Why 28V DC?

You used to be able to buy a "converter" for your alternator-equipped
car to turn it into a 110-volt DC supply. Good for running lights,
heaters, and series-wound motors such as those found in electric
drills, skilsaws and the like. No good for induction motors or
anything with a transformer (electronic stuff). I boought one of these
years ago, paid a handsome price for this "advanced" technology, then
took it apart to see what was in the box before I installed it.
It was the box, a household 110V duplex receptacle, a DPDT toggle
switch, an NE-2H bulb and 22K (might have been 27K) resistor. Total
value not more than eight or ten bucks, today's prices. The box would
be the most expensive item.
One pole of the switch switched the alternator field from the
regulator directly to the battery, and the other switched the
alternator output from the battery to the receptacle. A manual
throttle cable (not included) was used to set engine RPM to a value
that would fire the NE-2H, being about 110V with that resistor, and
you went to work. The battery would drain slowly as the field was
consuming about 5 or 6 amps, and you'd have to flip the switch once in
a while to recharge it. It didn't hurt the alternator, as it takes
amps, not volts, to burn it out. The diodes would suffer first, from
the voltage, I think. You'd have a hard time finding a tool that used
60 amps.
It was a handy doodad for anyone without power at the field. I
don't know if they're still made. It wouldn't work with today's
alternators with their built-in regulators.

Dan


Jim Weir wrote in message . ..
Jeez...that's something I've never considered. You are RIGHT, internal to that
alternator there IS 3-phase AC that is regulated and rectified to produce the
DC...

And now with three small transformers and a little surgery on the alternator, we
could have 3-phase 115VAC to run those military gyros...I wonder...yeah, I know
the frequency is a function of engine RPM...but still...

Jim


(Dan Thomas)
shared these priceless pearls of wisdom:

- That's probably why the alternators in your car and airplane both
-are three-phase. A 60-amp alternator weighs less than an old 25-amp
-generator.
-
- Dan

Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com