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."
They were both right...and wrong... (Score:5, Insightful)
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.
Re:Antistropic Magnetic Fields (Score:3, Insightful)
"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...
Re:Was Edison right? (Score:2, Insightful)
Just think, if he'd settled with Tesla back then, today they could be sending people to be killed on the Edison Chair.
Re:Antistropic Magnetic Fields (Score:3, Insightful)
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.
The trouble with 48VDC land (Score:3, Insightful)
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.
UPS? Of course DC is more efficient! (Score:3, Insightful)
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.
Re:Sensationalist, but effectively correct (Score:2, Insightful)
Re:Sensationalist, but effectively correct (Score:4, Insightful)
Re:Sensationalist, but effectively correct (Score:5, Insightful)
I'd love to see how Tesla would have applied his high frequency/voltage engineering to photonics.
Re:Someone has to say it... (Score:3, Insightful)
I fail it.