400Hz ?

[MANAGP]

Do any of you extremely clever folk know why aircraft use 400Hz ac? It must be nearly DC at that frequency?

Many thanks!

[woderick]

Size and Weight, for the same power 400hz motors and generators and motors are physically smaller and therefore lighter. 400hz is a very convenient frequency to generate, going to make a bigger idiot of myself now but IIRC it's an eight pole generator @ 6000 rpm - easy.

[Bre901]

MANAGP search and you will find, more here (http://www.pprune.org/forums/showthread.php?s=&threadid=125540&highlight=400hz)

BTW, no, it's not almost DC, the average value is still 0 as the waveform is a sinus curve.

woderick quite wight ;)

[411A]

Spot on, folks, and in addition, it was easier (lighter weight) to generate 400hz with non-static inverters (IE: DC motor driven).
A long time ago, but worked good....then.:ooh:

[Bumz_Rush]

on the G4 and similar there is a 400hz OP in galley...what modern kit uses 400hz.....

Bumz

[HEATHROW DIRECTOR]

<<Do any of you extremely clever folk know why aircraft use 400Hz ac? It must be nearly DC at that frequency>>

On the contrary, surely? It would be almost DC if it was 1 or 2Hz.... or is my aged brain playing tricks with my radio theory?

[Willit Run]

in a non-technical explanation to MANAGP, if you were to touch a 400 HZ live wire, it would feel more like DC, because the current reversal would be so fast you wouldn't feel the "buzz" like you would for a 60 or 50 HZ circuit.
Its all about power to weight ratio!

[146 reasons not to]

Oh dear Oh dear try touching a genny busbar dear boy 400Hz will hurt.
Amps Kill, volts and frequency dont .

[vapilot2004]

Much lighter power transformers in electronic equipment can be used with 400hz AC as opposed to 50/60 cycles.

Pre-1970, before electronic (transistorized) inverters were common, aircraft used mechanical inverters - A DC motor directly driving a 400hz generator.

[Fark'n'ell]

HD
On the contrary, surely? It would be almost DC if it was 1 or 2Hz.... or is my aged brain playing tricks with my radio theory?

If it were 1 or 2Ghz it still a'int nearly DC

Fark

[L Peacock]

Stick 400hz through a three phase rectifier and you do get a nice flat waveform with little smoothing required. In that sense, nearly d.c. I suppose.

[barit1]

When I read MY text (50 years old!!) I see that DC is ZERO hertz (except back then they called it zero cps...)

the waveform is a sinus curve
ROFL!!!

[146 reasons not to]

Here is a reason
1 Some equipment require a 3 phase supply ie motors
2 some use only one ie windscreen heaters
if you had single phase generators you would need 3 for 3 phase and then you would require a very complicated timing circuit to make sure the phases were the correct angle apart.You would also need to duplicate this system or triplicate it to get an airframe certified -the weight would be large .
So why not put a 3 phase genny on two engines and the apu that can supply all the power required and attach them to a constant speed drive that rotates the gen at a speed that gives you 400Hz depending on how many poles your choice of gen has because Freq= RPM x pairs of Poles
--------------------------
60
So know we know how to gear the constant speed drive to get 400 Hz whilst varying engine RPM ( still not said why 400Hz stay with me)
Now that we have gone to the effort of making 400Hz we can do a couple of things .
We tell everyone that's the standard so all equip is made to operate at 400 Hz and when they ask Why? we can say it's so we can use 3 phase generators to save weight and simplify power production because 400 Hz means a complete cycle occur's every 1/400 th of a second therefore a pole passes a coil of the gennny and generates max voltage every 1/1200 second (remeber 3 phase so 3 coils.The less the coils again the greater the weight saving but three is the minimum ) we therefore get the 3 phases 120 degrees apart because the same pole passes the next coil another 1/1200 of a sec later and then the third and all our electrical need's right down to Coffee is done on each 360 degree cycle of the genny or 400 times a second.

