115V/400Hz
 

Hello all,

A question for an electronics geek just out of curiosity... is there any particular reason why the standard aircraft power supply is 115Vac @ 400Hz? I ask this merely out of curiosity, since 400 Hz seems to be a high frequency, especially considering that your mains at home is only 50 Hz. Anyone have any explanation?:confused:

I work at an electronics factory that produces avionic systems, hence my interest!

Cheers

Smithy

@Rainboe
 

The gearing wouldn't be a problem, just a matter of number of teeth.

3 phase means basicly three AC circuits. On an oscilloscope you would see three sinus curves shifted by 120 degrees. Much more efficient than single phase and perfect for transmission to pointer gauges (28V AC).

Thanks folks.

Smithy

@Rainboe
 

Like I said, the answer is WEIGHT.

400 Hz alternators/generators and transformers need less copper and iron.

Hetfield is spot on. The reason is weight.

Do a yahoo search on three phase current images, there is a pix of three cycle sine wave. Why they picked 400 hz don't know, but the three phase also reduces the size of the wire thru out the a/c as each wire will carry more current/power.

Sorry for wrong spelling. Obviously it's sine not sinus.

Edit The thickness of a wire will be dictated by the current. Yes, if you have three wires instead of two for single phase, then they can be thinner...but then there is more of them!

Oddly, if there was a huge length of wire at 400 Htz there would be slightly more voltage drop on that line...therefore the wire would have to be thicker. A bit theoretical that.





Many of the voltage selections are somewhat historic.

The 400 Htz circuit is usually associated with electronics. As mentioned, smaller windings on transformers etc at 400 versus 50. The old flux-gate used to work at 400 samples per sec if I'm not mistaken.

115 is a handy voltage cos it is sometimes needed to put the voltage up rather than down...to 26v say, and this means that it can be transformed either way easily.

Now, the three phase AC coming off the engines is real man's electricity. It can cook your food (for those of you that can remember being fed) And it can pump enough power through your glass to keep it warm when it's -56c outside with a 500kt chill factor. It can run A/C hydraulic pumps powerful enough to drive flap motors in an emergency. That's a lot of power. However, whatever the nominal frequency, it's usually deemed to be ‘wild' cos at best it's only regulated by some sort of constant speed drive. This is a good device, but not up to controlling frequency to electronics accuracy. If you want to use some of this for electronics, it has to go through a Transformer Rectifier unit for DC or Transformer Inverter system for AC.

These devices are accurate, as the output is generated anew. 26v and 115v at 400 htz being an example.

In many cases, devices are so power hungry, (notably radar) that it might be that DC is taken from a substantial high priority buss, and the power cleaned up in the radar's internal power unit

But why 115 Volts?

400 Hz up in air BUT not on ground makes sense because of the inductivity losses. High frequency alternating current tends to have skin effects, proximity effects et cetera. When you have a plane at most 80 m long, the losses can be dealt with; in an extended network on ground you must have much lower frequency. High frequency limits the weight of transformers and other coils.

Now, as for voltage - the aircraft DC is said to be 28 V. Why this - not 24 or 36 V?

:confused:

Here's a typical 240 NM Weather Radar. Power Consumption: 4.2 amps @ 28 VAC 3.0 amps @ 115 VAC, 400 HZ Not much at all.

Not sure how theoretical. A problem with 400 Hz is that it easily induces currents elsewhere. Which means you have leak currents, you have proximity losses and skin effects... Those things get worse at low voltage - the lower your voltage, the stronger your current has to be, and therefore the stronger the induced currents.

A brief history
 

Cast your minds back folks...a w a y back to the good 'ole days, of straight wings, propellors and large piston engines.
Ahhh, feeling good already:ok:
Anyway, these aircraft were DC aeroplanes, normally using as ships batteries two large 12v batteries in series...and yes, 24v was used simply because the engine starting current draw was quite substantial, and 12v simply wouldn't cut it.
This was fine as far as it goes, but some instruments needed AC for proper operation, the 'ole Sperry C4 or C6 gyro compass systems, are a perfect example.
So, where to get the AC?
Some might say, well just hook up a solid state inverter, and all would be sweetness and light.
Problem was, there was no such animal at the time, as solid state electrics were quite rare.
Enter the DC driven electric motor, which in turn turned a small AC generator at a fixed RPM.
3 phase was choosen to keep the RPM's at a reasonable speed, so the unit, called the inverter, would not fly to bits...in normal operation, that is.
Single phase would have not worked well for the C4/C6 Sperry systems at any rate.
115v?
Common voltage in the USA.
Weight was a large factor as well.
If you ever have seen one of these motor-driven inverters, you would soon realise how heavy they were.
Three-phase and 115v helped with all this as well...made the units a practical weight.
However, not all is entirely sweetness and light.
IF, for example, one of the AC phases failed for whatever reason, many times the failure would not be known, IE: no red flags/lights to alert the crew, in the earlier days.
They were fitted later, however.
Why?
A couple of rather nasty accidents was the reason, which at the time were hard to identfy the probable cause.
Phase failed, Sperry compass indicated incorrectly, IMC (usually at night), and hard terrain was found.
Not good.:sad:

