Strange Phenominon or Tommy's Alternator

[jlandry@domain.elided (John A. Landry)]

In a brilliant stroke of genius, tsm1@domain.elided (Tom Mandera) blurted out:

>When I start, the alt gauge doesn't move... I have to punch the throttle
>once, then the alt gauge swings to charge until it makes up for the starter
>effort.  I'm curious why it takes this throttle spike to "wake up" the
>alternator.. it'll then charge at idle *after* I hit the gas once.

Tom,

Last night when I got home, I looked up the technical reasons why an
alternator does this and I thought you (or maybe someone else... such John
H.) might enjoy hearing about it.

To begin with the basics, as almost everyone knows, electrical current is
produced in a conductor when it passes through the lines of force in a
magnetic field.   It makes no difference whether the magnet is moving or
the conductor is moving.

For the magnet part, instead of using permanent type magnets, an automotive
alternator uses an electromagnet.  This electromagnet is in the rotating
part of the alternator called the "rotor."  Electromagnets offer the
potential for much higher voltages than would plain permanent magnets
alone.  The electromagnet consists of wire windings around an iron core.
Into the windings is introduced a current called the "excitation current."
The number of windings and the amount of current flowing through the
windings controls the strength of the magnetic field and thus helps to
control the output of the alternator.  If the current to the electromagnet
is completely shut off, the electromagnet will loose it's magnetism...
except for a slight residual magnetism.  This is why the alternator needs
either an external (as from the battery) or internal excitation current to
maintain the magnetic field in the alternator.

For the conductor part of the power generating system, the alternator has a
stationary assembly of windings called the "stator."  The stator has three
separate sets of windings that each produce an alternating current with a
sine wave peaking 120 degrees out of phase from each other.  These peaks
are combined together via diodes to yield a pulsing DC (direct current)
output.

In order for the alternator to get started producing current, the magnetic
field in the rotor first must be "pre-excited" in some way.  The stock IH
configuration is for battery current to be supplied to the alternator
through a small resistance wire.  This is because any residual magnetism
that remains in the rotor upon startup, or even low engine speeds, is
insufficient to cause current to be generated in the stator.  After the
initial pre-excitation, a "self-excitation" current (tapped off the output
of the alternator itself) takes over and supplies all the necessary current
to build and maintain the magnetic field in the rotor.

A set of diodes feeds the self-excitation current to the rotor's
electromagnet.  The self-excitation circuit voltage must be greater than
the combined voltage drop across the diodes before it will supply current
to the rotor.  This is precisely why the pre-excitation current is
necessary... to overcome this voltage drop and basically kick-start the
alternator's charging system.  The alternator must reach a certain speed
before the self-excitation current is reliably established... usually
somewhere above curb idle.  This is the "cut-in" speed we were talking
about in a previous post.  This cut-in speed varies depending on various
design factors such as size and number of windings.  Once the
self-excitation current is firmly established, the excitor field is strong
enough so that the alternator produces power even at idle.  But drop below
a certain point, and the self-excitation drops out again and the
pre-excitation current has to take over again.

In Tom's case, with his higher output alternator, the point at which the
initial self-excitation takes over is obviously higher than his idle speed.
I've always understood this effect is intensified the higher the alternator
output capacity.  This makes sense, considering there are undoubtedly more
windings which take more current to build a sufficient magnetic field to
insure self-excitation.

Regards,

John
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