Re: Alternators

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

In a brilliant stroke of genius, John Hofstetter <hofs@domain.elided>
blurted out:

>>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.

>Great post, John! I knew it was happening, in fact it was so bad in the 
>100 amp alternator that I had that I got rid of it, but I certainly 
>didn't know why. Not that's it relevant to much of anything, the 65 amp 
>alternator that I have now is a pretty good compromise for me. It starts 
>charging early, sparing me the anxious seconds on whether it was going to 
>charge or not, yet is capable of keeping all my accessories running 
>except for my winch. The 100 amp would be nice in the once-every-5-years 
>that I use my winch. 

Another interesting phenomenon I've observed in my Traveler's alternator
had me digging in the book again (that automotive electrical book I told
you about from Bosch) for an explanation.  At idle and when in drive with a
moderate to heavy current draw, my ammeter shows a fluctuating discharge.
The needle fluctuates more or less with the load (like for example if a
turn signal is on) and the rpm variations of the engine.  This appears to
be in every way normal, as the total vehicle electrical load exceeds the
output of the alternator at this low engine speed.

When I smoothly increase the engine rpm to a faster speed, the ammeter
shows an immediate and rapid increase to the charge side of the gauge.  As
the needle swings up well into the charge region of the gauge, it almost as
suddenly drops about half the way back from its position between neutral
and charge (still in the charge region of the gauge).  In this position,
the needle smoothly and slowly drops toward neutral as the battery becomes
fully charged.

After studying the book, it seems this phenomenon has to do with the way
the voltage regulator controls the alternator output.  The voltage
regulator actually controls the flow of self-excitation current to the
rotating electromagnet (the rotor).  The voltage regulator basically works
by very rapidly switching the self-excitation current on and off (on the
order of 1 to 3 millisecond cycles) to yield a mean current flow.  This
keeps the alternator output balanced between acceptable limits needed to
charge the battery and not damage electrical systems.  Just an aside... if
there were no voltage regulator, a modern alternator could easily hit 140
volts!  Ouch.

Anyway... back to the point... At very slow alternator speeds and/or
current loads, the "on" switching portion of the voltage regulator cycle
becomes longer and longer, until below some speed and above some current
load, the switch stays wide open.  The flow of current to the
self-excitation circuit then remains wide open as the alternator struggles
to keep up.  The alternator output at that point becomes unsteady and
fluctuates with the alternator (engine) speed and current load.  This is
because its output essentially unregulated and totally dependent on factors
outside the control of the regulator.

When the alternator rpm is then suddenly increased, the alternator output
just as quickly shoots rapidly upward.  The voltage regulator, which was
holding the self-excitation circuit wide open, has to suddenly catch up and
begin switching the current on an off properly.  It's the apparent lag in
the regulator's response time to the rapid output change that causes the
ammeter needle to swing quickly to charge and then be "caught" and brought
back to a controlled limit.

Pretty cool huh?

John L.
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