It takes 30 milli Amps to kill a person.
Just think of a person as a big resistor. The more Voltage you apply the bigger the current that flows through the resistor.
Ie Double the Volts 115 to 230 and double the current.
If don't apply the voltage at either end of the resistor (Wear rubber boots) It don't hurt.
As for 25000 Volts vs Humans If the contact is not enough ie very very big resistor you'll only get a little current.:eek:
On a slightly different topic
3 phase. The power delivered is constant that means for a motor it runs smoothly and very little vibration.
1 phase The power pulses at twice the mains frequency. Giving alot of vibration in a motor. You don't usually find a single phase motor bigger than 2 hp

High cycles/frequency equals smaller motors higher speed and smaller transformers less ripple in your converted DC
(from memory)
3 phase AC 115 Volts is 200V between phases but 115 to earth I can only guess why they picked this voltage but the USA background is a good guess.
Why 400 Htz don't know but a 6 pole generator spun at 12,000 rpm gives 400htz and 8 pole Generator at 8000 rpm gives 400 htz, nice round figures. etc
Higher frequency has side effect of a back emf against the flow of the current due to the collapsing electo-magnetic feild every time the sinusidal wave switches polarity ( AC ) it used to known as "reactance". Higher the frequency higher the reactance (resistance) until you reach radio wave frequencies then the whole theory changes again, but that is for another debate.
Just a little more
PS anyone whom thinks they can be less carefull around 115V than they would 240V at home may end up with an interesting epitaph

Yes, but why stop at 115V?

Think of the mathematics.

Double the voltage at a given power. You can halve your current.

Double the voltage and half the current means that for given acceptable resistivity losses, you can increase your resistance four times. Which means one-quarter the cross-section and weight of wires!

You do have to use thicker insulation, though. At some voltage, more insulator needed will compensate for saved conductor.

Now, in aircraft, you have high frequency. Which means high currents are a big problem - induction losses and leaks go up with the magnetic field!

You should like to have higher voltage on aircraft than on ground - exactly to save weight.

What are the AC voltages on a Comet or a Britannia or Caravelle? What about Tu-114 or Il-62?

There's both inductance and capacitance in power cables.
Don't loose site of what the power is going to be used for, The cost(weight) of beefing up the insulation on kettles Ovens DVD players would be off putting compared to the saving in weight of copper. We dont get 275KV electricity delivered to our door stop here in the UK.
:8

When building a transformer the core is of a certain Magnetical hardness. When changing the polarity in a transformer the core, like anything else, is resisting the change and and it takes a while before the core changes polarity as well, something called Hysteresis.

http://en.wikipedia.org/wiki/Hysteresis

To compensate for the losses the transformer designers have to build the transformers a bit bigger and thus heavier. Now, It turns out that at 50Hz (the normal european freq) the losses induced by Hysteresis are bigger than at 400Hz, so to keep the losses (and thus weight) as low as possible they settled on 400Hz.

This is all I remember from school, which was quite a while ago...

Rainboe

The risk is not really voltage related. For domestic supplies the current most likely to cause death is in the range of 0.1 amp - 1 amp, sometimes even lower currents can be fatal.

Higher currents, whilst still dangerous are less likely to upset the hearts rhythm.

(The other issue with AC is that it can cause horrendous burns, DC tends not to cause burns).

Just be careful and install earth leakage.

There is only one reason 115v 400hz is used and its..weight

Please no more comments about steadier power supplies or improved efficency....:ugh:

Its all to do with power to weight ratio, and 115v/400hz is most efficient

Volts/Amps
 

Being electrocuted is something one can get used to! I have a good friend who can take hold of domestic mains (240v/50Hz) and offer an opinion on whether the National Grid is delivering good quality power on any particular day! He was an electrician for many years but - crucially his skin/bodytissue resistance (a genetically endowed trait) was high. A potential difference of 250 volts could be applied across his fingers and because of his natural high resitance this was not enough to pass a lethal current through his body or vital organs (read heart)..

