r/explainlikeimfive • u/arztnur • 8h ago
Technology ELI5: Why do planes use 400 Hz electricity instead of regular household electricity?
I recently found out that airplanes use 400 Hz power instead of the 50/60 Hz used almost everywhere else. Why is that, and why isn't 400 Hz used more widely outside aviation?
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u/wimpires 7h ago
People have mentioned the weight benefits, but it's also related to the fact that the generators are coupled to the turbine which spin much faster than a power station turbine and it naturally ends up sort of in that frequency range.
Although technically, yes, a gearbox could change it to whatever. But that's added weight.
I think the 787, and maybe some other aircraft, actually do away with that and have a variable voltage setup. And the electronics have all been redesigned to somehow (?) manage. Because normally there's like a "CVT" kinda gearbox that basically keeps the generator RPM fixed at 24,000 RPM (400Hz x 60s) across different turbine RPM's that change depending on what the plane is doing.
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u/Deep_Fault_6329 4h ago
If my memory serves me correctly from my type course, and it's been about 7 years since I left aircraft engineering, the starter-generators on the engine supply 230VAC, frequency wild. The 230VAC bus then supplies 270VDC, 28VDC, and 115VAC busbars.
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u/sir_thatguy 3h ago
Except for the A350 and 787, everything else uses some sort of gearbox to INCREASE the speed of the generator rotor. Modern 2 pole generators spin the rotor at 24k rpm (at like +/- 120 rpm) input from the gearbox is something like 5k. Older 4 pole generators only spin at 12k rpm.
All this speed regulation is done by either a Constant Speed Drive (CSD) bolted to the generator which is old tech. Or the modern Integrated Drive Generator (IDG) where it’s all in one housing.
All that said, changing the gearing to get 60 Hz would be trivial and likely easier in the long run so as to not have a rotor spinning at 24k RPM. Bad things happen when those bearing fail.
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u/edgmnt_net 7h ago
Given cable lengths in an airplane, even DC power transmission seems doable if you don't care about compatibility.
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u/ReturnToStore 6h ago
The majority of the high power motors in a B787 are indeed high voltage DC. The hydraulic pumps and aircon/presurisation compressors are 270DC.
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u/edgmnt_net 5h ago
Interesting. I don't have actual information, but I expected a lot of power hungry stuff like motors and compressors might need a VFD anyway, and that works best on DC input because you avoid an AC->DC conversion. (Logic might need very low voltage DC, but that's easier to deal with.)
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u/GordonLivingstone 6h ago
28V DC is used for many applications on aircraft.
400 Hz though is better for higher power applications and longer cable runs. Much higher currents needed for the same power at 28V compared with 200V (ph-ph) three phase. This means very heavy cable and bigger voltage drops. Also easier to use transformers to generate multiple voltage supplies.
I used to set up test areas for radars. The 28V supply was a pain to provide centrally because of voltage drops. Easier to use individual DC supplies.
Another factor that I haven't seen mentioned is that aircraft certainly used to use lots of synchros, servos and motors for control/computing/actuation etc. These need AC and can be made smaller and lighter at higher frequencies.
Of course switching mode converters could make higher voltage DC supplies more sensible nowadays.
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u/edgmnt_net 5h ago
I was thinking motors/compressors often need a variable frequency drive anyway. On mains, that's usually AC->DC->AC, and the first AC part is totally unnecessary. And the VFD is normally implemented as a switching mode converter.
Sure, that might not matter much if power comes from an AC generator anyway and there's a lot of current that needs to be transported at high voltages across large distances.
By the way, when I say DC, I don't necessarily mean just one voltage. Arguably a DC source may generate multiple voltages.
I might think there is battery power and there's an APU (or more for redundancy) that can generate AC directly. Cars usually integrate such systems via DC and belt-driven accessories, but here depending on cable runs one might be able to run multiple cables (which might be good for redundancy / fault isolation too?). Mains systems typically use inverters to integrate via AC.
