Thread: Why 28V DC?
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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