5 ms·

In what way is DC better than AC? To my knowledge AC rightfully won - much easier to transport over long distances, whereas DC required a generator nearly on e

In what way is DC better than AC? To my knowledge AC rightfully won - much easier to transport over long distances, whereas DC required a generator nearly on every city block (among other reasons).

silvestrov 4y ago
From Wikipedia: > High-voltage direct current (HVDC) is used to transmit large amounts of power over long distances or for interconnections between asynchronous grids. When electrical energy is to be transmitted over very long distances, the power lost in AC transmission becomes appreciable and it is less expensive to use direct current instead of alternating current. For a very long transmission line, these lower losses (and reduced construction cost of a DC line) can offset the additional cost of the required converter stations at each end. https://en.wikipedia.org/wiki/Electric_power_transmission#High-voltage_direct_current https://en.wikipedia.org/wiki/Electric_power_transmission#Hi...
pletnes 4y ago
AC voltage can be changed up/down cheaply by making a couple of (relatively) simple wire coils. Also AC engines are simple, compact and reliable. Recall that the choice of AC was also taken over 100 years ago, before any electronics were around.
mananaysiempre 4y ago
AC is easy to change the voltage of with 20th-century tech (even high-power 21st-century tech for DC amounts to “convert to AC, shift that, filter it back” with some—crucial—improvements), so it works reasonably well over moderate distances at moderate power if you shift down as late as possible, but it’s absolutely awful for transporting large quantities of energy over long distances, because of the skin effect. Essentially, thick round wires are useless for AC because the current gets expelled to a thin layer on the surface (determined by the frequency and the properties of the material, roughly independent of how much you were trying to cram in there), thus the resistance goes down as the radius rather than the cross-sectional area, and you end up using absolutely asinine amounts of (nowadays very expensive) copper or (kind of expensive) aluminium in high-voltage lines. (Using flat buses instead of wires does work, but then sharp edges mean you get a lot more current leaking into the air.) If you don’t have high-power semiconductor components you can use to convert to AC and back in order to shift voltages or to just convert to AC in order to power industrial motors, you will have to grit your teeth and bear it—it seems that mechanical motor-generators are simply too unreliable. But for the last twenty or so years we’ve had them, and I understand that people are once again looking seriously at high-voltage DC. Of course, grids are huge and expensive, so it may be some time until implementations become common. I don’t think any of this counts as an instance of worse is better, though,—it’s just that the ease of building alternators changed dramatically over the last few decades.
- javajosh 4y ago
  >you end up using absolutely asinine amounts of (nowadays very expensive) copper or (kind of expensive) aluminium in high-voltage lines I assume there's some reason big hollow wires don't work.
  - tekchip 4y ago
    >I assume there's some reason big hollow wires don't work. So pipes then. The internet really would have been a series of tubes. ;-)
- mananaysiempre 4y ago
  s/alternators/inverters/, d’oh.
chaosite 4y ago
And of course, AC didn't beat DC. DC is still widely used, for example in whatever device you're using to post this comment.
- colejohnson66 4y ago
  AC is also used, in a sense, through every switching power supply. The phone you’re using that’s running on DC is actually taking the DC from battery, converting it to “AC” to buck it to the lower voltage, then rectifying it back to DC for the processors. Wall outlet switchers are also the same. The incoming 120V AC is rectified and smoothed to 170V DC (240V AC goes to ~335V DC) before being chopped up back to AC to lower the voltage. After, it’s rectified and smoothed to the desired voltage. The USB outlet does that to 5V, your phone takes that down to 3.7V, then takes that battery voltage down even lower (1V or whatever) Basically, AC is used everywhere DC is. People just don’t realize it. Everything runs on DC, but requires AC to change voltages.
nimish 4y ago
AC has to deal with reactance and those associated losses in addition to resistance. AC resistance goes up with frequency because of the skin effect too. This matters for underwater lines especially.
kragen 4y ago
DC needs a substantially higher voltage to kill you than AC. You can hold 100VDC wires in your fingers and be fine, though if you try this, I strongly recommend doing it with the other hand in your pocket and no path to ground, in case your hand is sweatier than average. The lower danger of transmission lines is less important once you're committed to stepping voltages up and down, because HVDC transmission lines can still kill you, and there's no real reason we couldn't be using 48VAC inside our houses instead of 120VAC or 240VAC, which would be a lot safer. Historically, though, AC came as a package with transformers and higher-voltage transmission/distribution to substations, and without such super low voltages in houses. You're exaggerating the technical advantage of AC by a couple of orders of magnitude; you can reasonably transmit 100VDC power about a kilometer, and Edison did, so it's a generator on every 314 city blocks or so.