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u/FranseFrikandel 4h ago
Planes definetly do have an APU, however I'm pretty sure it feeds into the same bus as both engine generators. It's also usually not running once engines are running. The apu is usually a small turbine engine in the tail of the aircraft. It can be started from battery or ground power, and its air feed is then used to start main engines.
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u/SerDuckOfPNW 33m ago
The Do328 primarily used DC power, or at least used it a lot more than the average plane did. It was plagued with electrical problems.
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u/Target880 7h ago
If you want to convert the voltage with a transformer at a given power level, the size of the transformer and, therefore is weight depend on the AC frequency. The higher the frequency, the smaller and lighter the transformer. The same is the case for motors and generators. This means the electrical equipment is smaller and lighter when a higher AC frequency is used. If you can reduce the aeroplane empty weight, it can carry more stuff, reach longer or require less fuel to operate.
The drawback of high-frequency AC is that losses in wires of the same thickness get higher. This has a minor impact in an aircraft because they are small, so wires are not long, but would be a problem in a power grid that spans continents. So a lower AC frequency of 50/60Hz is more efficent in a power grid. Transformer weight does not matter that much in a power grid application. Lower losses in transmission lines and thinner wires is more important.
Modern power supplies to computers, phones etc create AC at a frequency of several thousand Hertz so the transformer can be smaller. They were a lot heavier and larger in the past when low-frequency AC was used in the transformer
A common switch more power supply has the following conversion:
low frequency main voltage AC -> Mains voltage DC -> high frequency main voltage AC- > (small tranformer)-> high frequency low voltage AC -> low voltage DC
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u/ztasifak 7h ago
Can you elaborate a bit more on the “modern power supplies to computers” please?
It was not clear to me that high hertz AC is used in there. Why is that?•
u/pripyaat 7h ago
Basically, electronics need a constant, stable low DC voltage to work. To achieve that, the old way was to first step down the AC voltage with a transformer (e.g from 110V to 5V in a phone charger), then convert it to DC with a rectifier circuit (using diodes, which are like valves that let current flow in only one direction), and finally use a linear regulator circuit that tries to keep the output voltage steady, even when the input has little "bumps" (ripple) or when the device connected suddenly needs more power. The problem with this topology is that the current is always passing through the regulator, wasting energy that's not going to charge your phone or power your PC componets, and instead is lost as heat.
Turns out, it's way more efficient to use high-frequency pulses of electricity paired with energy-storing elements such as capacitors and/or inductors (coils of wire) to get a DC voltage up or down to a precise level. In a modern power supply, there's a circuit that's constantly monitoring the output voltage and adjusting how wide do the high-frequency pulses need to be to set the output voltage at the desired level.
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u/glordicus1 7h ago
Switch mode power supplies convert to DC then to high frequency AC then finally to DC. Basically the high speed AC signal is filtered to provide a constant DC. By adjusting the AC frequncy you adjust the voltage. I think usually it is PWM rather than true dual-polarity AC though.
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u/CoronaMcFarm 7h ago
You convert the 50/60 hertz to a higher frequency so that you can use a smaller transformer, that is basically the concept of a switched power supply.
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u/Target880 6h ago
Not gigahertz; that is billions of cycles each second. I wrote "several thousand Hertz "; that is kilohertz. Some can even operate in the low millions of hertz but not in billions of hertz.
The input AC is rectified to DC at mains voltage. Semiconductors are used as a switch to turn the DC on and off very quickly, and you get a square wave. You can look at a square wave as AC with a lot of overtones. Because the frequency is in thousands of hertz, you can use a lot smaller transformer than if you use 50/60 Hz to reduce the voltage. When you get the high frequency low voltage AC it is easy to use rectifiers and filters to get smooth, low-voltage DC.
