I am a former ramp and tech support engineer just looking in, I like to keep an eye on whats going on and have been looking for info on the strange hi volts DC I keep hearing about.
It seems that the +/- 270v 'DC' is of course actually AC, the square wave being very much easier, and cheaper, to generate electronically from rectified frequency wild AC.
They seem to be using this level of control to give high power motors, I assume they are induction, higher starting torque with the initial lower frequency.
It may be this basic or it could be a whole lot more complicated!
Square wave AC is also very messy, being every harmonic possible if I remember correctly, and that can interfere with delicate digital devices, like the rest of the plane.
Frequency wild is alright for galleys that use a lot of power in quiet cruise when other demands may be lower, they are resistive loads that are not greatly frequency sensitive and are designed for normal cruise power settings.
Having been on a couple of Boeing courses, 747-400 and 777, we found the instructor could be very insistent on some matters. We resolved it to 2 reasons.
First.B. S. E. Boeing Standard English was sometimes the cause, they are often teaching engineers for whom English (full of ambiguities) is a second, or even third, language. BSE is very straight forward, but sounds rather awkward to us
The other reason was Boeing proprietary, meaning they had some patent involved, or hoped to. Then the instructors would say quietly that there were 'lawyers involved' and we shut up, knowing that discussion was useless.