# Explain alternating current to me **URL:** \ **Category:** Factual Questions\ **Created:** [September 27, 2012, 12:45am UTC](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944 "2012-09-27T00:45:26Z")\ **Posts on this page:** 20\ **Page:** 4
**Author:** ![Michael63129](https://avatars.discourse-cdn.com/v4/letter/m/3da27b/32.png) [@Michael63129](https://boards.straightdope.com/u/Michael63129)\ **Post date:** [September 29, 2012, 3:01am UTC](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944/61 "2012-09-29T03:01:52Z")
> [@Jamicat](#): > > Oh… > > So, it’s a Mechanical wall? (spinny stoof) > > Would more (faster) = better, or it doesn’t matter? (use the same at any rate despite frequency, 50-60 Hz vs 1-100,000 Hz) :dubious: For power transmission, a lower frequency is better because capacitance and inductance have less effect at lower frequency, dropping to zero at DC. Also, even at 60 Hz, [skin effect](http://en.wikipedia.org/wiki/Skin_effect) becomes a factor for wires thicker than about half an inch, which increases the resistance compared to DC. On the other hand, transformers can be made much smaller and lighter (and cheaper) at higher frequencies; [this page](http://www.wcmagnetics.com/catalog/1plkw-switch_transformer.php#output) has a table (power output) that shows the maximum power output for a given frequency; the largest core shown (1.9x2.2x1.6 inches) can output only 22 watts at 60 Hz but over 1.6 kW at 500 kHz (power output isn’t linear because of losses which increase with frequency; 50-100 kHz is more typical in a SMPS, which still provides 700-1000 watts). ---
**Author:** ![Leo\_Bloom](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leo_bloom/32/10377_2.png) [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)\ **Post date:** [September 29, 2012, 3:40am UTC](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944/62 "2012-09-29T03:40:55Z")
> [@california\_jobcase](#): > > cjepson, you may have felt that movement if you’ve ever been shocked by house current. It causes an AC shock to feel vibratey. Huh. Neat. So what does a DC shock feel like? ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [September 29, 2012, 5:19am UTC](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944/63 "2012-09-29T05:19:23Z")
> [@Jamicat](#): > > When I first started understanding wiring…There were Two hot wires and One ground. > > Later on, there was Two hot wires and the ground was tied to the black wires at the box, meaning white = Hot. > > Now, it’s the Black wire is the Hot one tied to the Ground at the box and white is the neutral. > > Considering I played with electric for 20 years with the same conflicting knowledge. > > Are they just messing with my head, or am I remembering wrong from the get go (taught wrong)? 😕 I don’t know if you were taught wrong or if you are remembering it wrong, but that’s definitely wrong. If you have AC, then it doesn’t matter much which wire is which. The current flows one way and then the other, back and forth 60 times a second (50 in some parts of the world). If you touched either wire but not the other, then you would be ok. The only time you get in trouble is if you touch both wires. This is called an isolated system, and it is safer than a grounded system. But we use grounded systems. Now at first you might think that’s kinda stupid since isolated systems are safer, but in the real world, isolated systems are about impossible to keep isolated. Mother nature likes to randomly throw ground points into your system, by having tree branches grow into your wires and such things. So we intentionally ground one of the conductors. This is called the neutral conductor, since it is neutral with respect to the earth. You can (almost) safely the neutral and touch anything that is earth grounded (like a water pipe, your home’s metal siding, etc) and not get shocked. The other wire is the hot wire, since if you touch it and anything that is earth grounded you’ll get shocked. The standard colors in the U.S. are black for hot and white for neutral. For a while that’s all we had. If you had an appliance like an oven with a big metal case, you connected the case to neutral so that it was safe to touch. This could cause you problems, though. If the neutral wire broke, then when you turned on the oven the entire case would become electrically hot an unsafe to touch. So they figured out that it’s much safer to run a completely separate ground wire to use as your protective ground. If the hot wire breaks, nothing happens. If the neutral wire breaks, nothing happens. If the protective ground breaks, nothing happens. You only get bad things if you have multiple failures, such as the protective ground breaking and the hot wire breaking and shorting to the case. This is a much safer system. It also has the advantage that even if current is flowing through the circuit, there is no current flowing through the protective ground at it remains at true earth potential. In the U.S. the protective ground is green, or bare copper. You should never tie black and white wires together, and the green and white wires are only connected together at one place, and that’s near your main breaker box. The connection to earth ground should be somewhere near there as well. In the old days you were required