Warren Allen wrote:
>
> Fellow Scions,
>
> The recent cooling fan/generator/alternator thread brought
> back a question that I have wondered about for years: Does
> increasing electrical load really result in increased load
> on a generator/alternator, and therefore in increased gas
> mileage? Please hear me out before you answer, not because
> I'm right, but because I want a specific question answered.
> My argument is strictly intuitive, as I have no formal
> electrical education. Here goes: A generator consists of an
> armature (a metal shaft covered with wires) rotating between
> two magnets. There's no physical contact (other than
> brushes). How, therefore, can increased load make the
> armature harder to rotate? My intuition has always told me
> that a certain amount of electricity will be produced by
> this arrangement, and any excess beyond what's needed is
> (forgive me, this is all intuitive) bled off somehow, by
> sending it to ground..
>
> Related question: When I have my air conditioning on in the
> car, and I turn the temperature dial towards warmer, does
> the A/C compressor therefore start working less hard, and my
> gas mileage goes up, or does the compressor work just as
> hard, but the cool air get mixed with warm uncooled air in
> order to warm the air coming out of the vent?
>
> I somebody can explain this in simple terms, I would
> appreciate it. You will have cleared up one of the mysteries
> of the universe for me..
>
> Warren Allen
> 1960 TR3A (And no, it doesn't have A/C. That question
> concerns a BAC (Big American Car.)
Warren, it's been more than 30 years since I took motors and generators
in my college days but I'll give this a try in terms that even I can
understand. Duh!
First of all you don't get something for nothing. If you remember,
energy can neither be created nor destoryed. To drive more electrical
load such as the stereo, headlights, fog lamps, etc. you have to supply
more energy from somewhere. The generator changes the energy supplied by
the engine (mechanical) into electical energy based on the demand of the
load. The demand is sensed by the regulator. It attempts to keep the
voltage at a value above the battery (13.2V). It does this by adding
current to the field coil of the generator. When more current flows in
the field coil, a stronger magnetic force is generated. The rotating
part is trying to spin through this "force" field. (Star Trek stuff
here!) The more current in the field coil, the stronger the magnetic
"lines of flux", and the more force it takes to move the rotor. The
engine has to work harder to turn the generator and this takes gasoline.
No current is "wasted to ground" when there is no load. The field
current is regulated to a very small amount and the generator uses very
little energy to spin. Less gasoline is used so you save money.
Your air conditioning is a different kettle of fish though. When it's
on, it also uses energy because a pump (compressor) has to raise the
pressure of the refrigerant. When the temperature reaches a preset
point, a clutch turns off the pump at the pulley. Either the pump is on
or off, unlike the generator/regulator which varies levels in step with
the degree of load.
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