Why 28V DC?

On Sun, 18 Jan 2004 19:20:48 -0500, "Ron Natalie"
wrote:

"Jim Weir" wrote in message news
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...
\
You think that AC coming out of those inverters is all that stable? It tends to wander
with the input voltage. Of course the real problem is that if the alternator crumps, the
battery will not put out three phase.

The voltage is pretty stable, and is a square wave clipped by the
battery voltage ... at least while charging. I think the regulator
shuts it off when not, but am not sure. Years ago, when construction
of auto alternators was less optimized, I helped a freind rig his full
kW ham rig ... we took the alternator output into a 3 phase
transformer to get plate voltage. MUCH better than the vibrators of
the day. And we needed very little filtration with the 3 rectified
phase.

FYI, one, and perhaps the main, advantage of 3 phase rotating
machinery is that in theory, the developed torque of a motor, and load
torque of a generator is constant. Only manufacturing tolerances and
real conductors and iron give ripple.

"GeorgeB" wrote in message ...
On Sun, 18 Jan 2004 19:20:48 -0500, "Ron Natalie"
wrote:

"Jim Weir" wrote in message news
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...
\
You think that AC coming out of those inverters is all that stable? It tends to wander
with the input voltage. Of course the real problem is that if the alternator crumps, the
battery will not put out three phase.

The voltage is pretty stable

I was referring to the freqeuncy (in response to Jim's comment about the engine RPM's).
You can actually here the whine of the inverter change pitch with the change of input voltage.

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

And more easily filtered when rectified for DC.
--
Gene Seibel
Hangar 131 - http://pad39a.com/gene/plane.html
Because I fly, I envy no one.





"S Narayan" wrote in message ...

If memory serves, 3-phase power is more efficiently distributed, if all 3
phases have equal loads (either as a star or delta connected network), there
is no current in the return path (ground). That is, sum of all phases is 0.
So you can save one conductor for the same power transmitted. The generation
of 3 phase power is also easy and I believe it may also be more efficient in
terms of the generator design. The 400Hz transformer, compared to a 50/60Hz
one, requires less "iron" for the same flux generation (or less turns) since
the mutual inductance is proportional to frequency. Hence they are lighter.
However, they may have more losses due to eddy currents etc.