Why 28V DC?

"Larry Fransson" wrote in message
news:2004011412081416807%lfransson@comcastnet...
On 2004-01-14 10:34:04 -0800, "Charlie" said:

Jim Weir wrote an interesting response to the subject, then added,
"Then
there is the 115v 3ph 400Hz. discussion..."

Let's hear it!

Possibly because AC is a more efficient distribution technique that DC
for
long runs in large aircraft ? And 400Hz instead of 60Hz allows for
smaller
transformers to step down the voltage ? Just guessing here.

All of the gyros in the plane I fly (Lear 35) are powered by 400 Hz AC.
The inverters aren't more than 30 feet away. There are two transformers
(one on each bus) to provide 26 volts for the oil pressure guages, RMIs,
nav radios, and a few other things.

Back in my navy nuclear power days, I knew the reason for three phases. I
think it has something to do with power density - smaller, lighter,
cheaper
is the AC mantra. Anyway.... 400 Hz provides higher power density and is
much cleaner than 60 Hz.

Am I on the right track?

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.

In article ,
S Narayan wrote:

If memory serves, 3-phase power is more efficiently distributed

I thought the advantage of 3 phase was that the peaks of the sine waves
were evenly distributed making it more efficient for running motors.
But this was in the context of home/shop power where the single phase
in a standard home is inefficient with big motors.

--
Ben Jackson

http://www.ben.com/

(Ben Jackson) writes:

If memory serves, 3-phase power is more efficiently distributed

I thought the advantage of 3 phase was that the peaks of the sine waves
were evenly distributed making it more efficient for running motors.

I feel like one of the blind men describing an elephant, but...

I thought three-phase was used to eliminate the need for the (existance
and maintenance of) starting windings, etc.

(It's been years since my Ag. Electricity course but I'm going to take
it again next semester...long story.)

--kyler

"Kyler Laird" wrote in message
...
(Ben Jackson) writes:

If memory serves, 3-phase power is more efficiently distributed

I thought the advantage of 3 phase was that the peaks of the sine waves
were evenly distributed making it more efficient for running motors.

I feel like one of the blind men describing an elephant, but...

I thought three-phase was used to eliminate the need for the (existance
and maintenance of) starting windings, etc.


Maybe a good side effect of 3 phase power, but that's not the main reason
for switching to 3 phase power.

"Ben Jackson" wrote in message
news:veiNb.66937$xy6.125476@attbi_s02...
In article ,
S Narayan wrote:

If memory serves, 3-phase power is more efficiently distributed

I thought the advantage of 3 phase was that the peaks of the sine waves
were evenly distributed making it more efficient for running motors.
But this was in the context of home/shop power where the single phase
in a standard home is inefficient with big motors.


Yes, I imagine that's another reason to use 3phase for high power motors,
where the torque is more uniformly available. I am probably getting out of
my league here but, separate windings carry different current phases which
translates to thinner copper for the same power and smoother operation, if
this makes sense. The same argument applies for a generator.

"Ben Jackson" wrote in message
news:veiNb.66937$xy6.125476@attbi_s02...
In article ,
S Narayan wrote:

If memory serves, 3-phase power is more efficiently distributed

I thought the advantage of 3 phase was that the peaks of the sine waves
were evenly distributed making it more efficient for running motors.
But this was in the context of home/shop power where the single phase
in a standard home is inefficient with big motors.

I believe it's actually the converse on an aircraft. It's the generator
that's big and heavy and by generating 3 phase, you can get more power per
pound of generator.

To make things more interesting, the industry is now going to 120V, variable
frequency AC power. There are two variants, IIRC they are 320 to 480 Hz and
280 to 620 Hz (or thereabouts.)

Why? Because to get a fixed 400Hz, you have to have a constant speed drive
off the engine to drive the generator. Those drives are expensive, heavy,
and require lots of maintenance. Getting rid of them saves money, but now
frequency varies with engine speed.

So now we get to put variable frequency power supplies in all our new
avionics.

Gerry

"S Narayan" wrote in message ...
The generation
of 3 phase power is also easy and I believe it may also be more efficient in
terms of the generator design.

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

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

"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.

"Ron Natalie" wrote in message
. ..

"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.


What?!? Your plane doesn't have 3-phase batteries? Call CBS!