115 VAC at 400Hz , Why for airplanes?

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danny davis

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Why is 115 VAC at 400Hz use for airplane circuits?

Everything in the cockpit of airplanes use 115 VAC at 400Hz, any reason why?

Why at 400Hz?
 

Yes i know, but why does circuits use a low frequency transformer than? because most circuits like amplifiers use big transformers because they use 120VAC at 60hz

Any reason why they don't use 120VAC at 60hz?

why is it at 115VAC and not 120VAC?

Why is it at 400hz? why not higher? why did they pick it at 400hz?
 

Homes and businesses use 50Hz or 60Hz because the low frequency does not get attenuated when travelling far from a generating station. Also maybe the generators (actually they are alternators) turn slowly due to their huge size.
400Hz was selected for airplanes when airplanes and electricity were both new and 400Hz was a high frequency in those old days.

Maybe the voltage is 120V with no load then it drops to 115V when fully loaded.
 

So it was picked because they just wanted a smaller size AC power transformer? no other reason?
 

Rotary generators also have smaller core size and weight like transformers, an important feature for airplane instrumentation.
 

So these Rotary Generators output is 400hz 115VAC ? any reason why?
 

Danny , its a historical thing. At least as far back ass WW2, aircraft used 400 HZ, as explained, the generators were smaller and lighter, the mains transformers in the radio equipment were smaller and lighter, the HT smoothing components were smaller and lighter. The reason we use 50/60 HZ for land based mains is that the transmission losses are higher for 400 HZ, while the weight of electronic kit is not so important. Thats it !!
Frank
 

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