Beta

×

Welcome to the Slashdot Beta site -- learn more here. Use the link in the footer or click here to return to the Classic version of Slashdot.

Thank you!

Before you choose to head back to the Classic look of the site, we'd appreciate it if you share your thoughts on the Beta; your feedback is what drives our ongoing development.

Beta is different and we value you taking the time to try it out. Please take a look at the changes we've made in Beta and  learn more about it. Thanks for reading, and for making the site better!

cancel ×

168 comments

Sorry! There are no comments related to the filter you selected.

Current Wars 3: Revenge of Edison (0)

Anonymous Coward | more than 2 years ago | (#39357285)

Yeah, I know the article only mentions one prior Current War, but that doesn't fit the template correctly.

Re:Current Wars 3: Revenge of Edison (0)

Anonymous Coward | more than 2 years ago | (#39357369)

Current Wars V: Edison Strikes Back (which happened to be the second movie produced)

Re:Current Wars 3: Revenge of Edison (0)

Anonymous Coward | more than 2 years ago | (#39357473)

Prepare yourself for the shocking conclusion to the Current Wars Trilogy: Current Wars VI, wherein it is discovered that Edison and Tesla were really twin brothers!

Re:Current Wars 3: Revenge of Edison (0)

Anonymous Coward | more than 2 years ago | (#39357563)

"Nikoli, Darth Franklin is your father, just as he is mine."

"But... I totally frenched you at the beginning of the last movie to make Han Jealous. Eww!"

Auuuuuughh! (2, Funny)

Anonymous Coward | more than 2 years ago | (#39358817)

Must... unimagine... Tesla... frenching... Edison...

Edison invented FUD (3, Funny)

Thud457 (234763) | more than 2 years ago | (#39357607)

Thank god, I'm so tired of stray dogs wandering into my datamacenter and getting electromacuted by deadly AC currents!
Peta's really on our asses about that.

Re:Edison invented FUD (3, Informative)

Mashiki (184564) | more than 2 years ago | (#39357823)

Well if you live in Toronto, there's a very good chance that simply walking down the street you could get electrocuted by well anything. I'm not actually kidding, they had a serious problem with live plates and poles all over the city for the last couple of years.

Did anyone RTFA? (3, Informative)

goombah99 (560566) | more than 2 years ago | (#39358251)

Uh the article the post links to supports AC more than DC in case no one noticed. The article is about DC being hyped beyond the facts and that AC is claimed to be just as good. Sort of reverses the whole discussion here making it AD, alternating discussion. Edison gets the carbonite filament..

Re:Did anyone RTFA? (0)

Anonymous Coward | more than 2 years ago | (#39358785)

So long as we all just avoid "The Cloud", we won't have to concern ourselves with their choice of electrical supplies.

Makes sense. (5, Interesting)

Anonymous Coward | more than 2 years ago | (#39357313)

AC is better than DC for transporting electricity because you can convert between voltages with just a transformer. But in a data centre, when all the equipment will be powered by the same voltage, it makes sense to use one good efficient power supply for multiple computers, so that all the components don't have to be duplicated for each computer.

Re:Makes sense. (-1, Troll)

Dwedit (232252) | more than 2 years ago | (#39357349)

Yes, let's use big power supply for all computers, so they all share the same exact point of failure.

Re:Makes sense. (3, Insightful)

Microlith (54737) | more than 2 years ago | (#39357425)

As opposed to the transformer coming into your building? How about the UPS and HVAC units supporting your server room?

Obviously, you'll have redundant DC power supplies, just like you do now. Except instead of having two AC->DC power supplies per PC, you'll route two room-level DC power supplies to each machine in the room. Lots of little, less efficient, lower quality power supplies replaced by a pair of high quality, high efficiency supplies.

Re:Makes sense. (1, Funny)

cayenne8 (626475) | more than 2 years ago | (#39357791)

I like AC/DC too!! They totally rock!!

"I'm on the highway to hell......"

Re:Makes sense. (3, Interesting)

Skapare (16644) | more than 2 years ago | (#39357995)

And a big disaster waiting to happen with such large DC currents available on all the busses going all over the room. FYI, telco 48VDC systems addressed the dangers with resistive busses. But that was a huge efficiency loss. They didn't care so much about efficiency back then as all they wanted was a reliable battery backed up system. Making DC efficient is also making DC unsafe, at data center scale. AC is safer on that scale. Then do the conversion to DC at no larger than one rack, and put ride-through (2 minute) backup batteries in each rack (just need to be long enough for slow start generators or maybe a little longer for diversity loading systems so you don't slam the generators with load). I'd have a separate AC distribution system for the generator power and have each (two input) power converter switch over at randomized times over a 2 minute interval.

Re:Makes sense. (3, Insightful)

nschubach (922175) | more than 2 years ago | (#39358587)

I'm more concerned that I convert AC to DC to charge a battery, then convert it back to AC to power a power supply in my machine that outputs DC voltage. (Or, taking the DC battery output and inverting it to AC to run a computer.) Why can't I just run my PC off a battery that's kept charged by a DC current from a single power supply? I mean, I don't need the efficiency of AC for long distance transfer (we're talking maybe 3 feet) so why convert it back to AC?

Re:Makes sense. (3, Interesting)

Imagix (695350) | more than 2 years ago | (#39357429)

Because you've immediately forgotten the concept of redundant power supplies? In a rack of 48 1U computers, that could be 96 AC-DC converters. Or replace those 96 with 2 (or 3, or 4, depending on risk tolerance) big, high-efficiency AC-DC converters. Better efficiency, easier to cool.

Re:Makes sense. (2)

EdIII (1114411) | more than 2 years ago | (#39358721)

Thank you. This is why the debate always confuses me. The poster is not exactly trolling. A single AC-DC power converter is a single point of failure, which is bad. Typically you have two, or even three, power supplies on most servers.

