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Was Thomas Edison Right about DC Power? 545

Declan McCullagh writes "Everyone knows the alternating vs. direct current wars ended with Thomas Edison and Nikola Tesla. But now DC power is being seriously considered for data centers. DC advocates say that plugging servers into AC power is inefficient, and switching to DC cuts down on waste heat and component failure. The University of Florida has even bought 200 DC servers."
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Was Thomas Edison Right about DC Power?

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  • by davmoo (63521) on Thursday March 02, 2006 @09:22PM (#14839966)
    Tesla and Edison were both right...and wrong. Like many Slashdotters do when debating which operating system is best for any given job, Tesla and Edison wanted to apply one power system to every job. Its like having a toolbox with only a screwdriver in it. Ever try to drive a nail with a screwdriver?

    For moving power over long distances, AC is king. But for short distances with most modern electronics, DC would win. The first thing a desktop system or server does with AC is converts it to DC. So if you have a number of machines all in the same room, why not do the conversion in one spot, and eliminate the redundancy in every machine.

    Would it benefit the average user with one or two machines? Not at all. But for a major center with many machines in the same room, I can see quite a bit of benefit with going with DC.
  • by imsabbel (611519) on Thursday March 02, 2006 @09:24PM (#14839979)
    "Anisotropic Magnetic Field" has to be the worst offense in terms of technobabble i have seen recently.
    Newsflash: there are no magnetic monopoles, so EVERY magnetic field is anisotropic...
  • by AndroidCat (229562) on Thursday March 02, 2006 @09:31PM (#14840012) Homepage
    It was a proprietary patent pissing match too. Edison went with the system that he controlled the patents on, and didn't really care about the technical merits of each system.

    Just think, if he'd settled with Tesla back then, today they could be sending people to be killed on the Edison Chair.

  • by demonbug (309515) on Thursday March 02, 2006 @09:33PM (#14840026) Journal
    First, that would be anisotropic.
    It doesn't seem too surprising that AC power would produce an anisotropic field since the current keeps switching, so the magnetic field should be switching direction also. I suppose this would make the magnetic field from a DC current isotropic (invariant with direction, or I suppose in this context constant orientation), but I don't really see why either would be an issue (since you referred to Maxtor, I assume the issue was something that had to do with hard drives). Although if you have a weak, constantly switching magnetic field it might demagnetize (randomize) low-coercivity magnetic grains (domains, whatever - I work with sediment, dammit!), but unless it is a pretty strong field it shouldn't bother the relatively hard (magnetically) magnetic media in use.
    I'm too lazy to actually look up what you are referring to, though, so whatever.
  • by Animats (122034) on Thursday March 02, 2006 @10:02PM (#14840181) Homepage
    There are some advantages to operating on 48VDC, but unless you have the big battery room of a telephone central office, they're not that big.

    What Rackable is really pushing is a system where AC to 48VDC conversion takes place in a unit at the top of the rack, and 48VDC is local to the rack. That, at least, simplifies the cable management.

    One big advantage of 120/240VAC power distribution using US standards is that the connectors are standardized and reasonably idiot-proof. That is, if you can plug it in, you won't overload the power cord or the connector, and if you overload the branch circuit, a breaker will trip. Outlet strips have circuit breakers, so you can't overload the cord to the outlet strip without a breaker trip. There are NEMA standard power plugs [leviton.com] for 15A, 20A, and 30A circuits, 120/240VAC, and single and three phase configurations. All this is standardized nationally and enforced by the National Electrical Code.

    In contrast, there are no simple standards for 48VDC. Most 48VDC gear has big screw terminals. There are no standard plugs and sockets. Somebody, preferably a licensed electrician, has to check all the data plates, add up the current loads, calculate voltage drops, size the wire and breakers, and torque the big screw terminals to the correct torque, using the correct lockwashers. Every time you add or change a load, somebody has to recheck the math. Errors can cause a fire. None of this is all that hard if you have basic power technician skills, but you can't just go casually plugging stuff in.

