vapilot2004 you are quite correct many motors use one or two phase but if you want a motor that moves a load at a constant speed then it's the good old squirrel cage and that uses 3 phase also A TRU will use 3 phase AC to convert to DC to feed the DC Bus bars perhaps this is a better example. If anyone wants to understand the magic of this subject in greater depth may I suggest Aircraft Electrical Systems by EHJ Pallett he also does good books on Instruments and Autopilots.