Why 400hz for aircraft power?

[Paul Wilson]

Don't know if Tech Log would be better, but I'll give it a shot here.

Given that AC electrical power in domestic/industrial settings is either 50 or 60hz, why do aircraft generators put out power at 400hz? Is it just a function of bolting a generator onto a something rotating very fast, and it was easier to go with 400hz and produce electrics to fit? Or is maybe something to do with harmonics from 50/60hz upsetting other aircraft systems?

Seems needlessly complexity, when "off the shelf" type motors etc. that use 50/60hz could have been used instead of redesign to use 400hz. Possibly I'm barking up the wrong tree and the AC power is all converted to DC before being used anyway, so it wouldn't matter what frequency was used.

Thanks
Paul

[Bre901]

IIRC (and, as an electrical engineer, I should, but it's been a long time then) the weight of a rotating electrical generator or motor is in inverse relationship with frequency, at fixed power.

[Jet II]

Bre901 got it about right - its all down to weight.

The 400 HZ frequency is used in aircraft systems primarily because the generators are much lighter in weight.

50/60 HZ was originally specified by the US government (I think) as a standard back in the days at the start of widespread use of electricity - its hasn't been changed since then due to the costs involved.

Oh and a lot of services use AC.

[411A]

Large aircraft years ago (talking DC4's, DC6's, Constellation's etc) are DC aircraft, ie; the main power source are a 28volt battery and 28volt DC engine-driven generators (not alternators).
However, with these aircraft, 3phase AC current was required for some instruments, and this 3phase power was generated with 28volt DC driven AC generator(s), which are called, inverters. A small 28volt DC motor was connected to the small AC generator via a common shaft and driven at a constant speed, which produced 400 cycle, three phase AC current (usually 26volt).
400 cycles was choosen for reduced weight and size, both of the AC generator and the DC motor required to drive same.

Later DC aircraft (for example, Grumman Gulfstream) used solid state inverters for the required AC current.

[seacue]

400 Hz is used where weight is important because the magnetic cores can be smaller/lighter. This applies to transformers as well since a lower inductance (smaller core/number of turns) can be used than required by lower frequencies.

IIRC, the US Navy was using 800 Hz or higher at one time, but the "singing" from vibrations in the cores must have been very annoying.

In the early days there were eddy-current problems in the laminations of the magnetic stuctures of motors/transformers. This gets worse as frequency is increased. Improved magnetic materials helped a lot as did thinned laminations IIRC.
60 Hz, the North American household standard, allows slightly smaller components than 50 Hz, but the desire to suit world-wide markets has negated this in many cases.

And then there's 25 Hz. The early hydro plant at Niagara Falls was 25 Hz. This meant that a lot of New York state and Ontario lived on 25 Hz. Motors and transformers were BIG. Southern Ontario didn't switch to 60 Hz until afer WW2. I understand that the abrasive materials plant at the Falls still runs its furnaces from 25 Hz. Resistive heaters don't care about frequency.

[Paul Wilson]

Many thanks for the replies, never realised the weight thing, although thinking back to physics lessons of the past should have been able to deduce it.