115V/400Hz

[Captain Smithy]

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

[hetfield]

The advantage of high-frequency alternators is that they require fewer copper coils in order to generate the necessary electrical current. This reduction in material allows the alternator to become much smaller such that it takes up less space and weighs much less than it would otherwise.

[hetfield]

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).

[Captain Smithy]

Thanks folks.

Smithy

[hetfield]

Like I said, the answer is WEIGHT.

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

[alexban]

Hetfield is spot on. The reason is weight.

[rigpiggy]

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.

[hetfield]

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

[Loose rivets]

Edit 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.
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

[chornedsnorkack]

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?

[forget]

In many cases, devices are so power hungry, (notably radar) :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.

[chornedsnorkack]

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.

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.

[411A]

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:

[snips]

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

[Bolty McBolt]

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

[chornedsnorkack]

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.

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?

[snips]

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

[Coffee Black None]

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...

[ZFT]

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.

[ARINC]

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

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

[3bars]

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

[8846]

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:

[vapilot2004]

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/albums/userpics/10270/normal_sin.gif

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

http://www.shutter7.com/coppermine/albums/userpics/10270/normal_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

[Wodrick]

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 !

[Loose rivets]

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.

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.

'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.


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.

[barit1]

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:

[Dan Winterland]

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.

[GearDown&Locked]

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

[criticalmass]

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.

[barit1]

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

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:

[hetfield]

BTW

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

[ColdWarWimp]

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.

[javelin]

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.

[tom775257]

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....

[criticalmass]

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.

[ARINC]

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:

[Self Loading Freight]

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

[FullWings]

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!

[ChristiaanJ]

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).

[barit1]

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).

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

:confused: :confused:

:E

[ChristiaanJ]

barit1,How's that again? Long distances? Swiss railways?We were talking AC frequency, no?
In aircraft, where you transmit power only over a couple of hundred feet at the most, the losses in the wiring (skin effect, etc.) are far less significant than the gain in weight of transformers and other equipment.

When you have to transmit power to a train over what may be tens of kilometres (depending on the distance between substations), the equation flips the other way, and switching to a lower frequency than even the standard European 50Hz starts making sense. Which is was the Swiss did.

[barit1]

No technical quarrel, ChristiaanJ. I'm just not calibrated to think in terms of long distances in Switzerland.

Texas maybe, or certainly Oz, or... :cool:

(A terrifying thought: How did they know 16 2/3 hz wouldn't interfere with the alphorns? :ugh: )

[WHBM]

Train buffs department.

The Swiss (and German and Austrian) electric railways use 16 2/3 Hz supply because they were initially designed in 1910-1920, and were in fact the first major AC railways, previous ones being DC apart from some experiments. They only continue with this system because of what they already have in place.

The low frequency was chosen primarily because they were using comparatively large and heavy mechanical elements for which 1,000 rpm was OK but 3,000 rpm was not. Nothing to do with the transmission but to do with the various revolving parts in the substation and on the locomotive.

Since that time any new electric AC railway system has invariably used 50 Hz. The arrival of semiconductors in the 1950s to take the place of the rotating machines sealed the point.

The railway in the US from New York to Washington was electrified at 25 Hz in the early 1930s but in recent times they did do a conversion to 50 Hz (and had a lot of changeover work to do, it must have been worth it).

http://en.wikipedia.org/wiki/15_kV_AC

If you are concerned about long-distance transmission (or, on a railway, long distances between substations, which are expensive) you use as high a voltage as you can, hence the 25Kv standard. One or two new schemes have actually gone for 50 Kv.

[Jo90]

1. Many people have survived being struck by lightening. Some of them several times. Very high VOLTAGES are not necessarily fatal.

2. Grazing cattle are often killed by nearby - not direct - lightening strikes because of the steep voltage gradient in the earth between their front and rear feet. Moral : if caught out in a storm keep your feet close together.

3. Electric chairs used for killing people in certain barbaric cultures use DC.

4. Long undersea cables used to interconnect electricity grids in different countries eg France and UK use DC.

[dixi188]

Off the track a bit, but electric fencer to keep cattle, sheep, horses, etc. give 5-8000 volts. Caught it a few times and it makes you jump but no harm done.
This is because the current is very low.
Safe flying!

