Why we use 115v 400Hz power supply in aircraft?

Aircraft electrical components operate on many different voltages both AC and DC. However, most of the aircraft systems use 115 volts AC at 400 hertz or 28 volts DC. 26 volts AC is also used in some aircraft for lighting purposes, where the power is generated by a commutator which regulates the output voltage of 28 volts DC. AC power, normally at a phase voltage of 115V, is generated by an alternator, generally in a three-phase system and at a frequency of 400 Hz.

Running at 115v 400Hz allows the use of smaller transformers. Less energy has to be stored in the transformer core per cycle, so the core can be smaller. A smaller core means a lighter transformer, and reducing weight is a good thing in an aircraft. The more is the frequency, the more is the speed and relatively lesser structures of electrical equipment and lesser the weight, which is required for the aircraft, there may be mechanical problems and expensive solution would require for that but keeping in view the economic benefits/requirements of the aircraft it would be financially viable.

U.S. Air Force Aircraft Electrical and Environmental System Specialist F-4, F-16, A-10, UH-1, and others, not only does the 3-phase 400Hz reduce the size of the components on the AC portion of the power supply, but it also rectifies into a DC that requires smaller and fewer components for filtering. The period for rectified 3-phase 60Hz is 2.78mS, for 3-phase 115v 400Hz it is 0.417mS, so a lot less capacitance is needed to bridge the peaks and it is easier to filter the 2400Hz "noise" riding on top of the DC.

I seem to recall that the 400 Hz frequency was commonly used about 30 plus years ago for IBM computer systems as well as for aircraft. I worked on several 60 Hz to 400 Hz conversion systems for IBM main frame users then. The reason given to me was that the 400 Hz allowed for smaller components (capacitors and inductors) and for smoother AC to DC conversion.

The advantage of high-frequency alternators is that they require fewer copper coils in order to generate the necessary electrical current. This reduction in material allows the alternator to become much smaller such that it takes up less space and weighs much less than it would otherwise. A higher frequency also increases loss of power over long distances, so using it on long transmission lines would be a bad idea.