In my data center the AC is very clean, redundant, and has diesel fail over. Now if that is considered to be reliable, and as one poster suggested, we could use backup batteries for only a minute or two, why not convert all of the servers and supporting hardware to DC inputs and dump the AC-DC converters?

If you wanted to still make it redundant, you could build a 2U dual high-efficiency AC-DC converter with battery backup. That should be pretty reliable.

The benefit would be an easier hand off for power from the data center. You don't need expensive power strips taking up space and you can dump all of the power supplies in the rest of the equipment. Just agree on a standardized connector and even color code it to voltage.

It has never made sense to me in a data center setting to have that much space occupied by AC-DC converters.

Re:Makes sense. (0)

Anonymous Coward | more than 2 years ago | (#39357441)

Sort of like how they all run the same OS!

Re:Makes sense. (1)

eyenot (102141) | more than 2 years ago | (#39357443)

Great, let's convert off the grid into DC and rely on a single, bottle-necked power supply, then.

Re:Makes sense. (1)

leenks (906881) | more than 2 years ago | (#39358025)

Like the single, bottle-necked, AC power supply then?

Re:Makes sense. (2)

gparent (1242548) | more than 2 years ago | (#39357455)

Yeah, it'll be just like network routers and switch that always bring down the whole network with them! If only there was some way to prevent single points of failure...

Re:Makes sense. (1)

leenks (906881) | more than 2 years ago | (#39358049)

Yeah! Err. Oh yeah. Down with DC! Except the single AC supply into the building is already a single point of failure. There is no reason you can't have all the redundancy you have with AC phases / UPS / circuits, and have n redundant efficient PSUs powering m-racks, whatever works most efficiently.

Re:Makes sense. (1)

GodfatherofSoul (174979) | more than 2 years ago | (#39357469)

So, add 1 or 2 for backup. Still better than scores of separate, less efficient power supplies scattered all over your server room.

Re:Makes sense. (0)

Anonymous Coward | more than 2 years ago | (#39357529)

hehe sorta. You do things like have 5 computers all hooked up to the same one. These days the power supply is also a larger part of the cost of a computer. When a computer cost 5k just for the starting point of a good server it didnt matter. But now in the sub 1k market it starts to be a bigger cost of it. Then you are looking at over time what is the cost for ac vs dc. Is your data center already redundant? Then it may not matter to loose a cluster... But you can save a decent amount per month per server at the cost of some risk.

Some data centers are also power constrained. So if you can increase the number of computers you can use for the same amount of power (as you are not transforming it from AC to DC 2000 times)... You can also squeeze in a couple hundred more computers for that cost. In the same floor space.

Re:Makes sense. (1)

Nadaka (224565) | more than 2 years ago | (#39357647)

Even with centralized power supplies, you can still use built in redundancy in the rare case that one fails.

Re:Makes sense. (0)

Anonymous Coward | more than 2 years ago | (#39357739)

I suppose you could have two big supplies that automatically fail-over and still save money?

Re:Makes sense. (1)

fuzzyfuzzyfungus (1223518) | more than 2 years ago | (#39358427)

Yes, let's use big power supply for all computers, so they all share the same exact point of failure.

Eh, depends on the scale of your operation: Single computers only usually have one or two PSUs. Blade cages might have three or four; but serving 10+ PCs. If your infrastructure is in the thousands of racks, the savings on redundant power supplies might make a rack-level point of failure acceptable. Depends on what you are running and how much you want to pay for it...

What voltage would you use? (0)

Anonymous Coward | more than 2 years ago | (#39357427)

If you use a high voltage, you need power converters in each computer. If you use low voltage, you need thick, expensive, hard-to-run wire.

Most installations will be much better off sticking to conventional AC power.

Re:What voltage would you use? (1)

Skapare (16644) | more than 2 years ago | (#39357493)

Agreed. But it should be 240V everywhere, 50 to 60 Hz.

Re:Makes sense. (1)

Skapare (16644) | more than 2 years ago | (#39357445)

Yes, let's use big power supply for all computers, so they all share the same exact point of failure AND have a MASSIVE fault current when someone accidentally drops a piece of uninsulated wire across a bus bar, so we have have a couple racks of equipment meltdown and a techie vaporized to ash.

Re:Makes sense. (0)

Anonymous Coward | more than 2 years ago | (#39357531)

Well first of all, shorting is a problem inherent to electricity, not DC. An AC service can be shorted with spectacular results.

Secondly, ever heard of breakers and fuses?

Re:Makes sense. (1)

vlm (69642) | more than 2 years ago | (#39357755)

Thats why, like, I donno, 80 years ago, the telco business got in the habit of A and B power bus distribution. I worked at a place with a C bus which was pretty much a load balancing hack and confused the hell out of the CO techs and electricians... They actually shorted out the C bus one time because they didn't understand the concept of having three busses instead of the "standard" two.

Re:Makes sense. (4, Informative)

betterunixthanunix (980855) | more than 2 years ago | (#39357477)

AC is better than DC for transporting electricity because you can convert between voltages with just a transformer.

Which was a winning argument in the 19th century, but not anymore. The use of AC entails significant power loses, especially for cables that are immersed in salt water, which is why DC is used in such situations:

https://en.wikipedia.org/wiki/High_voltage_direct_current [wikipedia.org]

Re:Makes sense. (4, Informative)

tlhIngan (30335) | more than 2 years ago | (#39357483)

AC is better than DC for transporting electricity because you can convert between voltages with just a transformer. But in a data centre, when all the equipment will be powered by the same voltage, it makes sense to use one good efficient power supply for multiple computers, so that all the components don't have to be duplicated for each computer.

It depends.

