These alternators do not need a voltage to create the magnetic field - automotive ones do.

http://www.misterfixit.com/alterntr.htm 
Alternators use an "exciter" voltage to get the alternator working when you start your car.  Now get this!  About 90% of the cars made today run that 12 volts through the "battery" or "alternator" bulb (AKA the idiot light).  So you need to check to see if this bulb is not burned out.  It should light when the key is turned on!  If it doesn't then there is a very good chance that the alternator will not put out!!  Replace the bulb before beginning the rest of the diagnosis.

http://en.wikipedia.org/wiki/Alternator 
Automotive alternators invariably use a rotor winding, which allows control of the alternator generated voltage by varying the current in the rotor field winding. Permanent magnet machines avoid the loss due to magnetizing current in the rotor, but are restricted in size, owing to the cost of the magnet material. Since the permanent magnet field is constant, the terminal voltage varies directly with the speed of the generator.


>>> mailinglist4@farcite.net 21 May 08 15:42:58 >>>
On Wed, 2008-05-21 at 13:30 -0600, Cedric Chang wrote:
> Sorry if this has been asked.  Are these alternators more effective  
> than an automobile alternator ?
> cc

I was wondering the same thing.

The typical automotive alternator pumps out around 50amps at 12V, pretty
close to the 500W mentioned.

Heavier models can pump out 100amps or more.

Obviously they are not optimized for size/weight, and they also drop off
in output quite a bit at lower speeds.

TTYL
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