Alternator drain

[John Hess]

----------
From:  cloud at peaches.ph.utexas.edu[SMTP:cloud at peaches.ph.utexas.edu]
Sent:  Wednesday, April 02, 1997 10:41 AM
To:  diy_efi at coulomb.eng.ohio-state.edu
Subject:  Re: Alternator drain

>>  (I think) the alternator is the opposite: the *field* magnetism
>>  is produced in the spinning rotor and the output cometh from
>>  the stator (???).
>>
>>  At any rate, 'tis not true that either have permanent magnets.
>
>Its been so long since I've studied electricity, but I think the 
generator
>DOES have magnets, otherwise how would it power the lights and the
>accessories I have attached to it in my Humvee.  The generator, 
alternator,
>and everything for that matter share a common ground (vehicle frame), 
however
>the generator has no battery whatsoever in the loop.  Maybe I chose 
the wrong
>word (Generator), but its an isolated, batteryless circuit.  So the
>electricity comes from somewhere :).  I had assumed magnets that the
>rotor/stator rotates within its magnetic field.
>
>Fred

There can obviously be many variants.  Speaking in general
of U.S. vehicles made in the last 50 years:

 - the magnetism required to operate the alternator or
generator comes from current passing through wires/windings
in the device.  (Could use a permanent magnet -- I know of
no commercially available, automotive unit that does).

 - the output of either the alternator or the generator is
pulsating DC (i.e. it goes from 0 to some voltage of always
the same polarity).  As another writer points out, the
reason the alternator's output is DC is due to the fact that
there are rectifiers inside.  I didn't want to get that
technical (in fact, the reason the rectifiers are called
a "diode trio" is due to the fact that there is a 3-phase
AC signal to be rectified).

Why does your circuit work the way you describe?   I don't
know.  My first thought is that the circuit is not the way
you describe.  I suppose it's possible for some residual
magnetism to exist in the generator that would ("could ??")
eventually start itself once it was spinning.  I can tell
you from personal experience trying to do just that several
decades ago that it doesn't work -- least not with forties
through sixties Ford, GM or Chrysler generators.  (I
even checked out my '52 Studebaker Champion once too.)
If you wanted to pick up an old wreck (I bought a '48
Plymouth for $35), you took a battery to get it going
and then hoped it didn't die on you as you went home.
The generator can supply it's own field joltage once
it's going.

Tom Cloud <cloud at peaches.ph.utexas.edu>

As some of you might surmise from my other posts, I date far enough 
back to remember when ALL cars had generators (no alternators).  The 
fields in the generator, being energized under pretty much steady 
state DC conditions became (although weak) permanent magnets.  Turn a 
generator, and you get DC, whether you have the battery hooked up or 
not.  If you have a battery hooked up, the field is stronger, 
permitting greater amounts of current to drawn.