In a message dated 98-08-07 01:28:57 EDT, jmwagner@greenheart.com writes:
> HOW ON EARTH did the stock generator handle all that wattage?
Justin,
Theoretically, there is no limit to how much current a generator can be made
to produce. They can be, and have been, built large enough to power an entire
city. From a practical standpoint, though, it is hard to build one with enough
power for what you mentioned and still be small enough to place in a car.
Actually, there is no real difference in the production of electricity in a
generator and an alternator. In a generator, the field windings are fixed and
the output windings rotate, while the converse is true in an alternator. The
other differences between the two are as a result of that one change, and, as
a matter of fact, are what made that one change possible.
A little explaination is probably in order.
In order to produce electricity, one of the windings MUST rotate with respect
to the other. Getting current to/from a fixed winding is no problem - just
solder the wires to it. The rotating winding is another problem. How do you
connect wires to it, and still have it rotate? The answer is slip rings and
brushes.
Using slip rings and brushes created a problem and, at the same time, provided
a solution to another problem. The problem created was one of capacity - slip
rings and brushes had to be very large to handle any significant current. The
practical limit for an automobile application was around 20 - 25 amps. To get
around this limitation, the field windings could have been made to rotate,
while the output windings remained stationary, as in an alternator. This way,
only the low current used in the field winding would have to go through the
slip rings, and capacity would not be of concern. They could have, except for
one thing!
The voltage produced by rotating one winding with respect to another is AC
voltage. Always. Whether it is an alternator or a generator. Converting AC to
DC is a problem that the use of slip rings solved quite nicely. By segmenting
the slip rings, the AC voltage was converted into DC voltage, without the use
of external circuitry. Rectifiers, the fore-runners of diodes, were quite
large and bulky back then - too large and fragile to use in an automobile.
Since something had to be used to convert AC to DC, and segmented slip rings
were about the only practical means of doing it, the output windings, rather
than the field windings, wound up with them, and with the current limitations
imposed by them.
With the advent of solid state technology, it became possible to produce
diodes that were small enough, and sturdy enough, to be mounted in the body of
the generator itself. Now, it became possible to install the output windings
stationary, wiring them directly, and to use slip rings for the field
windings, which now rotated.
Diodes also have limits as to the amount of current they can carry and still
be small enough to go inside the case, but their limit is much higher than the
limits of the slip rings. Also, by rotating the smaller and lighter field
windings, the output windings can be made much larger, without the limits
imposed by the rotational forces previousy imposed.
As a result of these changes, generators now are capable of much higher
currents, while being even smaller than the low powered models of before. No,
that was not a typo - I really meant to say "generator" rather than
"alternator." The alternator in a modern car is in no fundamental way any
different than the old generators.
The term "alternator" was coined as nothing more than marketing buzz,
deliberately intended to persuade people to fork out money for a new car, when
the old one was still plenty good enough. When Dad wanted to buy a new car,
over the objections of Mom, one of the "persuasions" he could use was: "but,
honey, this new model has an alternator, instead of an old fashioned
generator, and..... safety...... reliability......and so on." Since neither
Mom nor Dad knew the difference, it was an easy sell.
So, to answer your original question (bet you thought I'd never get there,
didn't you?), the builders of rally cars could use a larger generator, and/or
put up with the hassles of changing brushes more often (or the entire
generator), just to be a little more competitive. That's just a guess on my
part, though, as I haven't actually seen anything in writing on this, nor have
I actually seen one of the generators used. For most normal automotive
applications, the current limitations of the older generators corresponded
quite nicely with the electrical demands of the cars of that time, so there
was really no great incentive to make them any larger or more powerful for use
by the general public, although they certainly could have been.
OK, I'll shut up now.
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 - slated for a V8 soon
'68 MGBGT---organ donor for the '74
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