Large aircraft years ago (talking DC4's, DC6's, Constellation's etc) are DC aircraft, ie; the main power source are a 28volt battery and 28volt DC engine-driven generators (not alternators).
However, with these aircraft, 3phase AC current was required for some instruments, and this 3phase power was generated with 28volt DC driven AC generator(s), which are called, inverters. A small 28volt DC motor was connected to the small AC generator via a common shaft and driven at a constant speed, which produced 400 cycle, three phase AC current (usually 26volt).
400 cycles was choosen for reduced weight and size, both of the AC generator and the DC motor required to drive same.
Later DC aircraft (for example, Grumman Gulfstream) used solid state inverters for the required AC current.