[vapilot2004]

1 Some equipment require a 3 phase supply ie motors

I never knew there where 3 phase motors on Aircraft - I'm not being cute - have there been 3 phase AC motors aboard aircraft and what are they used for ?

Freq= RPM x pairs of Poles

don't forget the /60 to allow for RPM ref minutes and HZ (or CPS) ref seconds.

FREQ = RPM x Pole Pairs / 60

It would be almost DC if it was 1 or 2Hz.... or is my aged brain playing tricks with my radio theory?

Now you'd be in the ballpark's home city :P if we're talking servo drive theory -from a standpoint of a PCM driven device (or motor) a higher freq = higher drive =kind of= more DC drive. but AC at any freq, because of pos & neg voltage excursions is not DC.

[146 reasons not to]

vapilot2004 you are quite correct many motors use one or two phase but if you want a motor that moves a load at a constant speed then it's the good old squirrel cage and that uses 3 phase also A TRU will use 3 phase AC to convert to DC to feed the DC Bus bars perhaps this is a better example. If anyone wants to understand the magic of this subject in greater depth may I suggest Aircraft Electrical Systems by EHJ Pallett he also does good books on Instruments and Autopilots.

[L Peacock]

3 phase fuel pumps and cooling fans are common.

146 reasons. You make the case for a.c., 3 phase and explain a.c. generator theory but, why 400Hz. Is it just an historic link to engine RPM and a convenient genny size/number of pole pairs?

Anyway 400hz makes a comforting whine in the headset.

[catchup]

400 Hz? In the A 380 it will become history.....

regards

[barit1]

400 Hz requires much less iron in the magnetic flux path of transformers and motors, compared to 50/60 Hz.

800 Hz was even lighter, and was used for a while in several military aircraft of the 50s & 60s. Never really caught on though.

But solid state inverters / DC-DC converters use much higher frequencies (several KHz) to achieve much lighter weight. The power supply in your computer is such a device.

[146 reasons not to]

LPeacock Cooling fan motors are a classic example you want it to switch on run at a constant speed so yes Brake fan motors and Hyd pump cooling fans and also fuel pumps are good examples.
Everything in an areoplane is a comprimise the aim being since the day's of Wilbur & Orville to get the maximum power for the minimum weight and from the minimum expended energy-switching on the genny means some Avtur gets used running it.
So 400Hz is the best fit at present as it gives us 3 phase 120 degrees apart with the least amount of iron and copper as barfit1 says. It also gives us the best Power (and I dread to go here because sums aint my strong point) but you must remember for a sine wave that we deal with rms because its impossible to measure max power like we would in a dc circuit where P=IV.
3 phase= 3 sine waves 120 degrees apart ( the next bit you cant do but I remember it from collage for the simple reason I spent a lot of time trying to work out what the F was going on) If you take a sine wave and sample the power at 6 points during 1 cycle square each value add them all together and divide by 6 then take the square root of the answer you get the rms or average power.
1.414 times rms gives you Peak power so do the sums for each phase for the total power of the gen in 1 revolution and that is the best fit for todays technoligy however with the 380 we may move on and good for us but will uncle sam certify it if he didn't invent it ?-now there's a whole new topic.

sorry for the misspelling barit1 to late to go back now :ok:

[L Peacock]

What philosophy is the 380 adopting?

[catchup]

The A380 will have a variable Frequency AC (3x115V) Elec Power System varying between 400 Hz and 800 Hz, depending on engine RPM.

No IDG (CSD).

regards

[FlyVMO]

What is the advantage to this new system on the A380? Are the constant speed units a common failure item?
What will be the effect of slightly different engine rpms, if the frequency varys with engine speed-will they be feeding separate buses? As I recall the generator frequencies and phases have to match before they can feed the same bus.

Also am I right in thinking the RMS voltage would vary with the frequency as the slope of the sine curve changes? (Or would a standard regulator also compensate?) Or am I completely off?

Possibly all easy problems for an electrical engineer these days, but you dont know until you ask!
Thanks,
FlyVMO

[MANAGP]

Thanks very much for all the answers.

[catchup]

What is the advantage to this new system on the A380? Are the constant speed units a common failure item?