to ground to the cold water pipe. Since the common use of plastic pipe, you are now required to have a separate ground, but you are also required to ground your water pipes to it so that they don’t become electrically hot if something were to short to them in a failure. Sometimes when you are running a switch and you need two hot wires, you can use a black and white wire as long as you cover the end of the white wire with black tape to indicate that it is hot. This is about the only time that you intentionally tie a white wire to a black wire. The only other time that comes to mind is if you have a dead circuit in the walls and you want to leave the wire in place. In order to assure that it’s dead, you tie the black and white wires together. That way if anyone ever connects the wire back up at the other end it will blow the breaker indicating that the wires aren’t connected to anything and shouldn’t be used. ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [September 29, 2012, 6:59am UTC](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944/64 "2012-09-29T06:59:50Z")
> [@Leo\_Bloom](#): > > Huh. Neat. So what does a DC shock feel like? It hurts more because it cancause you to lock down. ---
**Author:** ![Snnipe\_70E](https://avatars.discourse-cdn.com/v4/letter/s/b5a626/32.png) [@Snnipe\_70E](https://boards.straightdope.com/u/Snnipe_70E)\ **Post date:** [September 29, 2012, 7:12am UTC](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944/65 "2012-09-29T07:12:30Z")
> [@Bozuit](#): > > So how does that work? The thing that synchronizes the generator’s output is powered by the grid so it matches? What would happen if it weren’t in sync? Does it happen? When bring a generator on line adjust the govenor so the generator is running slightly faster than the system. As the output comes into sync with the system the breaker is closed as they match. With the slightly higher speed of the oncoming generator it will take a small load and slow down to sysstem cycles. The govenor of the oncoming generator is opened up and the generator will pick up more load. This is now done by computers and the whole system will ballance out at 60 cycles. I heard of one case where a generator’s breaker was pulled in about 180 degrees out of phase. Blew every breaker and fuse between San Francisco and the where the ship was plugged at the dock. I saw a 3000 shaft horsepower motor drop out of phase for a few seconds. That motor bounced about 6 to 12 inches. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [September 29, 2012, 12:26pm UTC](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944/66 "2012-09-29T12:26:50Z")
> [@engineer\_comp\_geek](#): > > The current flows one way and then the other, back and forth 60 times a second (50 in some parts of the world). No, the voltage is alternating. All the time. Regardless of the load (within reason). The current may be switching direction periodically, or it may _not_ be switching direction periodically – it all depends on the load. See my post above. The power company give you a stiff, constant, known, alternating voltage source. The current is freewheeling. Sorry to be such a nitpick on this. But I think we do a disservice to people when we state as fact the “current is alternating.” We should always start off our discussion with the fact the voltage is alternating. The signature of the current can be _anything_, depending on the load. Yes, the current is often alternating, but there are times when it is not. And often the current signature does not look like a nice, friendly sine wave. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [September 29, 2012, 12:38pm UTC](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944/67 "2012-09-29T12:38:42Z")
> [@Michael63129](#): > > …most electronic equipment now uses switch-mode power supplies instead of AC transformers; while more complex, they are cheaper because of the cost of all of that iron and copper (a SMPS transformer may be 100-1000 times smaller and lighter). Well, even most SMPSs have a transformer. But they’re much smaller and lighter (vs. a linear PS) due to the high switching frequency. And if you don’t care about galvanic isolation, you can buy non-isolated DC-to-DC boost converters that don’t even have a transformer. ---
**Author:** ![hibernicus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hibernicus/32/2802_2.png) [@hibernicus](https://boards.straightdope.com/u/hibernicus)\ **Post date:** [September 29, 2012, 1:17pm UTC](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944/68 "2012-09-29T13:17:13Z")
> [@ZenBeam](#): > > Yes. Just the voltage, or just the direction of current isn’t enough, but with both you can tell. > > In a two-wire transmission line, there will be a voltage between the two wires, which means there will be an electric field **E** pointing from one wire to the other. This will be strongest between the wires. > > There will also be current flowing on the two wires, in one direction on one, and in the opposite direction on the other. The magnetic field **H** curls around a wire with current flowing in it, following the right-hand-rule: with the thumb of your right hand pointing in the direction of current flow, your fingers point in the direction of the magnetic field. The magnetic field from the