115V/400Hz
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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?
I work at an electronics factory that produces avionic systems, hence my interest!
Cheers
Smithy
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?
I work at an electronics factory that produces avionic systems, hence my interest!
Cheers
Smithy
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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.
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@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).
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).
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@Rainboe
Like I said, the answer is WEIGHT.
400 Hz alternators/generators and transformers need less copper and iron.
400 Hz alternators/generators and transformers need less copper and iron.
Last edited by hetfield; 14th May 2007 at 14:43.
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.
Psychophysiological entity
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
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.
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
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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?
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?
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In many cases, devices are so power hungry, (notably radar)
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.
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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.
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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
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.
Ahhh, feeling good already
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.
Last edited by 411A; 14th May 2007 at 18:00.
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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.
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
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.
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
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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
(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
Last edited by Bolty McBolt; 15th May 2007 at 03:28.
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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.
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.
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?
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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.
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.
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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...
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...
N4790P
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.
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.