In what way is DC better than AC? To my knowledge AC rightfully won - much easier to transport over long distances, whereas DC required a generator nearly on every city block (among other reasons).

silvestrov 4y ago

From Wikipedia: > High-voltage direct current (HVDC) is used to transmit large amounts of power over long distances or for interconnections between asynchronous grids. When electrical energy is to be transmitted over very long distances, the power lost in AC transmission becomes appreciable and it is less expensive to use direct current instead of alternating current. For a very long transmission line, these lower losses (and reduced construction cost of a DC line) can offset the additional cost of the required converter stations at each end. https://en.wikipedia.org/wiki/Electric_power_transmission#High-voltage_direct_current https://en.wikipedia.org/wiki/Electric_power_transmission#Hi...

pletnes 4y ago

AC voltage can be changed up/down cheaply by making a couple of (relatively) simple wire coils. Also AC engines are simple, compact and reliable. Recall that the choice of AC was also taken over 100 years ago, before any electronics were around.

mananaysiempre 4y ago

AC is easy to change the voltage of with 20th-century tech (even high-power 21st-century tech for DC amounts to “convert to AC, shift that, filter it back” with some—crucial—improvements), so it works reasonably well over moderate distances at moderate power if you shift down as late as possible, but it’s absolutely awful for transporting large quantities of energy over long distances, because of the skin effect. Essentially, thick round wires are useless for AC because the current gets expelled to a thin layer on the surface (determined by the frequency and the properties of the material, roughly independent of how much you were trying to cram in there), thus the resistance goes down as the radius rather than the cross-sectional area, and you end up using absolutely asinine amounts of (nowadays very expensive) copper or (kind of expensive) aluminium in high-voltage lines. (Using flat buses instead of wires does work, but then sharp edges mean you get a lot more current leaking into the air.) If you don’t have high-power semiconductor components you can use to convert to AC and back in order to shift voltages or to just convert to AC in order to power industrial motors, you will have to grit your teeth and bear it—it seems that mechanical motor-generators are simply too unreliable. But for the last twenty or so years we’ve had them, and I understand that people are once again looking seriously at high-voltage DC. Of course, grids are huge and expensive, so it may be some time until implementations become common. I don’t think any of this counts as an instance of worse is better, though,—it’s just that the ease of building alternators changed dramatically over the last few decades.

javajosh 4y ago
>you end up using absolutely asinine amounts of (nowadays very expensive) copper or (kind of expensive) aluminium in high-voltage lines I assume there's some reason big hollow wires don't work.
- tekchip 4y ago
  >I assume there's some reason big hollow wires don't work. So pipes then. The internet really would have been a series of tubes. ;-)
mananaysiempre 4y ago
s/alternators/inverters/, d’oh.

chaosite 4y ago

And of course, AC didn't beat DC. DC is still widely used, for example in whatever device you're using to post this comment.

colejohnson66 4y ago
AC is also used, in a sense, through every switching power supply. The phone you’re using that’s running on DC is actually taking the DC from battery, converting it to “AC” to buck it to the lower voltage, then rectifying it back to DC for the processors. Wall outlet switchers are also the same. The incoming 120V AC is rectified and smoothed to 170V DC (240V AC goes to ~335V DC) before being chopped up back to AC to lower the voltage. After, it’s rectified and smoothed to the desired voltage. The USB outlet does that to 5V, your phone takes that down to 3.7V, then takes that battery voltage down even lower (1V or whatever) Basically, AC is used everywhere DC is. People just don’t realize it. Everything runs on DC, but requires AC to change voltages.

nimish 4y ago

AC has to deal with reactance and those associated losses in addition to resistance. AC resistance goes up with frequency because of the skin effect too. This matters for underwater lines especially.

kragen 4y ago

DC needs a substantially higher voltage to kill you than AC. You can hold 100VDC wires in your fingers and be fine, though if you try this, I strongly recommend doing it with the other hand in your pocket and no path to ground, in case your hand is sweatier than average. The lower danger of transmission lines is less important once you're committed to stepping voltages up and down, because HVDC transmission lines can still kill you, and there's no real reason we couldn't be using 48VAC inside our houses instead of 120VAC or 240VAC, which would be a lot safer. Historically, though, AC came as a package with transformers and higher-voltage transmission/distribution to substations, and without such super low voltages in houses. You're exaggerating the technical advantage of AC by a couple of orders of magnitude; you can reasonably transmit 100VDC power about a kilometer, and Edison did, so it's a generator on every 314 city blocks or so.