From Wikipedia.org
I ran across some cool pages while studying about AC vs. DC motors. I was interested in taking a small DC motor, and connecting a propeller to it. I want to stick the motor on the outside of my car, and generate a small DC voltage to power my toy, ie. an iPod, or my cell phone. I need 5v for the task. The question I have been having is if you take a DC motor, and spin it, will you be producing a DC voltage, or an AC voltage. The answer it seems is that you will produce a DC voltage. It is kind of complicated, but is very interesting. The key to understanding a DC motor vs. an AC motor is to see how they are wired. A DC motor has a commutator.

NNC gives a good definition of a commutator.
 

Here is a good site that explains a DC motor with its commutator.



You can see in this above image a diagram for a simple DC motor. It consists of a stationary magnet, a loop of wire, and the commutator. When you pass the current through the loop of wire, you cause a magnetic field to form in the wire. The loop of wires will allign up with the field of the stationary magnet. Now what the commutator does is it flips the field every half rotation. This causes the loop to continually spin.

To see a DC electric motor in motion go here.

Now if you take this simple DC electric motor with one coil, and spin it yourself, then you create a DC generator. As you spin the loop, it will cause a current in the wire, and the commutator will keep the current always positive. It won't be an exact DC current though. It is more like a Sin wave with all of the negative waves flipped positive. If you connected a capacitor to this output, you could smooth out the ripples and have a good DC voltage. Now if you add more loops of wire, you could make more ripples, and you would have a smoother DC output.


Now an AC motor is similar to a DC motor, but it does not have a commutator. Just as the commutator flips the direction of the current through the loop, if you have an AC current directly through the loop, you will be changing the direction of the current as the current goes positive or negative.

Below you can see a diagram of an AC motor. This is called a synchronous motor, and more about this picture can be found here.


 

Now if you turn this AC motor, you will get an AC output. For a visual aid, Sciencejoywagon has a nice flash animation that lets you visualize an AC or a DC generator. Just change it to with a commutator for an example of a DC generator, and change it to without a commutator for an AC generator.



Now there are some simple dc motors that you can build yourself to test out these new concepts. Go here to see for yourself.