Building a small electric model windmill

speed of a brushed motor would generally be proportional to voltage.
a solar cell should produce a reasonably steady voltage,
but if you need precise speed control, you may want a brushless or stepper motor with a controler.

Connections should be pretty simple.  The motor will have two wires.  The solar cell will have two wires.  Connect them up and the motor will turn.

You have to make sure the voltage and current specifications are compatible.

Note that the motor turns pretty fast, with very little torque.  You will need some sort of gearing to reduce the speed and increase the torque.  The easiest way would probably be a large pulley on the windmill and a smaller one on the motor shaft.  You might be able to run a rubber band directly around the shaft for example.

If you get it running too fast in bright sunlight, you could throttle it back by covering part of the solar cell with a piece of cardboard.

For the motor in the first link:  1040 RPM at 1.5 Volts with 0.08 shaft diameter.

If you want 3 RPM you need a reduction ratio of 1040/3 = 346.
If you use the shaft as the belt drive, your large pulley has to be 346*0.08 = 28 inch diameter.  So you probably need two stages of pulleys.

You could also consider the geared motors from Edmund:

     http://scientificsonline.com/product.asp_Q_pn_E_3052407
     http://scientificsonline.com/product.asp_Q_pn_E_3052408

>> Perhaps one rotation every 3 seconds.  ==> 20 RPM

If you want 20 RPM you need a reduction ratio of 1040/20 = 52.
If you use the shaft as the belt drive, your large pulley has to be 52*0.08 = 4 inch diameter.  You could use a lar lid or a cardboard ring.
So you

>> lar lid  

Or maybe  a jar lid.

Parts
Small dc electric motor  (radio Shack?)
3 volts worth of solar cells (radio shack)
Small pully for motor (Home Depot)
Large pulley for windmill (home Depot)
Sourced depend on you location
Hook all in series with switch if you want. If you want to run without the sun you need battery storage and things become more complicated. (solar path lights might help)

You're going to need some advanced engineering to get this to work.

Small motors need several hundred milliamps at 3 to 6 volts.

That's going to take a lot of solar cells.  your typical solar cell puts out about .6 volts at a few tens of milliamps.

That means to get 6 volts you have to put ten solar cells in series.

To get 100 milliamps you need to put ten of those series strings in parallel.

 

A small electric motor with a gear at the required ratio can probably be found at www.elfa.se and there's bound to be a supplier in the US as well, if that's where you are. Several ratios are available from 10:1 to 90:1 or even more for some models. The current drain at max load is is in the 90 - 200 mA range depending on model, but you don't need to consider max load in this project.
/RID

It should be powerful enough, put probably spins faster than 20RPM