115 volts applied to damp skin on a person with normal conductivity may kill them..


As we used to say when we went through electrician school - 'It's Volts that jolts and mills (Milliamps) that kills' Meaning a big voltage can give you a shock but unless the voltage is sustained and the power source can deliver a sizeable current it won't kill you.

:ok:

3 phase motors (and generators) are more compact and lighter above a given HP (or KW) rating. 3 phase equipment makes more efficient use of power than single phase motors/generators.
3 phase motors also run smoother, start up with more torque than their single phase counterparts and are more reliable.

A single phase current crosses zero twice per cycle and has two peaks.
One negative and one positive.

http://www.shutter7.com/coppermine/a...normal_sin.gif

With 3 phase power, these peaks and zero crossing points are staggered:

http://www.shutter7.com/coppermine/a...mal_3Phase.gif

The graphics above should illustrate how 3 phase current can provide a broader area of max power transfer than single phase current.


411A mentions the original inverters - converting power by driving a generator with a motor. This is why we refer to current day electronic inverters as static.


Blah blah blah...
During a summer job at a plastics factory belonging to a friend of the family, I learned wiring of 3 phase motors was fairly simple - if you got it wrong the motor would just spin backwards - swap any two power wires and you're in business. :8

Since you ask (post19) from memory, and it's a very long time ago, Comet was 115v frequency wild generation - this used for de-ice only, rectified and regulated to 28vDC. Instrument supplies from three? DC-AC 115v 400Hz motor driven inverters. Engine start was 120v DC. I Think the Brit was similar but might have been 28v starters. I seem to remember that the Caravelle had the same ground power requirements as the Comet which would imply the same starter. The GPU cables used to go rigid with the current on engine start !

LOL, I've become a total dinosaur. I had a gut feeling that someone would come up with figures from the modern world. Our retro-fitted 12" radar on the Viscount used to make the lights go dim.

I can recall one of my lecturers at Cambridge (I was being polished, not the full degree) telling me about the time he copped hold of a 9kv winding from a hard-wired EHT transformer. When he left hospital he went back to his lab and looked at the scuff marks from his shoes trailing across the ceiling. They gave us plenty of amps to play with in those days.

Brits........Geeeeeeees, I have an old pilot's hand book at home, and the schematic is mind-blowing, let alone the full circuit.



Talking of rotary generator days, hands up who can tell me what a torque switch does.

Post WWII the US military had some aircraft & systems that ran 800 Hz (they called it 800 cps back then). Same rationale, just taken a step further. I don't know why it was abandoned.

I used to use a pocket calculator in the jump seat to do engine performance checks. It had a wall wart to recharge the battery, said transformer labelled 120v 50/60 hz. It worked fine and ran cool on aircraft 400 hz. But don't try it the other way around!!! :eek:

I rememebr the cables need to feed juice to the Ecko 190 radar. Thick as my arm! And the rotary inverters on the Jet Provost accounted for about 30% of it's AUW!
Looking back to aviation electronics history, the Handly Page Victor Mk1 of the early 1950s had 'Alternators' producing 208V frequency wild AC. Rotary inverters then provided frequency stable AC at 400 and 1600hz. As the aircraft had electric powered flying controls, a backup was needed. So it had ten (yes, ten!) 28V batteries! Hardly a weight efficient solution, but the best the age could produce.

Then in the late 50s, the Victor Mk2 came along with CSDU driven generators which produced 200v 3 phase 400hz power with backups of an APU and two ram air turbines. This system was very similar to the VC10's, (which I flew later) and the VC10's was almost identical to the 747's which didn't change much on the introduction of the -400.

I seem to remeber the Jetstream 200 had both AC generators and DC generators. Not many people copied that system!

The torque switch: If I remember correctly, it was on a rotary inverter and was held open by the current induced by the rotaion of the generator part of the inverter. If the rpm dropped, the switch closed and put on a warning light on the flight deck to tell you the AC supplies to you instruments were suspect. As always, I'm open to the inevitable correction on that piece of information.

There's another good reason for the 400Hz freq. and that is to provide greater DC quality (smooth and clean) as opposed to 50Hz. Weight and Volume are the other main reasons.

GD&L

OK, the 400Hz and 115V Ac is for weight issues, but what I'd like to know is whether the 3-phase electric motors used in aircraft systems are delta or star-wound?

