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I was wondering why hard drives rotate at these specific speed (5400,7200, ...) and not other values.

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4 Answers 4

up vote 17 down vote accepted

A/C motors in many parts of the world commonly rotate at 3600 RPM because that's 60 Hz. 7200 rpm is obviously twice that, and 5400 rpm 1.5x.

I don't know the real reason, since HDD motors aren't driven from A/C, but it's likely IMHO that it's related to that. Like used to be possible with vinyl record decks, it's easy to check that something is running at the right speed by illuminating it with a strobe running at the required speed. If it's at the right speed (or a simple multiple thereof) then a mark on the motor will appear stationary.

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+1 He!!, sounds good to me :) –  squillman Jun 11 '09 at 16:23
    
Then where did they get 10,000 from. other than it's a nice round base 10 number? Why not 10,800, or is that the real speed and 10k is just used for marketing? –  Bratch Jun 11 '09 at 17:41
    
it probably is 10k, there's no real need (AFAIK) for the speed to be a harmonic of 60 Hz any more. –  Alnitak Jun 11 '09 at 19:28
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OK, could someone explain why some so-called 10k drives are in fact 10025 RPM (so slightly above 167 Hz)? –  kubanczyk Jun 22 '09 at 21:37
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10,000 rpm disks were designed in Europe, where we use 50Hz A/C :) –  Roger Lipscombe Jul 13 '09 at 9:52
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Thanks to this answer, I have found the explanation at http://books.google.com/books?id=Yu5SAAAAMAAJ&q=electric+engine+7200+3600

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I think that's only incidental - despite what I've written above HDD motors aren't driven from A/C. –  Alnitak Jun 11 '09 at 16:45
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(virtual -1) for not including a summary. –  Dennis Williamson Jun 11 '09 at 17:14
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Like many things in computing, the reason is historical. The design of early PC hard drives was based on earlier, large, mainframe hard drives which were powered by AC.

That is according to PCGuide.

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"At one time all PC hard disks spun at 3,600 RPM; in fact, for the first 10 years of the PC's existence, that was all there was. One reason for this is that their designs were based on the old designs of large, pre-PC hard disks that used AC motors, and standard North American AC power is 60 Hz per second: 3,600 RPM. In the early 1990s manufacturers began to realize how much performance could be improved by increasing spindle speeds." After switching from AC to DC they can make the speed anything. 20k rpm drives must be next. –  Bratch Jun 11 '09 at 18:07
    
@Bratch: just Hz, not Hz per second. Sorry for being pedantic ;) –  UpTheCreek Oct 17 '11 at 11:37
    
Everything between the double quotes was copied and pasted from PC Guide - pcguide.com/ref/hdd/op/spin_Speed.htm. "Hz per second" would be like saying, "RPM per minute." –  Bratch Oct 18 '11 at 20:11
    
Actually, "Hz per second", and "RPM per minute" aren't that invalid.. They just represent a rate of change, like "Metres/second per second" is a rate of acceleration. –  Tom O'Connor Apr 9 '12 at 0:11
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Correction: HD spin motors are driven by AC, very likely three-phase, but the AC comes from an inverter, although it's not commonly called that. It converts the DC power fed to the drive assembly into AC for the spin motor. That's done by an IC on the drive's circuit board.

Brushless DC motors could be used, but for mass-produced specific-purpose devices, that would'nt make sense. (DC motors with brushes are out of the question. No way to make the brushes and commutators last a long time.)

When you get up to 7,200 RPM, air resistance (and, probably turbulence) start to become significant; at 15,000 rpm, platters need to be smaller, afaik primarily because of air drag.

Some recent drives (2012, late 2011, perhaps slightly earlier) run at variable speeds, apparently dependent upon what they are doing; likely that the slowest is for standby.

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