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
| |