DANMAS@aol.com writes:
>In a message dated 97-11-01 14:01:02 EST, jruwaldt@indiana.edu writes:
>> Isn't the terminology technically that "dynamos" generate DC and
>> "alternators" generate AC? Having understood that, do the
"alternators"
>> in our cars generate AC or DC? What you're describing, Dan, sounds to
me
>> like the operation of a DC generator. In a DC generator, as in a DC
>> motor, there are two bent pieces of copper separated by a narrow
portion
>> of the plastic of the rotor. The brushes are directly opposed to each
>> other, so that each is always contacting the opposite plate or neither.
>> Since each brush is always connected to the end of the armature passing
>> through the same field (for that brush), the current flowing through
that
>> brush always flows in the same direction.
>> In an AC generator, as in an AC motor, there are two complete rings.
Each
>> brush is connected to a different ring. Since the rings, and therefore
the
>> brushes, never trade the ends of the armature to which they're
connected
>> while the ends of the armature move from one magnet to the next, the
>> current reverses direction.
>> So, what you're describing, Dan, doesn't sound like conversion to DC,
but
>> the actual generation of DC. Can anyone explain whether car
"alternators"
>> generate DC or rectified AC?
>Jim:
>Generators and alternators both *SUPPLY* DC voltage. They BOTH do this by
>*GENERATING* an AC voltage, and then rectifying this AC voltage,
internally,
>to create a DC voltage at the output terminals. Alternators use diodes to
do
>the rectification, while generators use the segmented slip rings for the
>conversion. The operation you described above for the generator is in fact
>rectification. Fundamentaly, no different than diode rectification.
>When you think about it, you will see that there is no practical way to
>produce DC directly with a rotating machine - it just can't be done. As
the
>machine rotates, first the north pole, and then the south pole, passes by
any
>given point. The polarity of the current generated by the north pole is
>opposite of that produced by the south pole. As long as the poles
alternate,
>AC will always be produced. If the machine doesn't rotate, no electricity
is
>produced at all.
>The difference between alternators and generators is in the details, not
in
>the concepts, as they both operate on PRECISELY the same concept. By using
>diodes instead of segmented slip rings, alternators can be made much more
>powerful, smaller, and more efficient than generators. Segmented slip
rings
>are basically mechanical switches, subject to wear, arcing and pitting,
just
>as any other mechanical switch. This places a severe limit on the power
that
>can be transfered through the "switches" while trying to maintain a
>reasonable size.
Please allow me to shed a little light on the subject.
The term "Dynamo" is a colloquialism that is more common in England but
is also used stateside. The term "alternator" is a composite of the words
"alternating" and "generator."
Dan's description of how a generator works is quite good. Voltage is
generated when a wire passes through a magnetic field and the voltage
changes polarity which requires a commutator to reverse the connections
to the wire to maintain the same polarity at the output.
One major difference between a traditional generator and an alternator
are the use of semiconductor diodes to rectify the voltage in lieu of the
brush/commutator set up. But the differences don't stop there. As you
may have noticed half of the system is stationary and the other half
rotates. It is necessary to get power to and from the rotating portion
and this is typically done using brushes and slip rings. The generator
uses brushes and a commutator as this coordinates the polarity switching
with the orientation of the armature coil and the field pole (the magnetic
field source).
An alternator is turned inside out. Instead of rotating the armature
within stationary field as is done in a generator, the field is rotated
within a stationary coil of wires. One advantage is the main power
output can be directly connected instead of passing through brushes.
The other advantage is that although there still are brushes, these
brushes only carry the field current which is much less than the generator
output. Consequently, the brushes outlast the car. Of course the
voltage from the stator (output) is now AC this is easily rectified
now that someone has invented solid state rectifiers and silicon diodes can
switch at much higher speeds and do not wear out. They sometimes burn out
but that is another story.
Q: Why does an alternator produce so much more power at idle than a
generator?
A: Because it turns faster. Look at the size of the pulleys. The
pulley on an alternator is significantly smaller than the one
on a generator. And that means that the alternator is running
faster at all times, even at idle.
Q: Why don't they run generators faster?
A: Because the brushes would wear out faster. Remember, the brushes
disconnect and reconnect the armature windings. That means the brushes
are switching the full output current on and off several times per
revolution. So alternators work better because they run faster and
they run faster because they can!
I try to keep it brief. Honest I do.
Dave Massey
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