Date: Tue, 22 Feb 2000 21:00:33 -0500
Reply-To: David Beierl <dbeierl@IBM.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@IBM.NET>
Subject: Re: Alternator not charging/ solenoid meltdown
In-Reply-To: <qyazOM8JIl=EiFn+p08Vk3ozrfDB@4ax.com>
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At 07:43 PM 2/22/2000 , Steve Green wrote:
>David, I think you are confusing the Vanagon alternator D+ terminal with
>the field winding terminal used on some other alternators. D+ is an
>output from the alternator, not an input to it. It runs the idiot light
>and the fridge relay. My van's alternator charges normally with the D+
>wire unplugged.
Steve, on the '80-'84s there is a 150 ohm, half-watt resistor paralleling
the idiot light LED and resistor. This is labeled "series resistor for
alternator pre-exciter circuit" in Bentley, and allows some large fraction
of 80 ma (in addition to the 15 ma or so through the LED) to flow from the
15-circuit (ignition-switched power) to ground through the D+
terminal. This gives a very small magnetic field from the field windings,
but it's enough to start the alternator charging enough to supply its own
field current which makes more output which makes more field current until
output is very rapidly up to normal. (which drives the D+ terminal high
which makes the light go out, so it's both an input and an output at
different times). They could get the current from somewhere else, but
since they need the identical function to drive the idiot light it makes
sense not to duplicate it. And it would have to be somewhere else on the
15-circuit, otherwise the battery would slowly drain through the field
coils while the van was sitting.
If the iron core of the rotor (pretty sure the field winding would be on
the rotor, that way the slip rings and brushes don't have to carry heavy
current) happened to retain a little magnetism then it would probably start
up on its own, but the tickler current makes it a sure thing. And I'm
guessing that the core material is chosen for very low magnetic resistance,
which would mean that it would retain very little magnetism. Also, (I'm
trying to visualize the magnetic fields in the alternator while it's
spinning down after you kill the motor, and not succeeding) if the
spin-down phase exerts an alternating and constantly weakening field on the
rotor -- well, that's the recipe for a demagnetizer.
Back in the days of DC generators in cars I remember that after taking one
apart you had to "polarize" it by passing a heavy current through it
briefly, so that it would charge in the right direction, and maybe so that
it would start at all -- obviously those puppies retained some magnetism in
the pole pieces. Of course I was working on <gag> Lucas stuff, maybe the
Americans did it differently. Them was the days of mechanical voltage
regulators, too, big as your fist. Ecchh. Worse than ignition points.
Anyhow, according to the circuit diagram for the '89s or whatever year he
has, by then they had two resistors paralleling the light circuit instead
of one, no doubt to spread the heat load -- 80 ma through 150 ohms is just
about one watt. The field circuit has its own internal resistance, so the
actual current would be less than 80 ma, but I don't know how much less.
cheers
david
David Beierl - Providence, RI
http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"
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