AC wins out because of ease of conversion, becaues the higher the voltage, the lower the current, and lower the current, the lower the IIR losses in the wire. DC didn't win because at the time, efficient (and cheap) voltage converters didn't exist. These days, a switching DC-DC supply can easily exceed 90% efficiency, and you can get solid-state converters that can handle transmission line powers easily. Hence the launching of HVDC transmission lines which don't have resonant losses and no phasing issues

In a datacenter, you'd probably take the incoming power and turn it into an intermediate voltage like 48VDC per rack or something - something that minimizes IIR losses (you want high voltages) and DC-DC converter losses (ideally you want output voltage and no converter).

It will have to be per-rack at the minimum purely because of the losses - if we did 12V lines and a few servers take 1200W total, we're talking 100A in current If we bump it to 48V, we're dealing with 25A (maybe 30A after inefficiencies), and IIR losses at 25A are lower than at 100A (it increases with the square of the current).

Also, the 100A cables are big and chunky (which you need because they reduce the "R" part of IIR losses).

Re:Makes sense. (5, Informative)

effigiate (1057610) | more than 2 years ago | (#39357633)

One of the challenges of HVDC, especially in the transmission/distribution world, is that normal switching happens on the line and not at the breaker. If you can switch futher down the line, you can leave all the people closer to the breaker with power. The issue is that this switching happens while current is flowing which requires that the device interrupts real current. In the AC system this is relatively easy because the arc created by opening a high voltage circuit under load goes out at every current zero. There is no current zero on DC, so you force the interrupting device to break current. An similar situation can be seen if you look at relay contacts. They may be rated at 20A @120VAC but only 0.5A at 12VDC.

Actually converting DC is pretty easy these days (1)

Sycraft-fu (314770) | more than 2 years ago | (#39357713)

The problem is it wasn't back when the grid was being made. There was no good, easy, efficient way to convert DC voltage. Now, not so hard.

Re:Actually converting DC is pretty easy these day (1)

NicknameAvailable (2581237) | more than 2 years ago | (#39357793)

The problem is it wasn't back when the grid was being made. There was no good, easy, efficient way to convert DC voltage. Now, not so hard.

AC won over DC primarily because of the distance it can be transmitted.

Re:Actually converting DC is pretty easy these day (4, Interesting)

marcosdumay (620877) | more than 2 years ago | (#39357973)

You are getting that wrong. DC can be transmitted farther than AC. DC has only resistive losses, while AC also has capacitive and inductive ones.

I'd sumarize it as the following:

DC is slighlty (just slightly) better for transmitting;
AC was easier to convert from one tension to the other (currently, we have the oposite situation);
AC is better to use on motors (it was much better, now it is just slightly better);
AC is easier to generate (it was much better, now it is just slightly better - except on photovoltaics);
AC is easier on the connectors (hight current DC connectors are a hell to maintain)

It is easy to see why AC won. I bet AC would win again just because of the connectors and generators, after all, converting it to DC is relatively cheap. The only problem is the low frequencies we currently use, it would be better to increase them a lot now that we have better materials.

Re:Actually converting DC is pretty easy these day (1)

Anonymous Coward | more than 2 years ago | (#39358395)

You need low frequencies for the grid, else the capacitive coupling to ground will limit the maximum AC cable length yet further and increase eddie current losses in the wiring..

Semiconductor technology has not advanced far enough to justify replacing HV transformers and overhead lines except for very long cable runs, underwater cable runs or connecting asynchronous grids.

AC is better to use on motors, however variable AC is better still - and this can be generated slightly easier with DC (although matrix converters are good, the current harmonics causes nasty problems...)
AC is better for generators, however this is at low voltage so power electronics works well.

Running a data center on DC makes a lot of sense. Even just using the same power supplies and wiring will save a huge amount of power - the rectifiers and the line filters could just be bypassed and the AC-DC conversion could be performed by multiple redundant 12 pulse rectifiers resulting in nice sinusoidal input currents for lower transformer losses.

However have you ever tried to fuse DC. You can get >50A breakers relatively cheaply but anything bigger will cost thousands...

Re:Makes sense. (1)

Chrisq (894406) | more than 2 years ago | (#39357729)

AC is better than DC for transporting electricity because you can convert between voltages with just a transformer. But in a data centre, when all the equipment will be powered by the same voltage, it makes sense to use one good efficient power supply for multiple computers, so that all the components don't have to be duplicated for each computer.

Unless you want to transmit with lower loss and send more current down the same cable. That's why high-voltage direct current [wikipedia.org] is used for most undersea cables

Re:Makes sense. (2)

vlm (69642) | more than 2 years ago | (#39357735)

AC is better than DC for transporting electricity because you can convert between voltages with just a transformer.

Not anymore. The greenies / cost cutting / etc means no more xfrmrs anymore. Bye bye to that technology. Whens the last time you bought a wall wart charged device with a transformer inside it (you'd know, it'll be cubical and heavy)? You have to be pretty old by /. terms to have bought a main desktop computer without a switcher, like early 1980s era pre-PC "home computers"... Ahh the old Altair with its smoking hot 7805 regulators...

Since you're gonna have a switching power supply anyway... why not skip the pesky rectifier diodes and feed in raw couple hundred volts DC? Quite a few PC power supplies work just fine off raw DC on the supposed "AC input"... good luck figuring out which work and which dont without some smoke events.

I can save everybody a lot of time by posting the summary from the last 50 /. discussions of DC data centers:
1) RMS voltage is always going to be lower than peak... insulation is cheaper, or for a given grade of insulation you can push more wattage in DC.
2) Switchgear is complicated. Labor cost goes up.
3) Expect the fire marshall to completely flip his lid unless he's up with the times.