A: Less weight and elimination of a possible failure source.

What will be the effect of slightly different engine rpms, if the frequency varys with engine speed-will they be feeding separate buses?

A: Yes, they will feed seperate buses.

Also am I right in thinking the RMS voltage would vary with the frequency as the slope of the sine curve changes? (Or would a standard regulator also compensate?)

A: Will be regulated to a constant voltage of 115V


regards

[barit1]

Many years ago a system called VSCF (variable speed, constant frequency) was proposed that used a "wild" alternator frequency, but processed this through a solid-state "chopper" to synthesize a constant-frequency output. Mechanically simple, let the electronics do the work.

I'm not sure what aircraft (if any) ever actually used this. Is it possible this is the A380 plan?

(Further research suggests Smiths and perhaps others are producing these systems)

[dixi188]

I see the new Boeing 787 is to be very electric. DC, AC, What volts, freq. etc?
Any body know?

[CaptainSandL]

barit1,

I have flown some 737-500's with VSCF's (http://www.b737.org.uk/generators.htm) fitted. Identical in operation as far as the pilot is concerned, so identical that I think some aircraft even had a VSCF on one side and a CSD on the other.

We did have a UKCAA restriction on aircraft with VSCF's fitted that we had to stay within 45mins of a suitable airport rather than 1hr with CSD a/c, so maybe they were less reliable?

S&L

[bobcat4]

Fark'n'ell wrote:


HD
On the contrary, surely? It would be almost DC if it was 1 or 2Hz.... or is my aged brain playing tricks with my radio theory?

If it were 1 or 2Ghz it still a'int nearly DC



Well, let's say we have 0.000011574 Hz. This would have been seen as DC "the entire day", and DC with reverse polarity during the night.

Now, the real philosophical question is: When (at what frequency) will AC become DC, and close to that limit isn't the AC "nearly DC" ?

I guess that depends who you are. For a mayfly (http://en.wikipedia.org/wiki/Mayfly) a frequency of 0.000011574 Hz would surely feel like DC as it would not alter in it's entire life-cycle.
:O

Bob.

[radeng]

I seem to remember reading that the 'new' F22 fighter uses 270v DC. Not too sure why - one problem with DC is the switchgear needed - with AC, as the volts go through zero, provided the power factor isn't too bad, any arc extinguishes.

800Hz was used in some WW2 airborne radar. There was appreciable weight saving in transformers - and, for that matter, a saving in transformer quality steel, which was another consideration.

[L Peacock]

radeng

Some general aircraft LRUs use 270VDC internally.
(3 phase 115V (200Vline) bridge rectifiers naturally produce 270V.)

[Blacksheep]

To answer some of the questions posed:

The earliest A.C. systems were frequency wild at 208v which was then rectified to provide 112V D.C. for the main power busses.

So why 400 Hz? Early inverters were mainly used to provide power supplies for radio and radar equipment that 'liked' to have that frequency. It then became a standard. We actually used 1600 Hz single phase inverters for one particular piece of electronic kit in common military use back in the fifties and sixties. 400 Hz can be fed at high current loads without significant line drop caused by power cable inductance. At higher frequencies like that 1600 Hz I mentioned, line drop due to inductance is unacceptably high at the current ratings need by the aircraft power systems. As with everything in engineering, it is a compromise. Nevertheless the generator output is generally regulated at 208V which equates to 200V at the bus bar.

Most of the more powerful motors on aircraft are three phase and have no need to vary their speed - hydraulic pumps, flap and slat drives, fuel pumps even powered flying contol units (PFCUs) Three phase synchronous motors are more efficient than non-synchronous types.

The 115V 400 Hz outlet in galleys is a power outlet. Most electrical accessories - especially heating units and the cleaners' vacuum cleaners - are not frequency sensitive and work well on any frequency, as long as the voltage is correct.

D.C. is often quoted as being Zero Hertz but tackle it by Fouriers analysis and you soon see that it contains an infinite number of harmonics. In other words, think of it as 'Infinity' Hz.