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Comment by ars | Hacker News Reader

They already have 400hz power (not sure of the voltage) from the engines.

Why not just use a switching power supply to change that to 60hz 110v? Why this whole business with inverters?

There may also be concerns about insulating the flight electrical system from the cabin electrical system. What if there's a surge or huge load in the cabin? You need to be sure that that sort of thing can't affect the engine operation. The easiest way to be sure is to put an air gap between them.

They already tell you to turn off your Gameboy. I doubt they'd let passengers get anywhere near plugging directly into the power from the engines.

They could run a generator off a turbine in the airstream. A lot of EW pods were powered this way. This would cost a bit of fuel, but there would be no direct connection between the cabin electrical system and the engines.

Switch-mode power supplies are essentially DC rectifiers, which will never ever give you 110V, 60Hz (an AC voltage).

One way to change the frequency of a power source is through a double-conversion type AC/AC converter, i.e. 1st stage is a rectifier which outputs a DC voltage - this is an arbitrary DC voltage, but it needs to be within the voltage tolerances of the 2nd stage - an inverter, which reconstructs an AC voltage from the DC link to whatever frequency you like.

This setup is also the basis of most UPS systems btw, with a battery connected in the DC link.

A switch mode power supply can do a whole lot more than act as a rectifier. "Switch mode" refers to the turning on and off of numerous silicon switches usually connected to some kind of LC energy storage/filter, and this is the basis of modern rectifiers, inverters, step-up and step-down converters, etc.

It is in fact possible to use a switching circuit to convert an arbitrary ac voltage/frequency to another arbitrary ac voltage/frequency directly without a DC link intermediary. This is called a "matrix converter." http://en.wikipedia.org/wiki/AC/AC_converter

So it should theoretically be possible to take say 200VAC at 400 Hz and convert this to 120VAC 60Hz. In fact this would probably be more compact/less expensive than the usual inverters mentioned in this article, since these are usually meant to be powered by a low voltage DC source and thus have to deal with taking a low voltage and very high current and first stepping this up to several hundred volts, then modulate it into an AC sine wave. A matrix converter (or even AC-DC-AC converter) would have no such requirement if it were powered from the already high voltage aircraft power bus.

At this point I think it's a matter of the technology catching up, since this is a highly specialized application that has just recently become an opportunity for innovation.

EDIT: Also - 240W per person? This seems outrageously high as a baseline load. I would estimate more like 50-100W per person on average, with 300W peak (like when the power supply is first plugged in.) The peak loads would not all happen at once unless everyone decides to plug in and turn on their computers at once, in which case the power limiting would kick in...

Fair enough, although I've never actually seen an AC/AC converter without a DC link in real life (the closest thing probably being a direct torque control VSD).

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