Re:Makes sense. (1)

marcosdumay (620877) | more than 2 years ago | (#39358047)

DC/DC converters are better than transformers in almost every way. They are lighter, smaller, and cheaper. Also, altought theoreticaly you could create a transformer that loses less power than a DC/DC conversor, in practice nobody did that, thus they also waste less power.

They would be even better (on all variables above) if they didn't need to deal with a low frequency AC supply. Either a high frequency AC or a DC one would do.

Re:Makes sense. (0)

Anonymous Coward | more than 2 years ago | (#39358375)

DC/DC converters are better than transformers in almost every way

Except in component count and circuit complexity. If you want reliability, nothing beats the simplicity of a couple of hunks of copper coiling around an iron laminate.

Re:Makes sense. (3, Informative)

Nethead (1563) | more than 2 years ago | (#39358635)

The last two data centers (Clearwire) I built out were DC. The only AC in the cage was for a video monitor and for the tech's wifi router. Very standard stuff, the telcos have always done it that way. Any bit of Cisco/Juniper/whatever kits can be ordered with DC power supplies. I see DC plants as more the standard now. And yes, the are still built using waxed string.

Even Power over Ethernet has it roots in telco -48VDC power. All the WAPs and fiber converters at a Lowe's are powered by a Valare DC power supply ( http://www.power-solutions.com/dc-power-systems/eltek-valere.php [power-solutions.com] ).

One nice thing about DC plants are the power cables are cut to length so you don't have all that extra line cord to bundle and hide.

Re:Makes sense. (0)

Anonymous Coward | more than 2 years ago | (#39358649)

The greenies / cost cutting / etc

Fuck you.

The side that wins the current wars will be (2)

sconeu (64226) | more than 2 years ago | (#39357849)

whichever one gets Dirty Deeds Done Dirt Cheap

Re:Makes sense. (1)

geekoid (135745) | more than 2 years ago | (#39357875)

but that's the proble. we aren't talking about 1 rack, we are talking about 10' of thousands of machines. How is the efficiency to the end of all the computers? what about heat? risk?

I honestly don't know the answer, and I look forward to datacenters data.

Re:Makes sense. (1)

viperidaenz (2515578) | more than 2 years ago | (#39357955)

Easier, not better. To carry the same power with the same losses, you need 1.4x more cross section in your cables. That's if you have perfect power factor. If you have poor power factor you need much thicker cables. Transformers are also not the most efficent at low frequencies like 50Hz. They require massive amounts of steel and copper too. Modern switch mode power supplies (like those used in computers, since transformers could not be made small enough the provide the required power) need to convert the AC into DC as the first step, resulting in losses in the diodes used. Around 1 watt is lost as heat for every amp fed in to it.

Re:Makes sense. (1)

fuzzyfuzzyfungus (1223518) | more than 2 years ago | (#39358387)

While it is true that AC can be voltage converted with nothing more than a transformer, it isn't really relevant: the old school AC transformer units have miserable efficiency and are both heavy and bulky. Basically all modern equipment is going to be using a switchmode power supply that is a great deal closer(in terms of complexity, cost, efficiency, and theory of operation) to a DC-DC converter.

Either way, you get to play the "let's balance transmission losses vs. redundancy vs. efficiency vs. component cost" game in terms of how many points of conversion you want and where you want them; but nothing that belongs in a datacenter is going to be using AC just for the ease of PSU construction.

War of the currents (1)

Anonymous Coward | more than 2 years ago | (#39357395)

It should be noted that this battle is on a different battlefield than the one in the 1880s. There the battlefield was distance and AC eliminated the need to have a power plant every few miles. In addition it takes lots of copper to carry lots of current any distance. Now a data center still is AC to the PDU or power switch board where it may well be converted. The outside power will still be AC and likley the generator set will be AC as an AC generator is lower maintenance than a DC one. On a related topic locomotives are switching to AC from DC as the weight of the motors and the generator set is less. It is perhaps more appropriate to say that the choice of which kind of current depends on the application. Due to better rectifiers and inverters very long distance power transmission is better done with DC, to avoid the capacitance problems with AC on long high voltage lines (The Dalles OR to LA for example)

Re:War of the currents (-1, Offtopic)

stanlyb (1839382) | more than 2 years ago | (#39357505)

You man, don't know anything about analog current and digital one. Sorry, go and take this course again. With A+. The cheapest way to transport electricity from point A to point B is to use, surprise, the "wave" format. The sinusoid one.

Re:War of the currents (0)

Anonymous Coward | more than 2 years ago | (#39357703)

I know that "transporting" and "distributing" seem similar, but I assure you that in this case there are important differences. When you need DC at 100 different drop points within 10 metres of each other, do you want to convert AC to DC at every node?

Re:War of the currents (1)

vlm (69642) | more than 2 years ago | (#39357767)

You man, don't know anything about analog current and digital one. Sorry, go and take this course again. With A+. The cheapest way to transport electricity from point A to point B is to use, surprise, the "wave" format. The sinusoid one.

LOL wake me when you have calculations showing RMS voltage is greater than peak voltage for a AC waveform...

Re:War of the currents (1)

geekoid (135745) | more than 2 years ago | (#39357893)

why is the relevant?

Re:War of the currents (1)

vlm (69642) | more than 2 years ago | (#39358085)

Cost of electricity dwarfs cost of endpoint components, at least now a days, so cheapest way to transport = most watts thru a piece of wire.
Watts is volts times amps
The insulation determines the peak voltage. For DC the peak is also the operating voltage. For AC the peak is the ... peak of the sine wave.

The graphical/intuitive answer is DC can run full output continuously, but a AC sine wave can only run full out for a zillionth of a second at the peak voltage. If somehow magically you made the AC signal run at the peak voltage all the time, then it could carry as much power as the DC line... it would also have become DC in the process.

