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name Punditsdkoslkdosdkoskdo

Why HDD rotate at 7200 rpm?

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

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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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.
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    • 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.

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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    • Re-correction: Hard disk drives are driven by brushless DC motors. From nidec.com/en-NA/product/no1 "Nidec holds the No. 1 position in the global brushless DC motor market. In particular, the company has shipped many world No. 1 products. For example, it has an 80% share of the global HDD spindle motor market."

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