Date:         Thu, 31 Jan 2002 17:34:03 -0500
Reply-To:     David Beierl <dbeierl@ATTGLOBAL.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Beierl <dbeierl@ATTGLOBAL.NET>
Subject:      Re: Red light rev
Comments: To: gary hradek <hradek@YAHOO.COM>
In-Reply-To:  <20020131000107.91036.qmail@web14703.mail.yahoo.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed

At 07:01 PM 1/30/2002, gary hradek wrote:
>Andrew,
>       I may be wrong about this but I do not think
>there exist a permanent magnet "weak or strong" in the
>alternator.   Increasing the engine rpm turns on a
>electrical loop from the dash that provides power to
>the alternator thus making the electromagnetic force
>needed to make the alternator charge.   The bearded
>guy from back east can explain this better than I.
>The jolly guy from cape cod.   regards gary

Glad they're not talking about me (the cranky guy from Rhode Island). <g>

But FWIW, you're right.  The old DC generators had a residual core
magnetism that was necessary to remind them which direction to charge in,
but alternators (except the "self-exciting" type) rely on an input current
on the D+ terminal to energize the field enough to begin charging.  The rpm
at which this happens appears to be dictated by the amount of current you
supply to the D+ terminal, other things being equal.  So it appears to be a
deliberate design choice by VW when they specified the bypass resistance
that parallels the LED in the panel.  Hanging relays on that line ought to
raise the required rpm by diverting some of the current intended for the
field (though probably not by much).

Ref. Bosch Automotive Handbook p. 809ff.

david


--
David Beierl - Providence, RI
  http://pws.prserv.net/synergy/Vanagon/
'84 Westy "Dutiful Passage"
'85 GL "Poor Relation"