You can also run some math. Obviously on long term average DC watts = volts which is constant times amps which is constant. Not so simple for AC. Turns out the equivalent power transfer of a AC wave is the RMS voltage. You can calculate it, but conceptually its the DC equivalent voltage.

USA wall outlet theoretically around 120 volts RMS which is 190 or so peak. Voltage is specd to plus or minus 10% so don't jump on me for being 5% low or whatever, I chose easy numbers because I'm lazy. You need to insulate to at least 190 volts. ten amps at 120 AC is 1200 watts. Those same wires could carry 190 volts of DC at ten amps or 1900 watts.

For a simple voltage limited transmission line, you'll always shove more power down the line if you go DC.

There are also AC power factor problems. And capacitive losses. and higher corona losses (well, that's debatable).

Things get complicated if you allow precisely 3 wires and no ground currents. Paralleling plus or minus won't help much. 3-phase AC might help. 3-phase is cool for other reasons like constant torque on a rotor shaft and stuff like that. Also it means you've got 50:50 chance the electricians hook up the motor to run in reverse which is always hilarious/tragic in industrial design.

Re:War of the currents (1)

vlm (69642) | more than 2 years ago | (#39358121)

Turns out the equivalent power transfer of a AC wave is the RMS voltage.

Err times the current, yeah. Ugh.

The point is the "average" of a DC line is ... the peak. The "average" of a AC wave is the RMS voltage which is about 70% of the peak.

I put "average" in scare quotes because the actual integrated voltage of a sine wave is zero. Or sometimes the "average" is calculated another way.
The number you're looking for is RMS root-mean-squared. take wild guess how you numerically calculate that...

Re:War of the currents (0)

Anonymous Coward | more than 2 years ago | (#39358109)

No, since the skin effect means you are not using the center of the conductor. That's not efficient. Sorry, *you* go learn some physics and some English too. There is no "digital" or "analog" current. You are an idiot.

Re:War of the currents (1)

c0lo (1497653) | more than 2 years ago | (#39358141)

You man, don't know anything about analog current and digital one..

Yes! Digital current!!! How about feeding the computers with AC at a frequency of whatever clock ticks/sec the CPU needs?

(duck... stop shooting)

Not really (2)

Anonymous Coward | more than 2 years ago | (#39357413)

which means that most modern data centers today run on AC power

Only if you ignore all telecom equipment which have run on -48VDC for decades. True, they're not really 'data centers' but it's not like they don't use massive amounts of electricity.

Re:Not really (1)

Skapare (16644) | more than 2 years ago | (#39357585)

48VDC also means a rather large amount of current. A data center in many cases these days is much, much larger than a telco switching center was (aside from maybe a few trunk points for large cities). They did, in many cases, divide up the electrical systems to avoid high fault currents. But it was well know the high battery currents involved could be a disaster if there was a short, even on a branch tap into equipment.

The benefit of DC distribution was NOT efficiency. They did use resistors and in some cases very large inductors, to reduce current faults and fault rise times. That would not fly today when the objective is efficiency.

Ultimately, a data center wide DC bus is very UNSAFE, even at 340V, but way more so at 48V. Don't even think of trying that at 12V. You need to segment the power systems to keep them safe. One segment per rack or two is about the way to go (just one rack if it's 12V). Even then, we're talking a few hundred amps of usage and a few thousand amps of fault.

telecom DC usage was based on a myth (0)

Anonymous Coward | more than 2 years ago | (#39358551)

In what should come as little surprise to slashdotters anywhere, the telecom equipment switch to -48VDC was driven by a myth propagated by PHB's who were measuring the wrong thing http://www.pipelinepub.com/0407/pdf/Article%204_Carrier%20Grade_LTC.pdf [pipelinepub.com]

Take some advice http://www.apcmedia.com/salestools/SADE-5TNRLG_R6_EN.pdf [apcmedia.com] from an engineer (not me) with 19 patents related to high-efficiency and high-density data center power and cooling infrastructure and read up on why so much of the non-US data centers use 400/230V AC because they are (a) nearly 6% more efficient than 48V DC systems and around 1% less efficient than 380V DC. Which helps explain why much more equipment is available for 230V AC than for 380V DC.

prior art 8) (1)

pbjones (315127) | more than 2 years ago | (#39357453)

most telephone exchange and related transmission hubs use DC 12, 24 and 48VDC are standard. This isn't anything new, and data centers have always been space and power inefficient, it's the nature of the beast, and method of construction.

Re:prior art 8) (1)

GerryGilmore (663905) | more than 2 years ago | (#39357567)

Along with the specialized telecom equipment, a few standard server vendors, including Intel when I was there, have models designed with 48VDC power, along with NEBS-compliant features like - not catching fire.

low-voltage vs. high-voltage DC (0)

Anonymous Coward | more than 2 years ago | (#39357461)

When talking about DC power distribution in data centers, people usually assume the DC will be at a low voltage, like 12, 24 or 48V. With such low voltages, the currents must be huge to supply the required power, which translates into large losses, or very big cables and busbars.

High voltage DC, on the other hand, really makes sense; instead of using power supplies with internal rectifiers and power factor correction, with the latter usually causing a significant loss in efficiency, one could build a big, central power factor correction device, which is probably more cost efficient en possible more cost effective than having a separate one in every single computer power supply.

AC makes sense when you need to use old style transformers (big iron cores, at 50 or 60Hz), while switch-mode power supplies would work a lot better with a high-voltage DC input. An added bonus is that the RMS current in the cables is smaller with high-voltage DC, reducing the losses even further (although the gain is probably rather small)

Ridiculous analysis (1)

Anonymous Coward | more than 2 years ago | (#39357465)

it has nothing to do with AC versus DC, but when and where the conversions are done.

In an old-style setup, you use high voltage AC for long distances and convert to DC just once, near the load.

