Just about all turbo jet engines that are installed on transport airplanes have 3 phase generators driven from the N2 accessory gearbox. Since Voltage and frequency need to be kept within fairly close tolerence, the generator is kept at a constant speed, being actually driven by a constant speed drive unit, very similar to an automatic transmission in your car. We are talking about 115 volts and 400 Hz. This is a very short and not very detailed description. but hope this helps!