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Why 28V DC?



 
 
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  #1  
Old January 14th 04, 03:19 PM
Bob Martin
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Default Why 28V DC?

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.
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  #2  
Old January 14th 04, 03:24 PM
Roger Hamlett
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"Bob Martin" wrote in message
om...
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.

Battery voltage.
The '12v' electrics on your motor car, actually run at typically 13.8v
(normal charging voltage of 6 lead acid cells). Hence much '12v' electronics
is 'mislabelled'. On aircraft, the naming was made to co-incide better with
the real voltage present.
In fact much industrial electronics running on '24v', is also a 28v system
(whenever the system has a lead-acid standby ability).

Best Wishes



  #4  
Old January 14th 04, 04:34 PM
Jim Weir
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Because the aircraft industry standardized on the nominal CHARGING voltage of 28
volts rather than the DISCHARGED voltage of 24 volts. 24-28?? Same animal with
a different nametag.

Now, why 24/28 volts? Because the aircraft needed to be lighter for military
performance reasons. Two 12 volt batteries in series comes nowhere near the
weight you can save in a fairly complex airplane (say, for example, a P-51) by
using a lighter weight copper wire for the same wattage load (double the voltage
= half the amperage for a given wattage). Remember, wire is sized by amperage,
not by voltage. INSULATION is sized by voltage.

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.

The REAL question is who decided on 6 volts (3 each 2 volt lead-acid cells in
series) to begin with.

And the inquisitive student might ask, if 24/28 was so good, why not go 3 in
series and get 36 volt systems...or like the phone company with 4 in series for
48 volts? Because, grasshopper, the calculation WAS made to find out the most
efficient combination of voltage/current/wire size and at the time (WWII) it
came out just shy of 30 volts. Rather than dick around with special 30 volt (15
cell) batteries, the decision was made to use off-the-shelf dual 12 or single 24
volt "industrial" batteries.

Then there is the 115v 3ph 400Hz. discussion...

Jim



(Bob Martin)
shared these priceless pearls of wisdom:

-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.

Jim Weir (A&P/IA, CFI, & other good alphabet soup)
VP Eng RST Pres. Cyberchapter EAA Tech. Counselor
http://www.rst-engr.com
  #5  
Old January 14th 04, 05:08 PM
Todd Pattist
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Default

Jim Weir wrote:

And the inquisitive student might ask, if 24/28 was so good, why not go 3 in
series and get 36 volt systems...or like the phone company with 4 in series for
48 volts? Because, grasshopper, the calculation WAS made to find out the most
efficient combination of voltage/current/wire size and at the time (WWII) it
came out just shy of 30 volts. Rather than dick around with special 30 volt (15
cell) batteries, the decision was made to use off-the-shelf dual 12 or single 24
volt "industrial" batteries.


And since WWII the calculation has shifted, so you now find
cars being designed for 36 or 48 volts. The reason is still
cost/weight/size of wiring. When you want lots of power,
and don't want heavy/expensive/fat copper wiring you want a
higher voltage. Engines are being sold with valves that are
lifted by electromechanical actuators instead of mechanical
cams. The designers would go to even higher voltage if not
for fear that the mechanics/owners out there who're only
familiar with 12/24 volt systems in vehicles would be
electrocuted.

Todd Pattist
(Remove DONTSPAMME from address to email reply.)
___
Make a commitment to learn something from every flight.
Share what you learn.
  #6  
Old January 14th 04, 05:31 PM
Roger Hamlett
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"Jim Weir" wrote in message
...
Because the aircraft industry standardized on the nominal CHARGING voltage

of 28
volts rather than the DISCHARGED voltage of 24 volts. 24-28?? Same

animal with
a different nametag.

Now, why 24/28 volts? Because the aircraft needed to be lighter for

military
performance reasons. Two 12 volt batteries in series comes nowhere near

the
weight you can save in a fairly complex airplane (say, for example, a

P-51) by
using a lighter weight copper wire for the same wattage load (double the

voltage
= half the amperage for a given wattage). Remember, wire is sized by

amperage,
not by voltage. INSULATION is sized by voltage.

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.

The REAL question is who decided on 6 volts (3 each 2 volt lead-acid cells

in
series) to begin with.

And the inquisitive student might ask, if 24/28 was so good, why not go 3

in
series and get 36 volt systems...or like the phone company with 4 in

series for
48 volts? Because, grasshopper, the calculation WAS made to find out the

most
efficient combination of voltage/current/wire size and at the time (WWII)

it
came out just shy of 30 volts. Rather than dick around with special 30

volt (15
cell) batteries, the decision was made to use off-the-shelf dual 12 or

single 24
volt "industrial" batteries.

Then there is the 115v 3ph 400Hz. discussion...

And the calculation here is shifting again.
With the increasing use of quite powerful electric parts in some new cars
(things like electric power-assisted steering), there is a new standard
making the rounds for systems operating just under 50v DC. However these
will use an unusual number of cells to avoid going over 50v (in some
countries the certifiation requirements increase massively at this point),
and are often based upon different battery types.

Best Wishes



  #7  
Old January 14th 04, 07:14 PM
Jon Woellhaf
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Default

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

Let's hear it!

Jon


  #8  
Old January 14th 04, 07:34 PM
Charlie
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Default

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

Let's hear it!

Jon


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.


  #9  
Old January 14th 04, 09:08 PM
Larry Fransson
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Default

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?

--
Larry Fransson
Seattle, WA
  #10  
Old January 14th 04, 09:43 PM
S Narayan
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Default


"Larry Fransson" wrote in message
news:2004011412081416807%[email protected]et...
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.


 




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