[Pinkman]

One of the more dubious changes I have seen is the new BA club world at-seat power supply which is now 115v at the floor rather than the 15v empower system.

Despite the fact that inevitably it will have trip protection, how can it be safer?

I used the system recently and I continually got 'belts' from touching the metal parts of the new seat. In the dark, upstairs on a 744, I rested my right hand on the metal of the table, and my left hand brushed the side of my laptop, one of these 'wee blue sparks" jumped from my hand to the unoccupied USB port on the machine, which promptly said "there has been a power surge and this point will be shut down as a precaution".

So I am not convinced its a positive development. Is this a grounding issue?

[Captain Smithy]

Jo90/Dixi188 - Correct, it's not the Voltage that kills you, it's the current.

Some of the nastiest shocks come from discharging Capacitors... ouch :ouch:

[IO540]

Going to 400Hz rather than say 50/60Hz does not save much copper weight but it saves the iron which makes up the magnetic components of transformers and motors.

In a typical 50Hz 3000VA transformer (that uses reasonable quality laminations) the core would weigh around 20kg.

At 400Hz this goes down to around 5kg; a big weight saving.

There is some copper weight saving because the turns per volt scale inversely with the cross sectional area of the core etc and a smaller transformer will need a shorter length of wire to make each turn around the core. But there is no wire diameter saving because the current is still the same.

Skin effect is negligible at 400Hz.

The other reason for 400Hz is that in the old days one did have solid state DC-AC inverters; one used a motor driving a generator to do such conversions. And the output frequency is the RPM divided by the # of poles of the rotor. The smallest number of poles physically possible is 2 and that means a 50Hz generator has to run at 3000rpm. This is inconveniently slow. A 400Hz generator can run at 24000rpm or, more usefully, can have more poles for the same rpm and thus be more compact.

Nowadays one would not bother with 400Hz because of solid state inverters using lightweight ferrite cores and operating at tens of kHz, but this is old history.

[west lakes]

Voltage
Usa does generally use 115v 60hz A.C. but has other voltages 220etc available in some areas. It is correct that the lower voltage is "safer" in similar conditions than say the european standard 230v. it is also down to the design of the transformers as in distrubution systems all voltages are referenced to earth. The neutral generally being connected to earth.
In many areas the earth connection is actually in the center of the transformer winding so halving the voltage to earth this in the UK is particularly on building sites where 230/110 volt transformers are used - so you get 50v from either output wire to earth but 110v between. Some U.S. systems use this.
The U.K. power system was designed for small numbers of large substations feeding an area so the higher voltage (240v) was used to reduce voltage drop problems on longer runs, though Blackpool for one originally was a 110v system. The U.S. system uses a mid level (possibly 1000v distrubution with smaller transformers feeding individual or small groups of properties.
Both work for them.
DC undersea cables i.e. France to U.K. is because France uses 60Hz frequency & U.K. 50 Hz - try connecting them together and it is BANG Big BANG. also losses in D.C are lower at Higher currents.

Swiss railways vwith 16 2/3 Hz was simply that before the invention of clever electronic kit A.C. motors were virtually impossible to speed control. It was found that by using that frequency D.C. controls & motors could be used thus saving the expense of costly A.C. to D.C. rotary convertors or latterly massive valve types.

3phase motors are much simpler as a single phase motor needs mulit sets of ciols and a capacitor to start it. the simplest 3 phase motors need 3 static coils and an aluminium (usually) cage that spins, the 3 pase sets up a rotating magnetic field, this is induced into the cage and it spins.

Sorry can'y help on the 400hz bit unless it is to do with connecting alternators in parrallel - but I don'y know if that is done on a/c

Sorry thats long & hope it helps

[ChristiaanJ]

Rainboe,
Good question, and I hope somebody well-informed can fill us in on the background.

My first reaction was "weight"....
"Hydraulic" is now a "mature" technology, and you'd need a breakthrough to significantly bring down the weight.
"Electric" is still evolving, with new materials, more efficient switching and control electronics, etc.

