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itsmoked (Electrical)
22 May 08 2:42
I am wondering why in the automotive world cars, with 12V batteries, are deemed 12V systems even though they are 13.2V to 14.7V typically but in the aviation world they call planes 28V even though they have 24V batteries and pushed in the charging realm may reach ~27V? Keith Cress
kcress - http://www.flaminsystems.com
IRstuff (Aerospace)
22 May 08 10:09
MIL-STD-704 and MIL-STD-1275 specify 28V. TTFN
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itsmoked (Electrical)
22 May 08 13:07
How so? (The short story.) Keith Cress
kcress - http://www.flaminsystems.com
IRstuff (Aerospace)
22 May 08 14:16
not sure what the whole story is; never tried to research it. 字串8
However, most aircraft rarely run on batteries for extended periods, and the power converters have to be sized for the alternator outputs, which, for MIL-STD-704 are quite ragged. Additionally, the overall voltage range of both mil specs cover 24V, since the standard voltage range is 22V to 29V.
Most airliners run on on-aircraft auxiliary power units (APUs) while on the ground, or directly off ground-based APUs.
On many aircraft, the batteries don't even go to all the electrical systems, thus making it imperative that they can reliably run on 28V from the APU as opposed to battery that might not even be there. TTFN
FAQ731-376: Eng-Tips.com Forum Policies
hgldr (Electrical)
22 May 08 19:54
TRU's in many aircraft are designed to output 28V nominal when the input is 115VAC. This value fluctuates widely as the 115VAC input could be anywhere from 85VAC to 122VAC. Different battery types also affect the standard "28V". For example, lead-acid batteries are kept charged at over 28V. NiCad aircraft batteries can be 27V to 30V for 19 cell or 25v to 28.5 for 18 cell, depending on the system. Boeings new 787 uses Li-ion batteries and the nominal voltage is set to approx 28V. 字串9
itsmoked (Electrical)
22 May 08 21:00
Ah, that's what I was looking for.
Thanks gentlemen. Keith Cress
kcress - http://www.flaminsystems.com
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