    Although, since the development of the low-cost clamp-around DC ammeter, things have become easier in the DC world.

  • by Omega Hacker (6676) <omegaNO@SPAMomegacs.net> on Thursday March 02, 2006 @10:18PM (#14840243)
    Last I checked, pretty much every data center worth its name has a bank of UPSs. That means that power is coming in AC, and being converted to DC to charge the batteries. AFAIK any decent UPS in use in a server these days is "on-line", which means that instead of attempting a fast switchout between mains and battery, all outgoing AC power is re-generated from the DC battery bus. If you assume a 10% loss in both steps, you're at 81% right away. Add a bazillion AC power supplies at 10% loss and you're down to less than 73% efficiency.

    Contrast this with a properly designed DC system a la old-school telco: The same front-end of the UPS is used, with a 10% loss converting AC to battery voltage. Then you run that into DC supplies that, with modern electronics, are going to be doing a lot better than an AC supply, so let's say 5% loss. That puts you at better than 85% efficiency.

    The critics cited in the article are actually probably not far off in calling the Rackable solution over-hyped, if you only take into account the isolated-rack design. Rackable puts 2-3U of beefy redundant supplies at the top of the rack and does DC to the servers. Efficiency-wise this is only fractionally better than a bazillion AC supplies, and quite possibly dead even because of the DC->DC losses in each server on top of the AC->DC->AC->DC setup implicit with AC-based UPS systems. However, AFAICT from a glance at their site, Rackable's systems are designed to drop right into existing DC datacenters, which eliminates the AC supplies at the top and the DC->AC->DC stages.

    The issue is what kind of infrastructure is needed to feed the selected DC voltage (which is going to be -48VDC) into the racks with the lowest bus losses, but this is someone I would expect is either a) already solved by the decades-old telco industry, or b) going to be solved in at the appropriate 384-cores-and-100TB-per-7ft-rack scale RSN, by "the market".

    I know that if I were in the position of designing a big datacenter right now, I would be looking very hard at DC systems.

  • by LoRdTAW (99712) on Thursday March 02, 2006 @10:28PM (#14840284)
    Great clicked submit by accident, link is in the wrong place. Too bad we cant edit our comments.
  • by Cobralisk (666114) on Thursday March 02, 2006 @11:31PM (#14840534)
    DC power is easier to deal with for backup redundant systems than AC. Simply put a battery (or 100 - as many as you like) in parallel and you're done. This is how telephony has always worked. Ampacity is your main concern. There are no frequency/phase angle/power factor related issues. DC is simpler than AC. The -48v in common use(actually closer to 52v, but who's counting) is safe to touch with your bare hands (although you can arc weld a screwdriver with it - melted metal==ouch). The major inconvenience is you just can't send it through a transformer. The added bonus is the internal capacitance of the batteries in parallel makes for a very stable power supply with a nice buffer for spikes/brownouts. The market for this equipment is pretty large and mature, since the entire PSTN(including all dsl service) runs on it already, and the big cable companies are migrating this way as well.
  • by Doc Ruby (173196) on Friday March 03, 2006 @12:03AM (#14840656) Homepage Journal
    Edison wanted to pump DC across longdistance lines, which would have consumed much more than the 10-20% losses in Tesla's AC. A hundred years ago, electronics complexity was so low that a single electric motor's mechanical power was often distributed around an entire factory by pulleys, rather than use multiple motors. Now we've got more complexity on a single square CPU inch than existed in the entire world when Tesla and Edison battled.

    I'd love to see how Tesla would have applied his high frequency/voltage engineering to photonics.
  • by MysteriousPreacher (702266) on Friday March 03, 2006 @09:53AM (#14842399) Journal
    Beowulf cluster != BSD dying.

    I fail it.

Marvelous! The super-user's going to boot me! What a finely tuned response to the situation!