"John Larkin" wrote in message
...
On Tue, 29 Nov 2011 16:06:51 -0700, Jim Thompson
wrote:

On Tue, 29 Nov 2011 16:50:02 -0600, John Fields
wrote:

On Mon, 28 Nov 2011 16:51:45 -0800, John Larkin
wrote:

On Sun, 27 Nov 2011 12:06:24 -0700, Jim Thompson
wrote:

On Sun, 27 Nov 2011 10:50:39 -0800, Fred Abse
wrote:

On Sun, 27 Nov 2011 10:28:17 -0800, John Larkin wrote:

Alternators like this are often designed with a lot of internal
series
inductance. As speed increases, open-circuit terminal voltage
increases, but the series inductance impedance increases too,
tending
to make them constant-current sources, ideal for driving light bulbs
at sorta constant brightness at various speeds.

What I was going to say.

I think there is an ancient patent somewhere about just that.

Well! Alternators _are_ current sources,

Ah, so their terminal voltage must be infinite.

John

---
And, indeed, it would be, would not Mother Nature conspire against it
with the spoiler, resistance.

When Larkin sets out to put his foot in it he just can't resist
squishing his foot around to make sure it sticks.

Suggestion to Larkin: Set up loose connection at battery post...
connects but you can pull connector off easily.

With engine running at 3000RPM (you'll need a helper, maybe Ian Field
:-), right hand on car frame, left hand at battery post, take firm
hold of connector and pull quickly off of battery.

Unless you happen to have an alternator with built-in zeners to limit
voltage, you will be dead... or at least multiple broken bones... it's
a pulse. You do know what a "pulse" is? Then come back and talk to
me about "infinite" ;-)

Pulse? There will be an initial ringing spike, then high voltage AC
for as long as the field holds up... forever for a PM alternator like
the one discussed here. "Pulse" is a cartoon of what the waveform will
be.

But it's not a current source.


Young bucks... DON'T try this... this instruction for Larkin-disposal
purposes only!

(Before zeners were added to alternators I had to design to a 400V
kick, and have the regulator survive. How did I do it?)

...Jim Thompson

As I said, it's an AC voltage in series with an inductance. The
current into a low-impedance load is relatively constant as speed
varies above some threshold. But that doesn't make it a current
source; at any speed and excitation, it has a finite impedance and a
finite open-circuit voltage.


I remember as a kid I borrowed a neighbour's (electrician) multimeter
intending to measure the current output of a Sturmey Archer hub dynamo
(6V/0.3A) and bent the pointer on the 10A range.

Regarding motorcycle alternators (PM type) I'm reasonably familiar with the
Honda one's - output rises steadily until it levels out at 5000rpm - pretty
sure thats down to inductance.