two wires will add together between the wires. > > So between the wires you’ve got an electric field pointing from one wire to the other, and a magnetic field pointing between the wires. The cross-product **E** X **H** , called the [Poynting vector](http://en.wikipedia.org/wiki/Poynting_vector), points in the direction of power flow. That page doesn’t have a good picture for a two-wire line, but it does have one for a [coaxial line](http://en.wikipedia.org/wiki/Poynting_vector#In_a_coaxial_cable), and the same thing applies there. > > In the picture, if you’re not familiar with the convention, a circle with a dot in it means the vector is pointing out of the screen, and the circle with an x in it means the vector is pointing into the screen. Supposedly representing an arrow point coming at you, and the arrow feathers going away from you. So in that picture, power is flowing into the screen. Thanks very much for this explanation. If I have understood you correctly, this works even though the current in each conductor is changing direction? So regardless of whether the positive direction of current in one conductor at any instant is towards the generator or towards the load, the Poynting vector is always in the direction of propagation? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 29, 2012, 2:25pm UTC](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944/69 "2012-09-29T14:25:25Z")
> [@](#): > > Quoth **Michael63129** : > > On the other hand, transformers can be made much smaller and lighter (and cheaper) at higher frequencies; this page has a table (power output) that shows the maximum power output for a given frequency; the largest core shown (1.9x2.2x1.6 inches) can output only 22 watts at 60 Hz but over 1.6 kW at 500 kHz (power output isn’t linear because of losses which increase with frequency; 50-100 kHz is more typical in a SMPS, which still provides 700-1000 watts). In fact, so-called “DC transformers” are really just AC transformers. You start with a DC source, and then use some fancy electronics to switch the direction of it back and forth really quickly, at a frequency far higher than the typical 60 Hz. Then you put that through a transformer that works exactly the same way as a normal AC transformer, but which can be much smaller and lighter due to the high frequency. Then you take the AC that comes out, and put it through some sort of rectifier to turn it back into DC. ---
**Author:** ![Michael63129](https://avatars.discourse-cdn.com/v4/letter/m/3da27b/32.png) [@Michael63129](https://boards.straightdope.com/u/Michael63129)\ **Post date:** [September 29, 2012, 3:48pm UTC](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944/70 "2012-09-29T15:48:05Z")
> [@Chronos](#): > > In fact, so-called “DC transformers” are really just AC transformers. You start with a DC source, and then use some fancy electronics to switch the direction of it back and forth really quickly, at a frequency far higher than the typical 60 Hz. Then you put that through a transformer that works exactly the same way as a normal AC transformer, but which can be much smaller and lighter due to the high frequency. Then you take the AC that comes out, and put it through some sort of rectifier to turn it back into DC. SMPS transformers are driven a bit differently though (I know this because I have designed numerous SMPSs myself); instead of an AC voltage/current into the primary, many converters ([flyback converters](http://en.wikipedia.org/wiki/Flyback_converter)) drive the primary only one way; that is, a transistor turns on and applies the supply voltage to the primary, then turns off and opens the primary circuit; energy stored in the core is then transferred to the output, which resembles inductor action more than transformer action (which transfers power but doesn’t store it, except for magnetizing current; output voltage also isn’t dependent on the input voltage / turns ratio). Thus, you never have current flowing in both the primary and secondary windings. Although other topologies (forward and push-pull/bridge converters) do use normal transformer action, but only push-pull converters actually use true bipolar drive, what you would call AC; the output voltage and current are symmetrical so you can generate +/- outputs with one winding by reversing diodes; for the other topologies, you need an additional winding wound in opposing direction (you actually don’t need to do this, but the output won’t be regulated, but can be used at low power levels with a post-regulator, like the -12v output in a computer PSU). ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [September 29, 2012, 4:14pm UTC](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944/71 "2012-09-29T16:14:32Z")
> [@Crafter\_Man](#): > > No, the voltage is alternating. Yep, that’s definitely a valid point. I’ll try to keep it in mind in future explanations. Thanks for the correction. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [September 29, 2012, 4:18pm UTC](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944/72 "2012-09-29T16:18:48Z")
Oh, as an aside: Once, I was teaching a physics lab on electromagnetic induction, and one of the questions at the end was “Why are there no DC transformers?”. One student answered “Because Marvel got the comic book rights”. ---