So possibly the "cross-over point" has been reached, where tossing out the entire hydraulic system and replacing everything, even the "big" stuff, with electrical systems, allows you to bring down the weight enough to make it worth the effort.

Happened years ago on choppers, when we started replacing hydraulic flying control actuators (autostab, autopilot) with electrical ones.

[ChristiaanJ]

Thanks west lakes,
At least the civil aircraft industry ended up standardising largely on 115V 400Hz and 28VDC, unlike the railways, where everybody went their own way.
1.5kV DC, 3kV DC, 750V DC third rail, 25kV 50Hz, 15kV 16 2/3 Hz.... take your pick. And there are certainly others.
Hence some TGVs runing on international routes, and the Thalys, and the Eurostar, which are all "tri-tension" (but not the same voltages, LOL)..

[west lakes]

yea not forgetting the Frence CC 40100 loco with 4 voltage choices!

Forget to mention the current bits
50mA (.05) can kill an adult!! especially if current flow is through the heart.
Myth - the electricity threw me
Sorry your own automatic reflexes couples with your muscle contaction does the throwing usually around 30mA. Lots of reports in my industry of severe injuries to the guy stood behind as someones arm retracts from the shock!
Cattle fences are usually around 50v as a voltage that low can produce enough current to kill a cow. warning they are very painful if you step across one and end up with one leg either side & contact is made - think about it!

[IO540]

50mA (.05) can kill an adult!! especially if current flow is through the heart.

There is almost no way to kill with electricity unless current passes through the heart. Anywhere else, you need a lot more current; enough to cause tissue damage through heating.

Cattle fences are about 10,000V - I have designed a few of the units :) One uses a car ignition coil, usually. The current is very low. But they can kill a horse (via a heart attack, presumably) if it gets tangled up in the wire and I am aware of one case near here.

[ChristiaanJ]

IO540 is right....and at voltages below 50V it becomes difficult to get enough current across a human body to kill, even with wet hands.
IIRC the standard for low-voltage power tools for hazardous environments is 48V, but I may be wrong.
115V is about the limit.... I've had the odd whack from 220V and 240V, but just been lucky.

Maybe a historian can tell us why America choose 100V (nominal, the 110V, then 115V, is just to account for the losses). Maybe that was OK for the insulation materials of the day, and by the time technology progressed, too many installations were in place to change?

Europe went for 200V (again 220V and 240V to account for the losses). Some low-voltage distribution systems went for 127V, which is the phase-to-neutral voltage for a three-phase network with 220V between phases.

As to cattle grids, yes, they're 5kV to 10kV but such a high internal resistance (that good old law of Ohm...) that they're not capable of delivering a mortal current, just a very nasty shock....

Sorry to hear about the horse.... I would have thought some sort of simple trip device in the supply could prevent that.

[james ozzie]

The comments on the dangers of voltages refer.
There are numerous accounts in the electricty industry of linesmen surviving high voltage accidents (11kV, 33kV and even higher) as a result of the discharge arc tending to track on the surface of the skin rather than through the heart. The bad news is that these accidents result in horrendous burns with subsequent death or disfigurement.
It is interesting to see that although high voltage network switching is rigourously controlled with SOPs, permits, cross checking, control rooms with radio links, radio procedures and readbacks etc etc, accidents still occur as result of a combination of things like time pressure, severe weather, taking shortcuts, attention lapses etc - sound familiar???

[west lakes]

I'll stand corrected on the fence question.

ChristiaanJ
Not sure I agree and your take on relation of phase to neutral & phase to phase Uk system is ph - ph is 1.732 (root3) times ph to n. i.e before beurope standardised 415/240.

James Ozzie
Yes burns are the bigger risk, injured persons have had to have limbs removed to avoid death.
Though all this is off the original thread hope it shows the dangers of electricity.
But the good news is that like airline industry such major incidents are rare biggest cause of injury to UK staff is slipping, tripping and falling over - is that familiar also

[ChristiaanJ]

james ozzie,
Only a few days ago, a kid got killed here in France, climbing on top of a goods wagon - to recover a football IIRC. Drew an arc from the live 25kV overhead catenary .... Got quite a lot of TV coverage, to alert as many people as possible.

