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.
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