Size of APU alternators; amount of frequency variation
Hamilton Sundstrand is the maker (possibly European sales use a different source) of the alternators driven by the 787 APU. These are 225 KVA units (each; I put that = 180 KW) intended to have a phase to phase voltage of 230. That make the phase current about 600 amps.
Hamilton also makes the blower/compressors for cabin air. They note these as being able to operate on a variable frequency feed with a range of 360 to 800 cycles. That gives us the alternator output frequency specification. Boeing has described it as a variable freqquency system.
Hamilton also makes the turbine main engine-driven alternators. There are four of these and I believe they are also each of the save 225 KVA output. They might differ in the gear ratio to the respective turbines.
Between 1974 and 1978, the labs at Curtis-Wright, with major assistance from GE, worked to build and test a turbine-driven alternator with a rating of 150 KVA. This had a permanent-magnet rotor with salient (misspelled 'sahlen' somewhere) poles of samurium-cobalt-- 14 rotor poles and a 9-phase stator winding, cycloconverted and filtered to a well-formed 3-phase output. Curtis-W labs had earlier tested an equal-sized and similarly constructed machine which had a wound rotor (i.e. suppled with external DC).
Of interest is the rotating speed range, from 12000 to 21000 rpm (that is, within 2:1). I'll have to clearify which unit had that speed as the last report page was a errata sheet dealing with that. I will explain the significance of all these variances, down the road.
What I am looking for is if and how these components and their operating ranges might cause variation of the voltage on the "hot" battery bus. Since we have minimal system details, we can just design what major detail is missing.
For starters, a major question is whether the APU altenators have a wound-rotor design (has then to be a DC winding). Anyone know?
OE