**Author:** ![Michael63129](https://avatars.discourse-cdn.com/v4/letter/m/3da27b/32.png) [@Michael63129](https://boards.straightdope.com/u/Michael63129)\ **Post date:** [September 29, 2012, 4:47pm UTC](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944/73 "2012-09-29T16:47:00Z")
> [@Chronos](#): > > Oh, as an aside: Once, I was teaching a physics lab on electromagnetic induction, and one of the questions at the end was “Why are there no DC transformers?”. One student answered “Because Marvel got the comic book rights”. Well, as I said, that isn’t entirely true; [net current flow](http://www.google.com/imgres?hl=en&client=firefox-a&hs=sJ1&rls=org.mozilla:en-US:official&biw=1010&bih=622&tbm=isch&tbnid=r5APmSq0rIUnOM:&imgrefurl=http://kralciplak.tripod.com/Electronic/smps.html&docid=P_ZX1Vqud2RE6M&imgurl=http://kralciplak.tripod.com/Electronic/flycnv.gif&w=470&h=225&ei=yyNnULqhFsONygH2vICQBw&zoom=1&iact=hc&vpx=69&vpy=170&dur=139&hovh=155&hovw=325&tx=160&ty=74&sig=108158042263569158783&page=1&tbnh=88&tbnw=183&start=0&ndsp=15&ved=1t:429,r:0,s:0,i:71) in the windings of a flyback or forward converter is DC; the only requirement for a transformer is that current flow can’t be continuous. ---
**Author:** ![ZenBeam](https://avatars.discourse-cdn.com/v4/letter/z/3ab097/32.png) [@ZenBeam](https://boards.straightdope.com/u/ZenBeam)\ **Post date:** [September 29, 2012, 6:15pm UTC](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944/74 "2012-09-29T18:15:37Z")
> [@hibernicus](#): > > Thanks very much for this explanation. > > If I have understood you correctly, this works even though the current in each conductor is changing direction? So regardless of whether the positive direction of current in one conductor at any instant is towards the generator or towards the load, the Poynting vector is always in the direction of propagation? Yes, that’s right. The current and voltage both change sign, so **E** and **H** both change sign, so the Poynting vector sign doesn’t change. That’s when the voltage and current are in phase with each other, both proportional to sin(120 Pi \* t), for example. If they are 90 degrees out of phase, say one is proportional to sin(120 Pi \* t) and the other proportional to cos(120 Pi \* t), then their product changes sign rapidly, and you have what’s called reactive power. Its time average is zero, even though there are both current and voltage. If the phase difference is somewhere between 0 and 90, you’ll have both real power and reactive power. This is relevant if you’re the electric company, since the total current causes your resistive losses, and limits your line capacity, but you only get paid for the real power. For industrial customers they’ll monitor the power factor, which is something like real power divided by total power, and charge extra if it’s too far from 1. ( **engineer\_comp\_geek** or **Crafter\_Man** could give more info on this than I.) ---
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [September 29, 2012, 8:42pm UTC](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944/75 "2012-09-29T20:42:45Z")
> [@Michael63129](#): > > > [@Chronos](#): > > > > Oh, as an aside: Once, I was teaching a physics lab on electromagnetic induction, and one of the questions at the end was “Why are there no DC transformers?”. One student answered “Because Marvel got the comic book rights”. > > Well, as I said, that isn’t entirely true; [net current flow](http://www.google.com/imgres?hl=en&client=firefox-a&hs=sJ1&rls=org.mozilla:en-US:official&biw=1010&bih=622&tbm=isch&tbnid=r5APmSq0rIUnOM:&imgrefurl=http://kralciplak.tripod.com/Electronic/smps.html&docid=P_ZX1Vqud2RE6M&imgurl=http://kralciplak.tripod.com/Electronic/flycnv.gif&w=470&h=225&ei=yyNnULqhFsONygH2vICQBw&zoom=1&iact=hc&vpx=69&vpy=170&dur=139&hovh=155&hovw=325&tx=160&ty=74&sig=108158042263569158783&page=1&tbnh=88&tbnw=183&start=0&ndsp=15&ved=1t:429,r:0,s:0,i:71) in the windings of a flyback or forward converter is DC; the only requirement for a transformer is that current flow can’t be continuous. [Foghorn Leghorn]I say, I say, that’s a joke, son![/FL] ---
**Author:** ![hibernicus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hibernicus/32/2802_2.png) [@hibernicus](https://boards.straightdope.com/u/hibernicus)\ **Post date:** [September 30, 2012, 1:50am UTC](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944/76 "2012-09-30T01:50:12Z")
> [@ZenBeam](#): > > Yes, that’s right. The current and voltage both change sign, so **E** and **H** both change sign, so the Poynting vector sign doesn’t change. > > That’s when the voltage and current are in phase with each other, both proportional to sin(120 Pi \* t), for example. If they are 90 degrees out of phase, say one is proportional to sin(120 Pi \* t) and the other proportional to cos(120 Pi \* t), then their product changes sign rapidly, and you have what’s called reactive power. Its time average is zero, even though there are both current and voltage. > > If the phase difference is somewhere between 0 and 90, you’ll have both real power and reactive power. This is relevant if you’re the electric company, since the total current causes your resistive losses, and limits your line capacity, but you only get paid for the real power. For industrial customers they’ll monitor the power factor, which is something like real power divided by total power, and charge extra if it’s too far from 1. ( **engineer\_comp\_geek** or **Crafter\_Man** could give more info on this than I.) Thanks again. I was thinking about this overnight and making the “right-hand rule” for each conductor and satisfied myself that the Poynting vector direction doesn’t change. It’s something I’ve wondered about for years and only now understand for the first time. As for reactive power, I do understand that. It’s worth saying that from the point of view of the electricity company, not only do they (we) have to carry the reactive power, we also have to burn fuel to generate it. ---