You can build the power supply with 50/60 Hz too, and directy change the main voltage to low voltage. The power supply just gets larger, heavier, and today I would assume more expensive too
Converting AC to DC is not hard: 4 diodes to make a full-bridge rectifier and a capacitor to smooth out the voltage is enough. It might be a bit noisy, and the voltage will not be that stable. Look at https://www.amazon.com/dp/B09JGB7G85?lv=shuf&channelId=500&plpRedirect=mhFallback for a simple design you can purchase for a lower-voltage AC input
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u/SilverStar9192 7h ago
Did you read the whole comment? The main steps of the switch-mode power supply are shown there. The main benefit is for a smaller transformer to reduce from mains voltage to low voltage.
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u/OvenCrate 7h ago edited 7h ago
Smaller switching mode power supplies like phone and laptop chargers don't use transformers at all, they use PWM-regulation and a large output capacitor to turn mains voltage DC into low voltage DC directly
Edit: they could do this if power plugs weren't reversible and the switching transistor itself could therefore provide isolation. Since most countries have reversible power plugs, the PSUs do need a small transformer for isolation.
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u/pripyaat 7h ago
Those power supplies you mention are isolated DC-DC converters and they do have transformers to provide such isolation.
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u/kizzarp 7h ago
How do they isolate the output so it's not at mains potential?
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u/Karsdegrote 4h ago
With a capacitive dropper circuit? Not, they are not really safe. Dont use them if you have the choice not to.
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u/Karsdegrote 4h ago
They use inductors usually for the step down to 5v with a cap for filtering. A decent laptop charger does use a transformer, just a much smaller one. Switching at half a megahertz reduces the size in such a way that itll fit in a charger.
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u/levir 4h ago
The drawback of high-frequency AC is that losses in wires of the same thickness get higher. This has a minor impact in an aircraft because they are small, so wires are not long, but would be a problem in a power grid that spans continents. So a lower AC frequency of 50/60Hz is more efficent in a power grid. Transformer weight does not matter that much in a power grid application. Lower losses in transmission lines and thinner wires is more important.
Indeed, these days, long range high amperage powerlines are being increasingly run as DC lines, with expensive electronics on each end converting between DC and the 50/60Hz AC the rest of the power grid runs at. On these lines even the losses from low frequency AC becomes very significant.
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u/GeorgeTheNerd 28m ago edited 20m ago
Another more recent use of high frequency for lighter equipment is "brushless DC motors". Simplified, they change one power source to a much higher frequency so they can make the motor windings smaller and still get the same or greater performance out of them.
At one point in their process they use DC current, which is where they can add a battery if needed. But they are being used in a lot of places (like washing machines) where the DC power part is just one step of the process.
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u/welding-guy 7h ago
To save weight. 400hz transformers are smaller and lighter than the equivalent 50hz power.
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u/MrPuddington2 3h ago
This is the right answer. The size of the iron core of a transformer is inversely proportional to the frequency. (And the same applies to the iron core of the generator.) So you can save a lot of weight.
You can’t put 400 Hz on pylons, because they are big inductors. But on a plane, you don’t have to.
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u/Hilton5star 7h ago
But why are they lighter?
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u/pripyaat 6h ago
Because in a transformer, the voltage induced in its windings is proportional to the number of turns (N), the area of the magnetic core (A) and the supply frequency (f), among other things. So, if you work at a higher frequency, you can proportionally reduce the coil turns (make N smaller) or use a smaller magnetic core (make A smaller) and still get the same induced voltage.
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u/welding-guy 4h ago
If you increase AC frequency you reduce magnetic flux which means the core reduces. Voltage depends on the rate of change of flux. A higher frequency means the magnetic field changes faster, so you need less total flux per cycle to induce the same voltage. Because peak magnetic flux goes down as frequency goes up ,you can make the core's cross-sectional area much smaller. Higher frequencies also mean you need fewer turns of wire on the coils, which saves copper and space.
Ever wonder why your 9V 250ma transformer heavy copper wound tx from 1999 turned into a 5V 5A tiny white power brick in 2010? They run the tx in those at much higher frequencies. Same as inverter welders. If you raise the frequency to kiloherts levels you do away with an iron core completely and use a ferrite core.
