Why 28V DC?

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

"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.

"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.

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

Jim Weir wrote:

So why was there 12 volts to begin with? Because Detroit started making cars
with a much higher compression ratio and to turn the starters over, the old 6
volt batteries weren't cutting it. Bingo. Two 6 volters in series gives 12
volts and that was close enough for Detroit gummint work.

Actually the auto industry is moving to a 42 volt system in the future to handle
increasing electrical requirements, plus more accessories (e.g. oil pump) may be
electric in the future. Where can you find more information on this? Why
http://www.42volt.org of course.

In article , Tiffani wrote:

Jim Weir wrote:

So why was there 12 volts to begin with? Because Detroit started making
cars
with a much higher compression ratio and to turn the starters over, the old
6
volt batteries weren't cutting it. Bingo. Two 6 volters in series gives
12
volts and that was close enough for Detroit gummint work.

Actually the auto industry is moving to a 42 volt system in the future to
handle
increasing electrical requirements, plus more accessories (e.g. oil pump) may
be
electric in the future. Where can you find more information on this? Why
http://www.42volt.org of course.



Just what we need in a car -- an electric oil pump! That would be fine
for pre-oiling, but for regular operation? It degrades reliability by
making the lubrication system dependent on another system.

"Tiffani" wrote in message ...


Jim Weir wrote:

So why was there 12 volts to begin with? Because Detroit started making
cars
with a much higher compression ratio and to turn the starters over, the
old 6
volt batteries weren't cutting it. Bingo. Two 6 volters in series
gives 12
volts and that was close enough for Detroit gummint work.

Actually the auto industry is moving to a 42 volt system in the future to
handle
increasing electrical requirements, plus more accessories (e.g. oil pump)
may be
electric in the future. Where can you find more information on this?
Why
http://www.42volt.org of course.


A 42V DC? shock could be pretty bad. The new Toyota Prius uses 273V for the
traction motor, but that is very local to the hybrid system. Maybe a low
voltage higher frequency AC system might be a solution as someone suggested
earlier in the thread. Conversion efficiencies are pretty good these days
with more advanced controllers. Then individual components could step up the
voltage and convert to higher DC voltages as needed.

Bob Martin wrote:
I'm working in the avionics integration test facility at Gulfstream...
one of the engineers had a question for me (being an airplane person
instead of an electrical/computer guy) that I couldn't answer... why
do airplanes use 28V DC systems (or 14V)? He says most industrial
applications use 24V DC. I tried googling on it but nowhere did it
suggest any reason why, just that it is.

Lead-Acid chemistry batteries need 13.8 to 14.4V for "charging". The
voltage regulator on the alternator is usually set to a "compromise"
setting of 14.25V, which causes the bus voltage of the aircraft to be
near this voltage all of the time the engine is running.

All of the avionics and instrumentation is running off said 14.2V for
the duration of the flight. Double all of the above voltages if the
airframe has a 12 cell battery instead of a six cell.

What happens if the alternator fails in flight? Almost immediately, the
battery voltage sags down to under 12.6V, so the avionics has to work
over a voltage range of 11.5 to 15V...

MikeM

"S Narayan" wrote in message
...

A 42V DC? shock could be pretty bad. The new Toyota Prius uses 273V for
the
traction motor, but that is very local to the hybrid system. Maybe a low
voltage higher frequency AC system might be a solution as someone
suggested
earlier in the thread. Conversion efficiencies are pretty good these days
with more advanced controllers. Then individual components could step up
the
voltage and convert to higher DC voltages as needed.


I've worked on a lot of 48 Volt systems and it is more like an unpleasant
tingle than a bad shock.

Higher voltage means thinner wiring, cutting weight.