Lyman
From the study guide, circa 2010:
The Start Power Unit (SPU) takes 28v dc from the battery and changes it to 115v ac. The 115v ac is sent to the R2 Autotransformer Rectifier Unit (ATRU) where it is changed to +/- 135v dc and then sent to the Common Motor Start Controller (CMSC) which then changes it back to 115v ac (!) to start the APU if 4 or so other things have happened in the meantime.
As I understand it, 115v ac is used to turn the ASG (which normally puts out 230v ac) and the generator neutral relay must be open.
The right-hand ASG is used to start from battery power. From ship or ground power, they alternate.