I like this thread, even if it deals with a lot of disjointed subjects....
The choice of 115V, of 400Hz, of single-phase and three-phase, of 28V DC, of DC for certain circuits and AC for others, are in a way all totally disconnected.

In a different world, we might well have seen 230V, 500Hz, and 48V DC as the aviation standard.....

And to add another spot of trivia....
Concorde electrical signalling (fly-by-wire if you prefer) used 1800Hz for the synchros, etc. to cut down as much as possible on interference from harmonics of the basic 400Hz systems.

[west lakes]

ChristiaanJ
The accident you describe is not uncommon over here, as well as the ill advised that try to walk?? along third rail DC lines in the south.
All absolutely tragic and unneccesary.

Saw a photo of two overhead transformers in Scotland this week, covered in gaffiti. The "artist" had been within 0.5m of live 11,000V (our safety approach is 1.2m

All this despite warning signs on all live equipment as per EEC

[ChristiaanJ]

Not sure I agree and your take on relation of phase to neutral & phase to phase Uk system is ph - ph is 1.732 (root3) times ph to n. i.e before europe standardised 415/240.The figures I quoted were Holland, not too long ago (I'm talking about the 220V, the 127V went out of use in the late sixties.... I think The Hague was about the last town to have it).

UK is nominally "240V". "Europe" is nominally "220V", although most has now shifted to "230V".
So 127V/220V is right. And "Europe" is standardised "220/380". At least that's still the terminology, although it's now really "230/400". Only the UK is 240/415 nominal.

And another spot of trivia....
Nowadays a lot of equipment is capable of working over a very wide range of voltages. Thingies like hair curlers or traveling irons, or battery chargers, will often work between 100V and 240V without even having to set a switch.

But light bulbs.... we brought a batch of nominally 240V light bulbs back from the UK to France.... and boyo, they lasted far longer than the nominally 230V bulbs we bought here..... Yet the difference is less than 5%.

[west lakes]

were all supposed to be 230/400, the main defference is the permitted variation. we had + or- 6% ion 240V its now +10 -6.

the lightbulb one is good one - you should see how short a life a 230v bulb has here! Actually saw on a US website advice to use europen bulbs there on 115V - apparently they last forever:)

[IO540]

The "artist" had been within 0.5m of live 11,000V (our safety approach is 1.2m

I used to design HV power supply units, and it's suprising how (not) far electricity will jump across a dry air barrier.

30kV DC will jump about 5mm; maybe 10mm if between sharp points. A 25kV AC wire would need to be practically touched before it will get you.

The highest I ever worked on was 500kV DC and that would jump about 1-2ft, between aluminium balls about 1ft diameter. The customer required it to withstand a number of short circuits - quite spectacular :)

1.2m at 11kV is hugely excessive but probably right if one is to assume the person might slip and fall over as well.

Back to the subject, 400Hz is also used to drive instrument movements that use the synchro principle; these would need to be much bigger if working at say 400Hz, and if working at much more, say 4000Hz, they would probably end up emitting noise. I believe 400Hz is also used for the fluxgate magnetometer. So there is so much stuff that has been designed around 400Hz over so many years, and I guess most of it is pretty proven and reliable, that nobody wants to change it. The electronics designers working on avionics are a long way from the sharpest - with annovation in avionics moving at snail's pace, most good people leave - and it pays to stick to tried and tested old principles.

[west lakes]

Hmm
Just remembered london underground used to use high frequency a.c. (and possibly british rail) when flourescent lights were first introduced. this was to keep size of rotary dc to ac convertors down & also to reduce the flicker effect from the lights (as the sine wave hits zero volts) in an enclosed space. 50hz was/is thought to be connected to epileptic fits. this does not affect filament bulbs. wouldn't surprise me if the same thoughts were applied as a factor in a/c

[ChristiaanJ]

west lakes,
Epileptic fits are associated with lower frequencies (several Hz, but not 50Hz). Fluorescent tubes flicker at 100Hz, BTW.

