Why HDD rotate at 7200 rpm ?

I was wondering why HDD rotate at these specific speed (5400,7200, ...) and not other values. Thanks in advance, from France

From serverfault Guillaume Rose

• 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.

  squillman : +1 He!!, sounds good to me :)

  Bratch : 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?

  Alnitak : it probably is 10k, there's no real need (AFAIK) for the speed to be a harmonic of 60 Hz any more.

  marc_s : sounds pretty convincing to me! :-)

  kubanczyk : OK, could someone explain why some so-called 10k drives are in fact 10025 RPM (so slightly above 167 Hz)?

  Roger Lipscombe : 10,000 rpm disks were designed in Europe, where we use 50Hz A/C :)
  From Alnitak

• Thanks to this answer, I have found the explanation at http://books.google.com/books?id=Yu5SAAAAMAAJ&q=electric+engine+7200+3600

  Alnitak : I think that's only incidental - despite what I've written above HDD motors aren't driven from A/C.

  Dennis Williamson : (virtual -1) for not including a summary.
  From Guillaume Rose

• 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.

  Bratch : "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.
  From zarkdav