PM? starter/generator
Since it is brushless and synchronous, i.e. frequency varies with speed, it would seem to be a permanent magnet rotor machine, with 3-phase wye winding plus neutral for the stator.
Looking at the schematic on page 11 of the Airworthiness report there is a functional block diagram for some of the power distribution of the forward and aft E/E.
The starter/generator functions as a starter for the APU (using 3-phase AC?) developed in the APU Controller fed from the P49 APU Batt Elect Panel. It then functions as an alternator/generator producing variable frequency 235VAC that is collected in the P150 APU AC Elect Panel. The AC panels feed the AC busses and the TRU rectifies AC to create the 28 VDC Bus.
Four electrical busses are described: 235 vac and 115 vac both variable frequency, plus 28 VDC and 270 VDC.
i didn't see anything about how grounding is done, but i would think that all the busses are tied and referenced to chassis. In electric cars the high-voltage traction battery packs are isolated from vehicle chassis--i was surprised to find that the Main and APU battery packs have their negative terminals grounded to the airframe chassis.