Re: Battery Light (+ good tech info) - techies MUST READ

[John Erickson <hombre@elp.rr.com>]

Damn twidget.

John
'87 16v
Former EW1 (SW)

At 08:30 PM 11/5/2001 -0500, Allyn wrote:
>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
>
>Allyn Malventano, ETC(SS), USN
>87 Rieger GTO Scirocco 16v (daily driver, 170k, rocco #6) running - for now
>87 Jetta 8v Wolfsburg 2dr (daily driver, 260k, 0 rattles, original clutch,
>driveshafts, wheels :)
>
>
>----- Original Message -----
>From: "Marc scirocco Québec" <marc_scirocco@yahoo.ca>
>To: <scirocco-l@scirocco.org>
>Sent: Monday, November 05, 2001 7:36 PM
>Subject: Re: Battery Light
>
>
> > This is normal on some alternators, depending on your
> > model.
> >
> > The alternator needs to get "excited" by some voltage
> > to start producing electricity. Without going into too
> > many details bla bla bla, it is normal for some of
> > them.
> >
> > Do not forget that even though it puts out DC Voltage,
> > it actually produce Alternative Current, which then
> > goes through the regulator who rectifies it (with
> > diodes), and regulates it so that it does not goes any
> > higher than 14V.
> >
> > Cheers.
> >
> > Marc
> > '83 Scirocco
> > '88 Scirocco Slegato
> >
> > --- Bill Collins <photo@intrex.net> a écrit : > This
> > comes up on the Vanagon list from time to time.
> > >  The consensus is that its just one of the things
> > > that makes VWs unique.
> > >
> > > I wouldn't worry about it as long as the light
> > > reliably goes out the first time you rev the engine.
> > >
> > > Bill
> > >
> > > > only going out after I've revved the engine past
> > > 3k.
> > > > Then it goes off and stays off.
> > >
> > >
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> > >
> >
> >
> >
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> >
>
>
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