Don't mess with an unknown fan - these days, they're not like motors used to be, assuming you're talking about computer fans.
For a science fair project, get a hobby motor for two or three bucks:
These, being permanent magnet motors, will not only reliably generate DC, but can show speed/torque/voltage/current relationships with a couple of meters and a little analysis, which should really dazzle the judges. :-)
Heck, why not give them a hand crank for a _real_ hands-on display? :-)
Well its not a computer fan, I was actually hoping to use an old room fan (the kind you can pickup and move around) Since I happen to have a junker on hand....
A neat idea, but the idea I'm going for is to recycle as much as I possibly can.... I want to demonstrate and measure how effectively a homes electrical costs can be offset (and If I can use junk parts to do this, then all the better!). A crank is a neat idea too! I might add one in, just for flare ;) but right now, I wanna see about getting it to run on natural resources, and if necesary, but having a flow run from a pair of buckets that can be raised or lowered, and then swaped.
Hope I can keep getting more usefull ideas!
Oh, by the way, I also have a junker freezer, could the compressor motor be scavanged?
In general motors intended for AC mains power will be more difficult to use as generators, so avoid them.
In principle any permanent magnet DC motor will do, for example from a R/C car, tape recorded, or the radiator fan motor of an old car, but you would need a lot of RPMs to get a decent voltage with any of these.
You might find that big stepper motors out of old printers (e.g. from the dump) are a good way of generating reasonable voltages without requiring high RPMs. With stepper motors, what comes out will be AC. You might need to experiment to find which wires are connected to the same winding using an ohm meter. If you want to power a light bulb then the fact that it is AC won't matter, but to run a radio etc. you will need a rectifier and smoothing capacitor, and ideally a zener shunt regulator or similar.
I assumed he wanted it to have a fan to be able to turn it with the wind, if not then any brushed dc motor would be easier, they are so cheap from many hobby shops for models rc, electronics etc, I gues the one your refering to is a brushed type.
so is it any use when theres a power cut or anything ?