Physics Lab B/C

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Glacierguy1
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Re: Physics Lab B/C

Post by Glacierguy1 » January 6th, 2010, 8:34 pm

Thats really good. What fan are you using?
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Re: Physics Lab B/C

Post by Greg Doe » January 6th, 2010, 8:38 pm

Glacierguy1
The simple answere to your question is yes. Last year I built 4 generators. Only two of them
showed similar characteristics. So of 4 motors, 3 were different. I also tried a CD motor from a
car CD player and a DVD motor. The latter two were very different, and we didn't use them. We
also checked and compared our window fans with a wind speed meter, and as you might guess
there was a lot of difference between fans. The reason we did this was because we had 4 different
teams doing research, and we wanted to be able to account for differences in performance when
we compared our results.
If you have more than one CD motor spin each one, and feel the difference. Than turn each slowly
and feel the "bump" from the magnets in the motor. The motor that has the strongest magnets
will generate the highest voltage. Of course you don't know what motor you will encounter at
your competition, but it will be the same for everyone.
I haven't seen any posts this year on how much voltage people are getting, but there were a lot
of posts last year, so check out last years forum. This years "fans" are larger than last years, so
all things being equal I believe the voltages will be lower. Having said that I will reverse my
prediction because I think that there is a lot of room for improvement in "fan" design. Good luck,
and let us know about your progress. One last thing to always keep in mind is that both B and C
division are doing this event this year, and the "fan" diameters are different . Last year the C
division did a similar, but different event. Be sure that when you take into consideration other
peoples performance, that you know which division they are supplying data from.
Greg Doe
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Re: Physics Lab B/C

Post by E Edgar » January 7th, 2010, 7:20 am

Greg Doe wrote:This years "fans" are larger than last years, so
all things being equal I believe the voltages will be lower.
But wouldn't a larger fan capture more kinetic energy from the wind and generate a higher voltage?
I mean, the wind power available increases proportionately with area.
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Re: Physics Lab B/C

Post by TwoplusTwoEqualsFive » January 7th, 2010, 8:07 am

As we approach closer to the competition, I just want to get a rougher estimate in how I was doing. I know it varies from fan to fan but I'm pulling roughly a 875mV on high and 550 mV on low. Anybody bold enough to give me a comparison? - Haha
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Re: Physics Lab B/C

Post by Greg Doe » January 7th, 2010, 9:54 am

E Edgar
You are thinking, and that's good. But I believe you have reached a wrong conclusion. Also there
has been some misinformation posted here. I am not comfortable saying someone is wrong, so sometimes I would rather just disregard there comments. If you read last years forum, you will
see that most people found that smaller, thinner "fan" blades generated more voltage. Here is
where you are having trouble with the concept. This Lab Event does not represent a
solution to the real world problem of cheep energy! In the real world we want to generate the
maximum amount of power, but for this event we want the highest voltage. The faster you
spin the motor the more voltage it will generate. Large wide "fan" blades will capture more wind
energy, but will turn slower. This detail has already been pointed out in this years forum postings.
Another tip has to do with the number of individual blades on the "fan". Less blades is less mass.
A two bladed fan is probably the best compromise. We experimented with a single bladed "fan"
last year, and it was our second best performer, but would not start without a flick.
This years rules allow the students to give the "fan" one flick to get it started. Last year this
wasn't specified, and from what I understand a few events allowed the practice, but most did
not. The highest voltage we obtained last year was from a 3 inch Cox .010 model airplane
propeller. It was done only as an experiment to see what would happened with a very small
"fan", and guess what, it did not want to start on it's own. It took a healthy flick to get it
started, but once going it spun very fast. We took a more conservative approach with "fan"
blades in the 6 to 6 1/2 inch range. Good luck to you, and as always let us know of your progress.
Greg Doe
Smyrna, TN

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Re: Physics Lab B/C

Post by Melkor6000 » January 7th, 2010, 2:06 pm

Large wide "fan" blades will capture more wind
energy, but will turn slower. This detail has already been pointed out in this years forum postings.
This is definitely true. I made a set of hand-sanded balsa blades in a great airfoil and at optimum angle of attack, but I got 4th out of 5 at an inter-school scrimmage because they were so heavy. Using only tow blades is something that would be worth pursuing.

Also, the best designs will probably taper the blades, which decreases rotational inertia, and lets the same amount of force from the wind exert more torque, and let the blades spin faster.

Lastly, the fan and test motor you use are factors to consider when comparing voltages generated. The bigger the motor (that is, the more turns of wire inside), the more voltage (Lenz's Law of Induction). Also, the fan makes a huge difference. For example, I generated almost 15 Volts with heavy blades with a Vornado fan running on high with a big motor attached to the blades, and barely scraped 1 volt with a box fan and CD motor. So be advised...
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Re: Physics Lab B/C

Post by E Edgar » January 7th, 2010, 4:10 pm

Thanks for your comment; that definitely agrees with my empirical observations.

However, wouldn't the weight of the blade only affect the rotational inertia and therefore acceleration? A heavier blade should theoretically take longer to accelerate but (I think) should spin with the same angular velocity. Considering a linear analogy, if one pushes a 1 pound brick with the same force as a 2 pound brick, only their acceleration is affected, not their velocity. The 2 pound brick would take longer to get the same velocity as the lighter brick but it eventually would. Now, the heavier brick would create a larger frictional force and so its maximum velocity would be less. However, with "fan" blades, since there is very little contact surface with the CD player, I don't see how friction could affect a heavy blade significantly differently to a lighter one.

Perhaps the observed superior performance of the lighter blades is due to some other factor (like, is it harder to balance a heavy blade?)?

The ideas presented in your last post may very well be correct, I just want to think it through a bit so that I can convince myself I'm not jumping to premature conclusions.
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Re: Physics Lab B/C

Post by Greg Doe » January 7th, 2010, 7:15 pm

E Edgar
If you have an idea, try it out. It's time to start building "fans"! By the way I'm not sure that I am
using the best word to describe our "windmill". If someone has a better word, please speak up.
Since we are using a FAN as in a box FAN, but we are making a "fan" as in a propeller, windmill,
or even impeller, it is possible to be writing and thinking about one kind of "FAN", but the reader
might misunderstand and think we are talking about another kind of "FAN"? Any suggestions?
I can't disagree with your theory, but I don't worry too much about theory when I can test the idea.
When you have built your first successful "fan" test it, and observe the results. Then glue some weight on the front of the CD disc ( a steel washer) and try it again. If your theory is correct,
get back to the forum with your results, and we will all go out and buy a bunch or round lead
fishing weights. Good luck.
Greg Doe
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Re: Physics Lab B/C

Post by Glacierguy1 » January 7th, 2010, 8:08 pm

I've been getting about 550 on high and 333 on low using a 4 blade fan. Are all of these higher numbers with 2 blade and 3 blade fans?
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Re: Physics Lab B/C

Post by fleet130 » January 8th, 2010, 1:22 am

it is possible to be writing and thinking about one kind of "FAN", but the reader might misunderstand and think we are talking about another kind of "FAN"? Any suggestions?
I suggest using the word "FAN" to refer to the device used to create the wind and the word "TURBINE" when referring to the device used to extract the energy from the wind and turn the generator to convert it to electricity. This is more in line with the way I generally see the words used.

Edit: Just accidentally came across this article: Why do wind turbines have three narrow blades, but ceiling fans have five wide blades?

And here's another that covers a little bit of everything: Science Fair Wind Generators
Information expressed here is solely the opinion of the author. Any similarity to that of the management or any official instrument is purely coincidental! Doing Science Olympiad since 1987!

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