I would think if they indeed used higher frequency AC, it was for the same reasons as in aircraft : lower weight, smaller transformer cores, smaller inverters, smaller chokes for the TL tubes, etc.

[west lakes]

ChristiaanJ

agreed thats why i said was/is (some people don't belive those who know).
Have seen old electrical books that used to advise spreading flourescents over 3 phases to reduce this effect which would work out at 300hz.
at the end of the day some of the answers sought will have
been lost in the mists of time

Bon Soir (about the only french I can remember)

[Loose rivets]

1. Many people have survived being struck by lightening. Some of them several times. Very high VOLTAGES are not necessarily fatal.



I'm not sure if it was in the Guinness book of records or some other source, but I recall reading that 11 times was the maximum recorded . I'm also not sure if it was the 11th or 12th strike, that hit his tombstone!:uhoh:

I got hit by a spray of secondaries, on a boat dock while cranking the jet ski into the water. It bl00dy hurt. :ooh:

[balsa model]

To all people arguing what kills and what doesn't (current vs. voltage):
If you're still young at heart (as I once was), you can go on a country ride and stop by a pasture with an electric fence. Grab a 2 ft long green (still moist, not dried out) grass blade by one end and rest the other end on the fence. You shouldn't feel anything. Slowly slide the grass blade towards the fence. At some point you should feel a somewhat unpleasant pulsed sensation. It will get stronger the closer you get to the fence wire. (If you are within 1 inch of the wire and still don't feel anything, the fence is OFF, or your grass is dead dry: abort.). If you did this experiment with a 10000V transmission line, you would likely be dead and learn a valuable lesson.
What protected you is the combination of your skin resistance, the (adjustable here) resistance of the grass blade, and the internal resistance of the fence generator. There is also pulsed, limited power nature of the generator, but let's not complicate things. The 10000V transmission line has pretty much no limits of its own. A capacitor charged to 10000V is also not likely to kill you because of the limited energy it can store and deliver.
Now, you can grab a 12V battery, one terminal with each hand and won't even notice. This battery could deliver over 100 Amps but here nothing happens.
You need a combination of sufficient voltage and low enough total path resistance to deliver the killing jolt. (Yes, it has to go through the heart. Time also matters.)
The resistance of your skin protects you up to about 40V. (It's just a rule of thumb, don't sue me.) Above that, please treat all wires as if they were spinning propellers.
So to conclude, yes, for academic purposes, it is the current through the heart of sufficient duration that kills. But when you face an exposed wire, for your safety you should mind its voltage. You will rarely know what limits are set in the generator circuits.
Poorly informed speculation:
Curiously, I used to think that you are probably more likely to be electrocuted by a household voltage of 110/220V than with a 10000V line because before your hand can get close enough to the 10000V line to convulse and latch on to it, it will arc through (high resistance) air gap of a few inches giving you a nasty warning.
BM
(edits in italics after learning from the wiser post below)

[james ozzie]

Balsa,
I think you summarised it very well but your last comment perhaps needs clarification. If you were to draw an arc off a high voltage utility conductor to your body, you WOULD be history. I have an earlier posting on this subject which points out the not infrequent cases of power arcs tracking over the skin surface, resulting in appalling burns but not sending the heart into fibrillation. The poor victim instead dies a very nasty death from the burns and associated infections. Of course, people have survived the burns but I have no statistics.

[ChristiaanJ]

....before your hand can get close enough to the 10000V line....People have climbed onto railway carriages, or goods wagons, with a live 25kV catenary overhead. No need to put out your hand.... the moment you stand upright, it's curtains.

