This all stems from lots of blank faces at work when these strange questions were asked:- 1. Why is aircraft dc voltage a strange 28v and not 6/12/24/36v etc. as most batteries are a multiple of 6. 2. Why is aircraft ac voltage 115v/200v and not any other voltage. 3. Why aircraft ac voltage frequency is a strange 400Hz and not 300Hz or 500Hz
PS we know it needs to be high frequency because it allows inductive components to be made smaller therefore less aircraft weight penalty.
I can take a stab at question no.1 for starters: I know that on some aircraft the battery is indeed 24V, the system as powered through the AC buses and a few TRUs uses 28V. The difference between the two makes it possible for the DC system to recharge the battery as the voltage for charging needs to be higher than the battery's own voltage.
As for the other questions, generally choices such are these were taken not because of some major issue but tended to evolve through the years. If you go back 60 years you'll find that the issue of voltages and frequencies was not as clear cut as it is now with more variations in use. As aviation matured some options survived and others were discarded. A bit like the Betamax/VHS struggle I guess.
OK this might make some sense.
This is taken from an article by Aspi Wadi
Copyright 2004, all rights reserved
Aircraft electrical systems and why they operate at 400 Hz frequency by Aspi Wadia of GE Aircraft Engines
Higher than usual frequencies, such as 400 Hz, offer several advantages over 60 Hz – notably in allowing smaller, lighter power supplies to be used for military hardware, commercial aircraft operations and computer applications. As aircraft space is at a premium and weight is critical to aircraft engine thrust and fuel burn (and thus the aircraft range and engine horsepower per pound), 115 volts at 400 Hz offers a distinct advantage and is much better than the usual 60 Hz used in utility power generation.
However, higher frequencies are also more sensitive to voltage drop problems. There are two types of drops: resistive and reactive. Resistive losses are a function of current flowing through a conductor with respect to the length and size of the conductor. This is the most important factor in controlling resistive power loss and applies regardless of frequency. The short transmission range of higher frequencies is not a factor in most airborne applications.
Reactive drops, on the other hand, are caused by the inductive properties of the conductor. Reactive drops are a function of both cable length and the AC frequency flowing through the conductor. With high frequencies such as 400 Hz, reactive drops are up to seven times greater at 60 Hz.
