Of Generators and Alternators

[jadney@vwtype3.org (Melissa & Jim)]

>From: Toby Erkson <Toby_Erkson@ccm2.hf.intel.com>

>I some solid answers (i.e. no guesses, please) on the rpm ratio between the 
>generator and the crank.  I also would like to know at what rpm does the 
>generator begin to produce current.  I believe at an idle the stock 
>generator isn't rotating fast enough to charge the system -- is this true?

Type III pulley ratio is 2.3:1.  Type I/II ratio is 1.9:1.

There are 4 regions of operation for any generator system:

1)  Low Speed.  The generator light is on and the generator is neither a 
source nor a drain for current.
2)  Medium Speed.  The generator light goes out, but the speed is still too 
low to pump charge into the electrical system.  The generator is a drain on 
the electrical system.
3)  High Speed.  The generator light is out, and the generator voltage is 
higher than the battery voltage.  The generator is a source of electrical power.
4)  Very High Speed.  The generator starts to lose efficiency due to a 
magnetic field phase shift in the armature.  This means that the brushes are 
no longer in the best place to pick up the generator output.  Output declines.

I'm not sure at what RPMs the transitions occur.  It is clear that idle is 
above 1), possibly above 2).  Note that type IIIs are better off due to 
their increased pulley ratio.  4) comes on only at very high RPM, but again 
I'm not sure where that is.

It used to be the habit among shade tree mechanics to set the idle up just 
enough to make the generator light go out.  This is really the wrong thing 
to do because this is the point where the backward current flow in the 
generator, and the drain on the system, is at a maximum.

SHAMELESS PLUG:  2) won't exist with my solid state regulator.

>I talked to a custom alternator/starter shop and the owner was very helpful 
>in educating me on the differences between the alternator and generator, 
>beyond the obvious.  I didn't know that if a generator isn't rotating fast 
>enough that it will stop producing current and it could even turn into a 
>motor, thus *using* current from the battery instead of charging it (so 
>keep that idle up)!

You can drive any generator as a motor if you connect it up right.  On a VW 
generator just connect the + lead to the D+ term and the - lead to the DF 
AND the case.  Unfortunately, it will turn in the opposite direction that 
you would like it to.  This is both a good way to make sure the generator is 
properly polarized and a way to check it out:  if you hold it in your hand 
while it's running, a good one will run smoothly while a bad one will 
"twitch" once per revolution as it encounters the shorted or open turn in 
the armature.  You can do this with a battery and jumper cables or with a 
battery charger.

Jim
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       Melissa Kepner                                    Jim Adney
       jadney@vwtype3.org              jadney@vwtype3.org
                             Laura Kepner-Adney
                             Madison, Wisconsin
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