[balsa model]

Thanks for the warning. I wasn't going to test this theory.
I am not an expert by any means in this field, but reading survivor stories, it seems to me that in the case of "arcing electrocution" (where there is no direct contact with the conductor), something else is happenning, at least some of the time. Is it that it is the arcing plasma that causes all the burns? Does it have some momentum?
I would expect the skin to be penetrated in two spots only and our body fluids to form an attractive path of least resistance in between. (Sorry to be so non-chalantly graphic about it.) So why such extensive skin burns?
BM
(not morbid for its own sake, just curious)

[james ozzie]

BM,

Yes, it is the plasma that burns (not the electricty). The plasma is the "4th state of matter" and is a very good conductor of electricity. The initial microscopic breakdown of air into ionised components allows a very rapid build up of heat therefore more ions therefore more conduction therefore more heat - hence a "power arc" develops in microseconds after the initial discharge (much faster than you can move your hand away...). A similar sequence applies with lightning strikes.

The very nature of this stuff is such that there is little analytical information available - it is all empirical/anecdotal. My guess that the reason power arcs track the skin surface is that the plasma is so hot and so well established that it is actually a lower resistance path than the wet body fluids. This would fit the instances of these observed surface burn injuries.

[west lakes]

Some additional bits and pieces.
IO540
Just checked clearances in the UK supply industry are: -
up to 33kV .8m, 132kV 0.7m added to these is an application factor dependant on the work involved e.g. when climbing a pole add .3m, working platform level on a pole add 2.1m. I'd added an extra .1 earlier - coward. these apply in all conditions and are err on the side of safety.

Plasma

Probably the major factor, try putting your bare hand near a welding arc see what happens (definately don't try this at home). As you say a lot of data is not analyitical. We are advised to wear cottton clothing (synthetics melt into the skin - seen this on advice for air travel as well) and are issued with Nomex Arc-Resistant clothing. This is tested, in Canada, to 25kV. we are told that if worn over no other clothing body protected from heat effects of arc.

As B.M. says depends on Ohms law voltage/resistance = current. Whist the 50mA figure I quoted earlier is accepted its only an average, similarly body resistence varys on the individual and on external e.g. weather conditions.

Deep burns seem to be one of the major causes of severe effects. I am sure we all have burnt the odd finger etc. over our lives these cause a blister and swelling. Current flow through the body causes the same under the skin E.G. to muscles etc. these cause limbs to swell and can only be relieved by surgically cutting the skin or ulimately amputation.


Curiously, I used to think that you are probably more likely to be electrocuted by a household voltage of 110/220V

B.M.
actually not far wrong, far more killed by electrical shock in domestic incidents at lower voltages than work related at higher voltages. Also at lower voltages the arc & burning effects are less likely and less evident. At higher voltages the shear ammount of burns can hide the fact that the heart was affected by electrical shock, it could have been physical shock!


I would expect the skin to be penetrated in two spots


If one or both them spots are fingers or toes you can also expect to lose them, heating effect of current flow through a resistance. In most UK systems the live to earth current can be up to 1000A (we limit it)

Some anecdotes

Early 80's a worker came into contact with an 11kV line whilst erecting an aluminuim flag pole at an agricultural show (about 15 miles from where I'm sat) I wasn't involved with the investigation, too junior at the time, but he survived but lost a number of toes in the incident and sufferd burns to the palms of his hands.
6 months earlier a 14 yr old killed erecting a CB aerial contact with an 11kv line.

A worker wheeling a portable scaffold came into contact with a 132kv line, survived, current flow burnt off parts of limbs, deep burns led to amputation of lower arms and legs. Appeared on some safety fims telling his story. Sadly I have been told he eventually took his own life.

90's A mentally ill patient left a local hospital deliberately climbed a 132kV pylon, the barbed wire and climbing guard didn't bother him, leant out and lost his life with contact with live conductor. The steelwork apparently was a mess afterwards.



Only a few days ago, a kid got killed here in France, climbing on top of a goods wagon - to recover a football IIRC. Drew an arc from the live 25kV overhead catenary .... Got quite a lot of TV coverage, to alert as many people as possible.



Christaan J
2 weeks later a "drunken" young person climbed an 11kv pole in southern england same result.