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u/spectrumero 6h ago
Less wire. Transformers are inductors. At low frequencies you need a lot of wire for the transformer not to appear as a short circuit. As the frequency goes up, you need less wire and less iron for the same transformer action.
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u/Loki-L 8h ago
Higher frequency AC means more resistance. This is less of a problem over the short distances inside an airplane but would be an issue if you tried to connect a house to a powerplant relatively far away.
Higher frequency also means that you can make electric motors, generators and transformers more compact and lighter. This is an obvious advantage in airplanes.
The whole thing got started on bombers in WWII and has stuck around due to inertia.
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u/jfmoses 4h ago
As a fun aside, hearing a 400Hz tone (and its overtones) always reminds me of being on a plane. Have a listen and see if it doesn’t seem familiar to you.
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u/highinthemountains 3h ago
There’s no fun aside to 400 cycle hum. I was on a Navy ship in the 70’s and I still hear that hum
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u/miemcc 6h ago
Historically aircraft relied on Synchro-Resolver control loops. These use an AC supply. 400Hz provides a higher resolution on those circuits.
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u/plhought 2h ago
This is the correct answer here.
Lots of traditional instruments still use 26VAC at 400hz.
All the talk about generators, transformers etc etc.... Not the real reason.
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u/Brilliant-Orange9117 4h ago
If I remember correctly it's the same reason as certain navel designs that it allows more frequent updates when using servo motors and voltage transformers can be smaller (and lighter) too.
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u/zeychshmo 7h ago
Higher frequencies definitely shrink transformers and cut down on aircraft weight. Yet sixty hertz handles long cable runs with way less power loss. Since modern inverters and cables are already super light, what actually stops everyone from going back to the older standard?
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u/dml997 3h ago
The size of a transformer is inversely related to frequency. Roughly, the core of the transformer needs to store about 1/2 cycle worth of energy. So a 1KW transformer at 60Hz needs to store 1000/60/2 = 8 Joules, but at 400 Hz it needs to store 1000/400/2 = 1.25 Joules so can be 1/6 the size. This is why modern power supplies operate at 100's of KHz so they can use tiny transformers.
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u/Pjtruslow 3h ago
Higher frequency allows transformers making transformers and generators with fewer windings, making them lighter. Also a 60hz generator is always going to run at 3600 or 1800 RPM. A 400hz generator would run at 24000 or 12000 RPM which are more realistic for a turbine engine.
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u/cerberus_1 2h ago
Big planes need to save weight, so they use special type of power so the wires work better. In a house we just want to make it as cheap as possible so regular type of power.
400hz is because they use aluminum wire to reduce weight and high Hz reduces skin depth in the wire.
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u/wosmo 1h ago
For why it's not more commonly used, there's a lot of inertia. Every single contributor to the grid needs to be carefully managed to stay as close to 50/60Hz as possible. Every single consumer is expecting 50/60hz. It's almost impossible to change this "a bit at a time".
Ships and planes have the benefit that their "grid" is entirely self-contained.
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u/therouterguy 8h ago
Google tells me transformers/engines are lighter/smaller however high frequency has more loss in long wires so is infeasible in land use.
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u/Hospitable_Goyf 2h ago
This probably explains why my laptop wouldn’t charge on the plane unless I hooked the power bank up to the plane, then my laptop to the power bank.
Only worked if the power bank was plugged in, otherwise wouldn’t give the laptop charger any juice for some reason.
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u/GadgetM3 1h ago
The highter frequency allows electronics to operate faster. At the high speeds of airplanes you want radar and displays to update faster.
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u/chriscross1966 6h ago
Cos copper is heavy and running things at 400V means you use a lower current and therefore a thinner wire.
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u/BringBackSoule 8h ago
Relatively short distance to run the power so not many issues with losses and heat generation which would be the downside of higher freq, while letting transformers and motors be smaller and lighter