Re: Alternator Light

["Bill Collins" <photo@intrex.net>]

The initial excitation comes not only from the permanent magnet, but also from current flowing through the warning light.  On many VWs, this light is an LED, which allows much less current to flow than a light bulb does.  I assume this is part of the reason this seems to be a VW only problem.

Bill

>I'll go into 'the details' for marc (p.s. marc, you know all this stuff 
>already). 
> 
>An alternator has rotating winding (armature), as well as stationary 
>windings (stator). 
>     Current goes through the armature winding (via the rings/brushes many= 
of 
>you have seen), making it one big rotating magnet. This magnetic field is 
>felt by the stator windings, causing them to produce an AC output. This AC 
>is then rectified (converted to DC) and sent out to your battery. The 
>regulator 'senses' the DC output voltage and varies the current to the 
>armature, varying the intinsity of the field, thus varying the output 
>voltage. You may be wondering 'what the heck does the regulator part have= 
to 
>do with the light staying on till i rev the engine*&#$^@', here's your 
>answer: 
>     Picture the car off, alternator is not spinning, nothing 'generated'= 
by 
>the stator, since the armature is not rotating. The reverse is true here:= 
no 
>generated voltage, regulator has no voltage to make the armature current 
>from (dont go thinking it has battery voltage, the battery cannot supply 
>current TO the alternator, since it would be going backwards through the 
>rectifier (just go with me on this, it doesnt work that way)). Compared to= 
a 
>gas grille, the gas is off and there is no flame. 
>     Now, you start your car. Belts start spinning the armature. Wait a= 
sec, 
>there is no armature current, and no stator voltage to make it. So now,= 
"The 
>gas is on, but there is no flame". What the heck%*&@)%. How does this thing 
>ever get going you might wonder??? 
>     Answer: There is a small magnet built into the center of the armature 
>winding. It supplies the initial field, just enough for the stator to 
>generate some voltage (our "spark"), just enough for the regulator to send 
>current to the armature and 'jack up' the output voltage to the 14VDC we're 
>all happy to see from a working alternator. The whole process is called 
>'flashing the field', which is why i related it all to a gas grille. 
>     Question #2: Why does my alternator do this flashing light thing now, 
>when it didnt do it when the car was new? 
>     Answer #2: Several possibilities: 
>         1. Magnet is getting weak: rust, heat cycles (remember, from high 
>school, heh), vibration. 
>         2. Rectifier diode(s) are open: the stator on our alternators are= 
'3 
>phase AC generators' to rectify this type of AC down to single phase DC,= 
you 
>need a group of 6 diodes. 3 phase systems are high reliability (the entire 
>power distribution system on my submarine was 3 phase), portions of them= 
can 
>totally not work, but there will still be at least some output. 1 or 2 of 
>these diodes can be broken causing the DC output to be weakened. While this 
>may be ok once the field has flashed, it will require a higher rotating 
>speed to get it flashed to begin with. 
>         3. Stator field winding open: one of the 2 windings could have an 
>open wire. The effect is the same as open diode(s) associated with that 
>field. 
>     You can 'knock out' one of the 3 phases and still get nearly 50% power 
>output. Thank VW for overengineering everything, since your scirocco can 
>probably still run just fine on the 3 phases. 
>Hope this info helps everyone. 
>Al 
>

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