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Tom,
You are right, of coarse, about the alternator field being
either on or off.
I may have implied that the field current was a linear function but
I was only trying to
simplify the explanation.
However, by switching the field on and off at a fairly fast
rate and varying the pulse
width duration of the field current, a linear approximation can be
achieved with a constant voltage as the result, within the limits of the
system of course.
That is in essence how the voltage regulator achieves its
function.
As for your "simple test" it may not give the expected results, as
most modern car engines, those that are controlled by computers, will
have the idle speed determined by the computer as well.
So increasing the load at idle, either mechanical or electrical,
will cause the
computer to sense a decrease in RPM and if it goes below a
predetermined value the computer will open the throttle (or idle speed
valve) to increase the RPM to the correct idle speed.
Jim
--- On Sun, 3/22/09, thomas walter
<roundrocktom@yahoo.com> wrote:
From:
thomas walter <roundrocktom@yahoo.com>
Subject: [FlyRotary]
Re: Alternator (Off topic)
To: "Rotary motors in aircraft"
<flyrotary@lancaironline.net>
Date: Sunday, March 22, 2009,
10:03 AM
Jim,
The field is either on or off.
The regulator would create too much heat with a linear
function, so solution is just a 'switch' function in the
regulator IC.
Battery does the trick in smoothing the
voltage.
George -- simple test is with the engine idling,
kick on the lights/fan... note the rpm drop.
That is the load
of the alternator turning on. Newer vehicles will have
additional inputs to the alternator so it knows when the AC is
on, or when you kick WOT (wide open throttle) as it will
disconnect the alternator and A/C off for a brief amount of
time. Also a simple battery test is to (in the
car) kick on all loads at idle.... if the engine dies, battery
reserve isn't there and time to replace 'em.
Tom
1) Re: Alternator (Off
topic) by James Maher < delta11xd@att.net>
George,
I'm sorry to disagree with you but you are quite mistaken
in your belief about the
alternator function.
The alternator output is mostly a function of load
because of the voltage regulator,
whose function is to keep the voltage constant with
changes in load and RPM.
It performs this function by varying the field current in
the alternator.
If fact this is one of the many advantages of the
alternator over a generator.
The alternator's voltage output is also more or less
constant regardless of engine RPM, also due to the regulator's
function.
You said "The alternator charges the battery - which in
turn runs the accessories".
This is not exactly true. If the battery voltage is below
that of the alternator output
then yes the alternator will function to charge the
battery, however if the battery is fully charged then the
alternator will not be charging the battery but will provide
the necessary power to drive any other loads connected to the
system.
So reducing the electrical load on the system will indeed
reduce the mechanical load on
the engine thereby using less overall energy.
Hope this clarifies your misconception.
Don't feel bad, many people do not fully understand even
the most simple of electrical devices and the
alternator/regulator charging system is fairly
complex. Jim
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