In a message dated 97-08-18 15:46:48 EDT, Warren.Allen@infores.com writes:
> 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.
Warren:
I don't know of any way to explain it so that it is intuitive, as one might
explain a mechanical process. It may help to compare it to the operation of a
motor.
For comparison, think of a large motor driving a conveyor belt. Assume the
motor is connected to the belt by a large chain, similar to a bicycle chain.
Turn the motor on with the belt empty. The motor will hum along quietly, and
the chain will have a little slack in it. Now, start loading the belt with
packages. As you load the belt, you will notice the sound of the motor
changes, and the drive chain begins to tighten up as the motor works harder.
As you keep adding packages, the motor will begin to sound strained, and the
chain will became very taut, until enough load is added to trip the circuit
breaker. The circuit breaker tripped because the motor drew more and more
current to keep up with the load. The motor is constructed just the same as
the alternator - the only connection between the electrical input and the
mechanical output is the electrical/magnetic fields, just as in the
alternator. Just as the motor draws more current as it works harder, the
alternator requires more work input (HP) to produce more current.
As for your assumption that the excess current is bled off to ground, that is
a reasonable assumption - it's just wrong. The alternator generates just the
exact amount of current being drawn by the loads.
> 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.
That depends on the arrangement in your car. There are different ways of
controlling the temperature. In all cars that I am aware of, the air
conditioner is an on-off device. In general, without allowing for different
engine speeds, outside temperature, etc, when the A/C is on, it draws the
same power from the engine regardless of the temperature setting. If you turn
the control to a lower temperature, the ON time is reduced, compared to the
OFF time. As a result, the engine is spending power to drive the compressor
for shorter periods of time, and, therfore, uses less gas overall. While it
is driving the compressor, though, it uses the same amount of gas.
Conversley, if you turn the temperature control to a colder setting, the ON
time is increased, with a corresponding decrease in overall gas mileage.
In some cars, the only temperature control is the above mentioned ON/OFF
cycling. In others, turning the temperature control to a warmer setting
actually turns the heater on. As a result, you have both the A/C and the
heater going at the same time - not a very efficient set-up. In my Toyota
pickup, for example, the A/C control cycles the compressor, and a separate
control operates the heater. If I leave the heater control to the OFF
position, and use only the A/C control, after a while, I can make ice in the
cab. As much as I hate to do it, sometimes I have to use the heater control.
In some of the fancier cars, there is only one control, and you have no
option. Adjusting the temperature control varies the ratio of A/C to heater
operation, with the A/C control still being ON/OFF.
I hope this has been of some help. Electrical engineering is no more
difficult than mechanical engineering, but there is one significant
difference - If you observe a mechanical process for a while, you can watch
the gears rotate, the pistons move up and down, the valves open, etc, and get
a pretty good idea of how the thing works, without any outside help. Pick up
an electric circuit, however, and it doesn't do anything that you can see. It
pretty much looks the same at shutdown as it does in operation. It is almost
neccessary to have someone explain to you what is going on to understand it.
Dan Masters,
Alcoa, TN
'71 TR6---------3000mile/year driver, fully restored
'71 TR6---------undergoing full restoration and Ford 5.0 V8 insertion - see:
http://www.sky.net/~boballen/mg/Masters/
'74 MGBGT---3000mile/year driver, original condition
'68 MGBGT---organ donor for the '74
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