**Author:** ![Michael63129](https://avatars.discourse-cdn.com/v4/letter/m/3da27b/32.png) [@Michael63129](https://boards.straightdope.com/u/Michael63129)\ **Post date:** [September 30, 2012, 4:31pm UTC](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944/77 "2012-09-30T16:31:24Z")
A big concern with utilities today with the proliferation of electronic equipment (including CFL/LED light bulbs and electronic motor controllers) is not necessarily reactive power per se, as in inductive or capacitive reactance, but [nonlinear loads](http://electronicdesign.com/article/power/whats-difference-reactive-power-factor-acdc-supply-power-factor-73569), such as when you use a rectifier and capacitor filter to convert AC to DC; it results in current being drawn in short spikes only at the peak voltage and besides the low power factor, it can also cause distortion of the waveform, flattening out the peaks and interference due to the sharp current spikes which causes (harmonic distortion, a pure sine wave only contains its fundamental frequency). As a result, many power supplies are required to have power factor correction, which requires an electronic circuit (an inductive load can be corrected with a capacitor or vice-versa but nonlinear loads need special circuitry). ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [October 1, 2012, 12:07pm UTC](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944/78 "2012-10-01T12:07:27Z")
> [@Michael63129](#): > > On the other hand, transformers can be made much smaller and lighter (and cheaper) at higher frequencies… I gather this is why [aircraft electrics operate on a 400-Hz system.](http://www.aerospaceweb.org/question/electronics/q0219.shtml) Why not even higher than that? ---
**Author:** ![Francis\_Vaughan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/francis_vaughan/32/3093_2.png) [@Francis\_Vaughan](https://boards.straightdope.com/u/Francis_Vaughan)\ **Post date:** [October 1, 2012, 1:17pm UTC](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944/79 "2012-10-01T13:17:45Z")
> [@Machine\_Elf](#): > > I gather this is why [aircraft electrics operate on a 400-Hz system.](http://www.aerospaceweb.org/question/electronics/q0219.shtml) Why not even higher than that? One would imagine that it comes about for a range of reasons. 400Hz is 24,000 cycles per minute. Clearly a simple alternator isn’t going to be viable, well certainly not reliable running that fast, so a multi-pole alternator will already be necessary, which reduces the physical speed it must rotate at to something sane, and something you would be prepared to trust on an aircraft. Pushing the design either in terms of higher rotational speed or more and more poles was probably the dominant limited factor. Iron losses increase with increasing frequency, so you don’t get all the efficiency you would hope for. Ameliorating these adds cost, uses more exotic core materials, and may start to increase weight again. Eventually I suspect the gains simply didn’t get much better, as the fraction of mass of the equipment occupied by the transformers dropped low enough that it no longer mattered enough. At 50Hz a transformer can occupy well over half the mass of many devices, but at 400Hz it may well have been good enough. Once it dropped much below say a quarter of the device’s mass is probably was as close to mission accomplished as was needed. Not just aircraft. A lot of the the old CDC (Control Data Corporation) and I suspect early Cray computer systems ran off 400 Hz power. Computer installations included a reasonably large genemotor to power the system. This had the advantage of providing superb power isolation. ---
**Author:** ![engineer\_comp\_geek](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/engineer_comp_geek/32/504_2.png) [@engineer\_comp\_geek](https://boards.straightdope.com/u/engineer_comp_geek)\ **Post date:** [October 1, 2012, 3:54pm UTC](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944/80 "2012-10-01T15:54:35Z")
> [@Machine\_Elf](#): > > I gather this is why [aircraft electrics operate on a 400-Hz system.](http://www.aerospaceweb.org/question/electronics/q0219.shtml) Why not even higher than that? Higher frequencies allow for smaller transformers, but they also have higher inductive and capacitive losses in longer wires. 400 Hz is a good tradeoff between smaller and lighter components and a reasonable practical length for the wires. [Previous page](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944.md?page=3) [Next page](https://boards.straightdope.com/t/explain-alternating-current-to-me/635944.md?page=5)