If I recall, delta-wound motors don't require a neutral wire, thus saving weight again. I think US 3-phase domestic electric motors are always delta-wound whereas in the Antipodes where we have 415V 3-phase power we use star-winding which requires a fourth wire, the neutral - and woe betide your equipment if you have the dreaded "floating neutral"!

I'm guessing the aircraft 3-phase motors would therefore be delta-wound to save wire and thus save weight.

Full-wave rectification on a single-phase source yields a ripple frequency 2X the line frequency. A three-phase rectifier gives ripple freq. 6X the line freq., and yes, that's a lot easier to filter to pure DC. This further reduces weight and volume. :cool:

BTW

A380 and B787 will have 380Hz - 800Hz AC power

Star/Delta Motors
 

Both configurations are used. Heavy duty motors (hydraulic pump drives etc.) are often started in Star to limit starting current. As the motor approaches full speed the connection is switched to Delta allowing full current and maximum torque.

WRT to battery voltage.

Batteries are nominally 12 or 24 volt.

The busses that they run are normally powered from a generator or alternator source which provides 14 or 28 volt in order to be able to charge the battery in the circuit.

So a car or aeroplane may be 12 volt but it runs on 14 volt.

Yay, I've had both US and UK mains shocks messing around with stuff in a stupid way. The only thing that scared me (and I never did) was catching hold of the high voltage DC line within a valve amp. People suggested this would cause a 'hold on until you are dead' situation....

Got bitten by the stored voltage in the cable of a colour TV tripler that had been switched off for 12 hours - just the voltage due to the stored charge in the capacitance of the cable. Dunno what it was (15Kv?) but it threw me backwards across the workshop, my hand was numb and the arm didn't work from the elbow down for about 12 hours.

Not intending to do that again in a hurry.

If we're talking belts....As a young sprog was absolutely fascinated by the rotating lights on a phase rotation meter placed across 3 phase 115v 4OOhz. Why I decided to remove it without isolating the power supply I will never know....ouch :eek:

If we're talking HV anecdotes (glances warily at TPTB)...

I used to work in a TV repair shop in the early 80s in Plymouth. One of the owners came from the Rank Bush Murphy factory there, and _really_ should have known better, but...

There was a TV on the bench, back off, facing the wall, with a line output fault, and he wasn't sure whether it was it was in the EHT tripler/smoother circuit or earlier. I can't remember which chassis it was, but it was one of those more recent jobs where the LOPT also provided most of the rest of the voltages for the set, so all manner of problems could manifest as low/no line output.

Anyway, he had the EHT anode lead off, and the set on. He went to get the EHT meter, which was on a shelf above the bench, and in so doing bumped the chassis with his crotch. This had two immediate consequences, and one rather more long-term effect.

in short order, the EHT connector moved and made contact with the metal fly on his trousers, and the fault - subsequently traced to an intermittent solder joint in the line oscillator - temporarily cleared. The longer term consequences mostly wore off after a night in Freedom Fields hospital 'under observation' from a curiously attentive medical staff. Mostly.

Relevance to aviation? Yer man certainly got airborne.

R

A long time ago in a high-energy lab far, far away...

We were testing a transmitter for a phased-array military radar. These devices were pretty heavy duty as the peak power output ran into the MW range and the power supplies were just as beefy.

Because of the lethal potential (ha ha) of a lot of the equipment inside the room (about 30m x 20m x 10m, fully shielded), one of the rules was you didn't ever work on something on your own. I remember a pair of us coming in one afternoon and hearing this muffled rhythmical thumping from the other side of the room; on closer inspection we saw a twitching pair of legs sticking out from under the transmitter. :ooh: We immediately hit all the emergency cutoffs, then went in with the earthing sticks...

It turned out that one of our more senior engineers (who should have known better) had decided to do a bit of work on this device. After he came back from hospital, badly bruised but without long-term injury, he explained that he'd brushed a high voltage terminal whilst working under the radar and that had flung him away. Unfortunately, he bounced off the floor and straight back into the wiring, which threw him back at the floor etc. Very luckily for him we had chosen to walk into the lab, pretty much at the same time as this started - I've no doubt he would have been killed within minutes without intervention. :ouch:

KIDS! DON'T DO THIS AT HOME!

Two spots of trivia....

German aircraft in WWII used 500Hz AC systems.
So why do we now use 400Hz? Guess who won the war....

To transport electric power over long distances, the lower the frequency the better. That's why Swiss railways use 16 2/3 Hz (one-third of 50Hz).
(DC would be even better, but unfortunately the wooden-core DC transformer is still a myth).

How's that again? Long distances? Swiss railways?

:confused: :confused:

:E