In a medium-new design, you convert the 120 or 220 VAC to DC by rectification at each PC, then use a switching-mode power supply to down-convert that to the required DC levels.

In an even newer design, you notice that the rectifiers are semi-redundant, you can factor that out and have one or a few larger rectifiers and distribute medium-voltage DC.

You will still need switching-mode regulators at the card or CPU level.

Not a very big difference. Factoring out the rectifiers lets you use slightly more efficient synchronous rectifiers. The money you save there you can apply to buying slightly more efficient switching regulators.

Yay, another volt standard... (4, Informative)

mlts (1038732) | more than 2 years ago | (#39357467)

There was an article about using 380 volts a couple weeks ago on /. in the data center.

Having DC brings some benefits, mainly just needing to step down voltage and not have to rectify it smoothly with capacitors to even out the output current.

However, there are some downsides:

1: AC power supplies in devices tend to be more tolerant of power fluctuations. An all DC shop might completely be halted by a power surge/spike that wouldn't bother a data center on AC.

2: DC sparks a lot when connecting/disconnecting. AC has plenty of zero-crossings a second (120 or so), so it won't make the fireworks show when plugging/unplugging. This makes switches rated for DC a lot more expensive than AC.

3: There is no such thing as a NEMA 380VDC connector. So, either items would have to be wired up to a bus bar similar to how 48VDC telco stuff gets, or it will end up like 12VDC with at least 5+ connectors (direct wires, cig lighter, airplane, marine connector, male/female combined connector, motorcycle accessory connector, banana plugs.)

4: Safety. 12 VDC shocks are annoying; a shock from 380VDC will be fatal, especially because of DC's tendency to get muscles to "lock". (This is why stun fences uses AC, while kill electric fences use DC so they can keep the target locked on the wires long enough to get the amps across the heart.)

5: Issues with wire length. AC, it isn't hard to use a transformer to deal with voltage drop. DC, that will be a lot harder.

All and all, 380VDC seems like a solution in search for a problem. We really don't need another standard. Heck, just pointing out 120VAC in the US means I have to doublecheck if I'm dealing with 15 amps, 20 amps, 30 amps, or 50 amps, and the locking versions of each, which means six plug types and minimum wire gauges.

Re:Yay, another volt standard... (1)

Microlith (54737) | more than 2 years ago | (#39357601)

1: AC power supplies in devices tend to be more tolerant of power fluctuations. An all DC shop might completely be halted by a power surge/spike that wouldn't bother a data center on AC.

All this does is require that the conditioning for power be done well before it reaches the machines. There will be an AC->DC power supply regardless, it'll just be much, much larger and could probably supply even more resilience than a bunch of smaller power supplies.

2: DC sparks a lot when connecting/disconnecting. AC has plenty of zero-crossings a second (120 or so), so it won't make the fireworks show when plugging/unplugging. This makes switches rated for DC a lot more expensive than AC.

So you'll handle it much like most hotplug PC hardware is these days, with latches and mechanical disconnects that ensure + and - are disconnected simultaneously.

3: There is no such thing as a NEMA 380VDC connector. So, either items would have to be wired up to a bus bar similar to how 48VDC telco stuff gets, or it will end up like 12VDC with at least 5+ connectors (direct wires, cig lighter, airplane, marine connector, male/female combined connector, motorcycle accessory connector, banana plugs.)

Just because one doesn't exist now doesn't mean won't be brought into existence. Virtually all modern PC interconnects are standardized, even before products are on the market.

5: Issues with wire length. AC, it isn't hard to use a transformer to deal with voltage drop. DC, that will be a lot harder.

So we might see these on a rack-by-rack basis, or high voltage to the racks at which point we step down. Not an insoluble problem.

Really, the problem being approached here is that of power efficiency. A pile of 80 PLUS Gold power supplies spread across hundreds or thousands of machines is a pretty significant drop in efficiency compared to doing the AC->DC conversion in fewer places and stepping down. If it wasn't, people wouldn't be considering solutions like this.

Re:Yay, another volt standard... (1)

vlm (69642) | more than 2 years ago | (#39357839)

AC power supplies in devices tend to be more tolerant of power fluctuations. An all DC shop might completely be halted by a power surge/spike that wouldn't bother a data center on AC.

Essentially you're just removing the rectifier from the power supply, putting it outside, and feeding the same old switching supply indoors. Not so. You could design a system that intentionally was more sensitive, but no one would intentionally do that.

or it will end up like 12VDC with at least 5+ connectors

The world seems to be converging on the Anderson Power Pole connector (which I believe is a (TM)). Cheap, high current, tough, reasonable simple to assemble...

All and all, 380VDC seems like a solution in search for a problem

See the above. Basically you're doing a lot of foolishness to remotely mount the rectifier diodes. Hard to buy ones below 99% efficient at that operation, so lots of Fing around for not much. I'd have to tentatively agree with you.

Why use electricity at all? (0)

Anonymous Coward | more than 2 years ago | (#39357521)

Why not send prototachyons through the main deflector array?

Re:Why use electricity at all? (1)

webnut77 (1326189) | more than 2 years ago | (#39358203)

Why not send prototachyons through the main deflector array?

Because it interferes with the tachyon beam, silly.

6 one way, half a dozen the other (4, Interesting)

thsths (31372) | more than 2 years ago | (#39357535)

AC, DC, it does not make a difference any more. Yes, you have to rectify AC before it powers a computer, but the rectification costs less than 1% of the energy. Power factor compensation can be more costly, but it could be avoided by going to a 3 phase rectifier. There are also serious distribution advantages in 3 phase electricity, but it is not used because of the extra complexity, despite being cheap.

DC distribution is expensive, and 1% gain is just not enough to pay for it. Once we have intelligent grids, the situation may be different, but for now there is just no business case.

