -
Great to see kids and parents work on wind power projects! I work for http://www.otherpower.com/ and we have a ton of them posted on our site.
A general rule is that to get the rated voltage out of a DC motor, you have to spin it about 20% faster than the rated RPMs. So you can see the problem...very hard to get 5500+ rpm out of even a very strong wind, and for sure not from a table fan. The lowest-voltage red LED still needs 1.7 volts.
You can punt and just hook the windturbine to an analog milliamp or volt meter....everything will still be proportional to your available power in the wind.
For an LED, you'll have to find a DC hobby motor that is lower RPM. Every so often I find them, but I dont' have any around now....seems like every time I find one, it's surplus and the seller runs out of stock.
I just tested a nice hobby motor that's used in the http://www.kidwind.org/ project....not bad on the rpms, and enough to make over an amp at 2.0 volts at 1100 rpm....kidwind.org links to it also, it's from http://www.shop-pitsco.com/
You could also try a low-voltage 'grain of wheat' incandescent lamp -- radioshack sells them, and I've seen them as low as 0.5 volts rating.
The big problem is that most hobby motors are very high RPM, and many are not marked as to their RPM rating -- I tested 15 motors for our kids mill projects before I found one that was low RPM.
Hope this helps, feel free to ask more questions as you keep experimenting.
DANF
Answer -
You are correct on the turbulence issue; it makes using fans in wind power experiments difficult....the way to get around it is to just use your own experimental measurements.
As a wind power experimentor, I use lots of different formulas, but airspeed vs. distance from the fan is not one I've ever run into :-( Your best bets are an anemometer (like one of those handheld ones for firefighters and backpackers) or the car method. You could always buy one (like the Brunton handheld), get your measurements, and return it intact!
I think with your pingpong ball idea, the scale would be compressed at the high end, since the force need to hold the ball up horizontally at max speed would be more than the force when the ball is hanging vertically.
Sorry I can't be of more help....airspeed vs. distance from a fan is something I've never seen a formula for either!
DAN
Answer Hi Ray and Reanne
You got it exactly right. Measuring voltage in an open circuit is useful, since it tells you at what RPM the generator will cut in and start charging a battery. Measuring current from an open circuit uses your multimeter to complete the circuit, and the numbers won't match up with a real-life load. Measuring current thru the bulb won't be the same as thru a battery, either....with a battery there's no resistance on the generator until you reach the battery voltage with your open-circuit measurement.
So, yes, you would always make your current measurements in a closed circuit (but not a short circuit).
The higher the resistance of the bulb, the more it will load down the generator. If it slows it down too much, you need bigger blades.
Basically, your project has now hit on the biggest challenge in deisgning a BIG wind turbine, too: matching the generator to the blades. If the generator is too powerful for the size of the blades, as soon as it cuts in, it will slow down and start to stall the blades = less power. If the generator is not powerful enough for the blades, they will overspeed if the wind gets too high....not an issue if you are using a fan to power it, but a serious issue in real winds.
When working with brand-new alternator designs, we actually take torque measurements of the alternator for this reason to help us design the blades. Powering a wind turbine project with a fan actually means you have to have a *bad* blade and alternator design to make it work! I discovered this after doing some testing on the kidwind.org parts. I'm currently working on designing the 'perfect,' most efficient set of blades for their generator. They will be tiny, fast and of small diameter -- and won't work worth beans with a table fan...probably will cut in at 25 mph wind. But they'll be VERY efficient! Then, I'll try to optimize the blades for use with a fan -- they will be MUCH bigger, and would blow apart in a 25MPH wind. Interesting, eh?
After that, my project will be to design an alternator that works at very low RPMs for table fan use. It will be similar to our Hamster Alternator (http://www.otherpower.com/hamster.html/
I hope that info helps. Great project you are working on!
DANF
