Thread: Why 28V DC?
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  #10  
Old January 14th 04, 09:43 PM
S Narayan
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"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.