Re:6 one way, half a dozen the other (0)

Anonymous Coward | more than 2 years ago | (#39358927)

Why do you have to do power factor compensation? I thought that "that was done by the power company", which is why the various devices you can use for it are basically scams.

I say this as someone who admittedly doesn't really understand the compensation in the first place.

Yes, but (1)

Hatta (162192) | more than 2 years ago | (#39357537)

What if you want to electrocute an elephant?

Re:Yes, but (3, Funny)

KBehemoth (2519358) | more than 2 years ago | (#39357579)

The appropriate demo of the dangers of AC data center power will be to show an elephant losing his entire database due to a power failure. Ominous voiceover: "Unlike an elephant... AC-driven data centers always forget!"

Re:Yes, but (1)

idontgno (624372) | more than 2 years ago | (#39358545)

AAAH! My database got Westinghoused! [urbandictionary.com] Where're my backups?!??!

You're gonna spend your savings on copper (3, Interesting)

ZorinLynx (31751) | more than 2 years ago | (#39357549)

Standard -48VDC current distribution requires four times the current as 208V AC distribution for the same amount of power. Have you seen DC cabling at data centers that use it? If we're going to start using DC in data centers we need to come up with a higher voltage standard, otherwise we're going to spend all the savings on more copper (which is expensive!) to carry those extra amps.

Re:You're gonna spend your savings on copper (0)

Anonymous Coward | more than 2 years ago | (#39357575)

Wanna know how I know you didn't read the article? The study referenced 380VDC distribution, pal.

Re:You're gonna spend your savings on copper (1)

Skapare (16644) | more than 2 years ago | (#39357609)

380VDC is still horribly unsafe without proper segmenting. And that means a lot of efficiency if you segment from a single large massive conversion system. You need to segment at the rack level. And then you end up with the double conversion scenario.

wire loss is I^2R (0)

Anonymous Coward | more than 2 years ago | (#39357553)

there is something to be said for higher voltages for transport. line loss is I^2R where R is the resistance of the wire. if you need a LOT of power moving it high voltage to where you need it is best. is DC better than AC? i wouldnt say that is true. low voltage DC is going to be a killer to distribute efficiently given the wire losses.

you forgot the lightning bolt (1)

Joe_Dragon (2206452) | more than 2 years ago | (#39357557)

you forgot the lightning bolt

Let's electrocute some elephants just to be sure! (1)

Chas (5144) | more than 2 years ago | (#39357595)

If it was good enough for Edison, it's good enough for me!

Slashad (3, Insightful)

Shadowhawk (30195) | more than 2 years ago | (#39357641)

Articles main source says modern AC and theoretical DC are about the same. By the way, he has a product to sell...

Re:Slashad (1)

Skapare (16644) | more than 2 years ago | (#39357885)

And you don't literally need, or need all of, his product, to make a very efficient AC-based data center.

I am concerned about his brief mention of cooling that seemed to be based on using a single system. There, I would want multiple redundancy at N(4)+2. The more discrete units you have, the more STABLE you can hold the temperature. The more stable the temperature, the higher temperature you can run it at. UNSTABLE temperatures cause damage to equipment as much as too high a temperature.

This is New? (2)

Pontiac (135778) | more than 2 years ago | (#39357661)

In 2005 we started looking at blade chassis and tested a rack of HP BL series blades.

That system came with a 48v DC power enclosure with 6 hot swap power supplies. It sat in the bottom of the rack and had a buss bar system to feed every chassis in the rack.

As others have stated.. 48v is a long standing standard for telecom power.

Re:This is New? (2)

Skapare (16644) | more than 2 years ago | (#39357811)

But 48VDC also means dual conversion. Convert the AC to 48VDC, then do the conversion again with the PSU in each chassis. You have to get both conversions to be very, very efficient to make that worthwhile.

Everything from Cisco can be had with 240VAC. Very little telco equipment these days actually requires a 48VDC power source. And most of that is for telcos, not for web site providers (for example). And where big network providers do need some 48VDC-only equipment, that can usually be put in the northeast corner of the room and limited in current.

Re:This is New? (1)

PPH (736903) | more than 2 years ago | (#39358169)

But 48VDC also means dual conversion.

Not really. Every switched mode power supply converts AC to DC, then back to AC (at a very high frequency) and then back to DC (at several voltages). The whole DC buss distribution idea pulls the first AC to DC conversion out of every individual supply and centralizes them. This makes it possible to back up the DC buss with batteries. But as others have noted, the high fault energies available on these busses are harder to deal with using common circuit breakers.

AC/DC? (1)

Anonymous Coward | more than 2 years ago | (#39357717)

I was shakin' at the knees Could I come again please? Yeah the ladies were too kind You've been - thunderstruck, thunderstruck Yeah yeah yeah, thunderstruck

I'll stick with AC through the data center and... (2)

Skapare (16644) | more than 2 years ago | (#39357753)

... convert that AC to DC at a "blade rack". That would be a rack designed to take blades. But the blades would be a mix of

  • Processor blades (mostly)
  • Power conversion blades
  • Battery backup blades

This will safely segment the power, leaving the DC busses limited to the amperage needed for one rack ... or even partial rack. It also has the flexibility of balancing power conversion vs. 1st tier power backup (at the point of use). Increasing the backup times to a couple minutes allows slow start generators, which are more reliable.

I would run 416/240 three phase everywhere in the data center (even in North America ... transformers for this are readily available). Where equipment isn't on the DC system, run it on 240VLN. The AC/DC converters might run on 240VLN or 416VLL. In countries with 400/230 or 380/220, just use it that way direct.