Don't forget also on high structures if the current doesn't kill you the fall might!
On that note and slightly lighter, in the 70's a person in the Bristol area decided to take his life by climbing a 400kV pylon, climbed the structure, climbed down the insulator string ans sat on the conductors - nothing happenned. Decided he had rubber soled shoes on so climbed back up the insulayors took them off and climbed back down - nothing happenned. was eventually rescued from the position.
It was a dry day the insultator strings are long enough not to be bridged when climbing down them.
On returning him to ground level a remark was made he would have been better jumping he hasd been at 100m agl

[Santy]

Hi


It would be my pleasure if you help me in my problem.
I am working on an Alternating current motor for use in an actuator for using an a model. (115v 400Hz a tiny motor)

But my customer wants me to pass all test that a real airplane have.
I have a problem in High temp stall. In this test when the shaft of rotor is locked, we put the motor in a high temp chamber and energized it. The motor should be bear about 1 hour without any problem.
but after about 15 min when the temp raise to 140 C it is failed, May you please help me in this problem?


Best regard
Eng, Santi Chou

[Intruder]

Simple approach: A slow-blow fuse or circuit breaker, near the normal max draw of the motor.

[MurphyWasRight]

On why 400hz
The original reason was a tradeoff between weight ( higher frequency is better) and iron core losses due to hysteresis where lower is better. At the time initial systems were designed ~400 was the sweet spot.
Core material has evolved but too much installed base to make it worth changing.

[EDDT]

1.
My wall charger (iPhone, Laptop) says it would accept 50-60Hz.
I have never found a problem with using them on any plane outlet though. Cockpit, Galley, Lavatory.
Why is it that way? Does it not matter if it's 50, 400 or 10000 Hz ?

2.
Are the shaving outlets in the Lav any different?
Because old shavers used to run a little faster in the USA (60Hz) than back home in Germany (50Hz).

[vapilot2004]

The motor should be bear about 1 hour without any problem.
but after about 15 min when the temp raise to 140 C it is failed, May you please help me in this problem?

A thermal cutoff, mounted inside the motor frame should do the trick. Your mileage may vary depending on motor quality and chosen cutoff temperature. Good luck, M. Chou.

[MurphyWasRight]

1.
My wall charger (iPhone, Laptop) says it would accept 50-60Hz.
I have never found a problem with using them on any plane outlet though. Cockpit, Galley, Lavatory.
Why is it that way? Does it not matter if it's 50, 400 or 10000 Hz ?

2.
Are the shaving outlets in the Lav any different?
Because old shavers used to run a little faster in the USA (60Hz) than back home in Germany (50Hz).
Most modern 'wall warts' use high frequency switching supplies, in these the first step is to convert the incoming power to DC. That is why they work just fine at 400.

As to shavers, good question. I believe old shaver did not use a motor as such, just a coil wiggling a metal armature that moved the blades so would would run -much- faster at 400hz.

[Chu Chu]

I've still got a couple of those cheap old clocks with the synchronous AC motors -- they keep perfect time! I guess if I took one on board and plugged into the shaver outlet, I'd really see time fly. :O

[MurphyWasRight]

I think EDDT's point was that shavers tend to work as expected when connected to the dedicated 'shaver' outlets. I suspect there must be a 50/60hz inverter on those for this to be the case.

BTW: Trivia point on electric clocks, the power companies don't actually hold 50/60 super accurately, can be a little fast as load goes down etc.

What they do is adjust the frequency so on average the clocks keep perfect time, using their own clock and an external standard.

[Chu Chu]

I remember seeing a 120V/400HZ sticker in an aircraft lav and wondering about it, long before this thread even started. But things may well have changed, especially in these days of cheap inverters.

Much more recently I read about an experiment they planned on some portion of the U.S. grid, where they would relax the relatively tight existing tolerances on frequency. Apparently, it saves energy if they don't have to pour on the coal (perhaps literally) to hold frequency when the demand surges.

[Sevarg]

On early intercoms the band was 500 to 3500 cycles (no hzs in those days). To stop the leakage of noise from the A/C power to the I/c 400 c/s was used. Greater than 3500 and the loses became too high.
All before my time but I remember am old crusty one telling me.

[rmm]

Two 737 BBJ's that I've worked on had 240V 50Hz single phase outlets throughout the cabin. They had two inverters mounted in the stab compartment.

From memory it becomes a problem if try and run a 400Hz device off 50/60Hz rather than the other way around.