Why 28V DC?

"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