Sorry for dragging up a year-old topic, but we (I work for a subcontractor of American Airlines at PDX) use a solid-state electronic converter, manufactured by FMC technologies. It converts standard 480V 60Hz to 3-phase 110/208V at 400Hz via a 6-wire connector. Most of the systems on the aircraft run on the 400Hz AC, but there are a few things (like lighting) that run on 28VDC. I'm not entirely sure what the sixth conductor on the plug is for, but I supsect it's probably a second ground.
Here is the converter:
http://www.fmctechnologies.com/AirportEquipmentServices/GroundSupportEquipment/GroundPower.aspx Here is the connector:
http://www.usairmotivegse.com/imagenes/producto/3989R676XXE7198.jpg There are other manufacturers for this equipment as well.
