Cast your minds back folks...a w a y back to the good 'ole days, of straight wings, propellors and large piston engines.
Ahhh, feeling good already
Anyway, these aircraft were DC aeroplanes, normally using as ships batteries two large 12v batteries in series...and yes, 24v was used simply because the engine starting current draw was quite substantial, and 12v simply wouldn't cut it.
This was fine as far as it goes, but some instruments needed AC for proper operation, the 'ole Sperry C4 or C6 gyro compass systems, are a perfect example.
So, where to get the AC?
Some might say, well just hook up a solid state inverter, and all would be sweetness and light.
Problem was, there was no such animal at the time, as solid state electrics were quite rare.
Enter the DC driven electric motor, which in turn turned a small AC generator at a fixed RPM.
3 phase was choosen to keep the RPM's at a reasonable speed, so the unit, called the inverter, would not fly to bits...in normal operation, that is.
Single phase would have not worked well for the C4/C6 Sperry systems at any rate.
115v?
Common voltage in the USA.
Weight was a large factor as well.
If you ever have seen one of these motor-driven inverters, you would soon realise how heavy they were.
Three-phase and 115v helped with all this as well...made the units a practical weight.
However, not all is entirely sweetness and light.
IF, for example, one of the AC phases failed for whatever reason, many times the failure would not be known, IE: no red flags/lights to alert the crew, in the earlier days.
They were fitted later, however.
Why?
A couple of rather nasty accidents was the reason, which at the time were hard to identfy the probable cause.
Phase failed, Sperry compass indicated incorrectly, IMC (usually at night), and hard terrain was found.
Not good.