AC is safer due to the zero crossing. Circuit breakers can break a lot more power (usually 5x the voltage) with the advantage of AC, as compared to DC. A 380VDC breaker for a rack would be HUGE, especially if it has to handle a data center level of fault current.

what about PSU with buildin UPS hook ups? (1)

Joe_Dragon (2206452) | more than 2 years ago | (#39357869)

what about PSU with build in UPS hook ups? so you can get rid of the AC to DC to AC to DC part and make it just AC to DC? at each system?

Re:what about PSU with buildin UPS hook ups? (1)

Skapare (16644) | more than 2 years ago | (#39357923)

Are you talking about a 2nd AC input, or a separate DC input which can be supplied direct from battery?

Re:what about PSU with buildin UPS hook ups? (1)

nschubach (922175) | more than 2 years ago | (#39358749)

I assume a DC connection that you can plug into a bank of batteries that can be used for power if the AC should fail (and charging the batteries when the AC is on).

No AC/DC comments? I'm saddened. (0)

Anonymous Coward | more than 2 years ago | (#39358001)

I don't want to live on this planet anymore.

Direct Current is localized current (0)

Anonymous Coward | more than 2 years ago | (#39358039)

What was missed in the 100+ years that electrictity has been in use was the adaptability of use on a broad scale. N. Tesla was right; unfortunately; the 'powers that be' were interested in profit. The 'Wardenclyffe Tower' Experiment should have been a continued. The realization that ' Earth is one big battery' would have changed our perception of electricity, for ever.

Phone COs are already DC (0)

Anonymous Coward | more than 2 years ago | (#39358045)

AC is what is normal for power companies to provide in the US.

DC is what all of the phone company equipment runs off of in the mini-C.O.s, though.

So, I'd say that using DC to power electronics is hardly a novel concept.

Verari Systems (4, Interesting)

sdguero (1112795) | more than 2 years ago | (#39358165)

I worked there for 7 years. I'm not going to get into specifics but I will say:

Verari tried to take advantage of the efficiency gains in DC with exotic power supplies etc... And that company went the way of the dodo bird after trying to force 800V, 48V, and 12V DC power distribution systems in customer data centers. The fact is, everything already out there (switches, routers, servers, etc) uses AC-DC power supplies in each unit and it works in 99% of power outlets with pretty good uptime. The added complexity of running DC infrastructure isn't worth the efficiency gains (which on paper sound like a lot but theory rarely translates to reality the way we think it will), and when one DC rectifier burns up and takes down a hundred servers (vs 1 server with an AC-DC supply), customers aren't happy. Between the uptime issues and employee safety concerns (high amperage DC power is more dangerous than AC for a variety of reasons) it's also a liability nightmare

Again, I don't feel like getting into specifics but modern datacenters != underground telco installations and DC power distribution has a LOT of challenges that are often overlooked when marketing types start squawking about efficiency gains.

If you ever designed a data center ... (0)

Anonymous Coward | more than 2 years ago | (#39358441)

you would know:
- Adding batteries to a central redundant DC power supply makes a wonderful online UPS.
- DC/DC converters are small and efficient.
- PC/Server AC power supplies are not so efficient and not so small.
- AC power supply efficiency is not linear (power usage vs. max. power offered).

A central DC power supply will have a shared high power usage causing a higher efficiency than having an AC power supply for each server.

And for SPOF people: A & B paths are standard for DC power.

There is only one way to settle this... (1)

WombleGoneBad (2591287) | more than 2 years ago | (#39358719)

I am strapping the electrodes to the elephant now...

DC advocates trying to get on the same page (3, Informative)

1sockchuck (826398) | more than 2 years ago | (#39358843)

The effort to gain acceptance for DC distribution in data centers is being helped by a series of investments by ABB [datacenterknowledge.com] , and the growth of the EMerge Alliance [datacenterknowledge.com] , which is trying to unify DC proponents around a 380V standard. The challenge for DC is that customers don't ask for it, meaning multi-tenant facilities aren't likely to offer it. Also, Schneider says it is "not aware of any data centers moving off of their established, traditional power distribution to DC." In fact, NTT has at least five DC data centers in Japan, and ABB is backing a DC distribution project at a Swiss hosting company [datacenterknowledge.com] . In the US, there are numerous sites testing DC power, which is widely used in telecom infrastructure.

I build data centers.. (1)

Anonymous Coward | more than 2 years ago | (#39358891)

and you can bet your ass that DC wont make its way into any of them anytime soon. Transmission line efficiency is the same for DC as it is for AC. Yes AC can be transmitted at higher voltages and easily converted reducing the current carried by the transmission, but power is the product of voltage and current and thus the loss is still there regardless - you're just using more voltage and less current. The only real benefit of using a higher voltage is that the transmission line can be smaller.

Not mentioned yet that I see is installation cost. The biggest cost of building a data center is electrical. The current standard is 3-phase 480vac power. 3 hot wires and a shared neutral. In a DC system the hots cant share a neutral. At face value you just increased the cost of wire and bus-duct by 50%. And believe you me, there are tons of it in a data center. If you've never seen one you would not believe how much of that there is in a data center. On a balanced load you really only have to contend with wire resistance to the equipment, and not coming back on the neutral. That's not the case in a DC system. In a DC system, your wire distance is effectively doubled over a balanced 3-phase system, and the DC system needs 50% more to wire to boot.

Load More Comments
Slashdot Login

Need an Account?

Forgot your password?
or Connect with...

Don't worry, we never post anything without your permission.

Submission Text Formatting Tips

We support a small subset of HTML, namely these tags:

  • b
  • i
  • p
  • br
  • a
  • ol
  • ul
  • li
  • dl
  • dt
  • dd
  • em
  • strong
  • tt
  • blockquote
  • div
  • quote
  • ecode

"ecode" can be used for code snippets, for example:

<ecode>    while(1) { do_something(); } </ecode>