In: Root » Computers and technology » Storage Devices
The first disk drives (back in the era of the original IBM Winchester) used synchronous motors. That is, the motor was designed to lock its rotation rate to the frequency of the AC power line supplying the disk drive. As a result, most motors of early hard disk drives spun the disk at the same rate as the power line frequency, 3600 revolutions per minute, which equals the 60 cycles per second of commercial power in the United States. Synchronous motors are typically big, heavy, and expensive. They also run on normal line voltage-117 volts AC-which is not desirable to have floating around inside computer equipment where a couple of errant volts can cause a system crash. As hard disks were miniaturized, disk makers adopted a new technology-the servo-controlled DC motor-that eliminated these problems. A servo-controlled motor uses feedback to maintain a constant and accurate rotation rate. That is, a sensor in the disk drive constantly monitors how fast the drive spins and adjusts the spin rate should the disk vary from its design specifications. Because servo motor technology does not depend on the power line frequency, manufacturers are free to use any rotation rate they want for drives that use it. Early hard disks with servo motors stuck with the standard 3600 RPM spin to match their signal interfaces designed around that rotation rate. Once interface standards shifted from the device level to the system level, however, matching rotation speed to data rate became irrelevant. With system-level interfaces, the raw data is already separated, deserialized, and buffered on the drive itself. The data speeds inside the drive are entirely independent from those outside. With this design, engineers have a strong incentive for increasing the spin rate of the disk platter: The faster the drive rotates, the shorter the time that passes between the scan of any two points on the surface of the disk. A faster spinning platter makes a faster responding drive and one that can transfer information more quickly. With the design freedom afforded by modern disk interfaces, disk designers can choose any spin speed without worrying about signal compatibility. As a result, the highest performing hard disks have spins substantially higher than the old standard-some rotate as quickly as 5400 or 7200 RPM. Note that disk rotation speed cannot be increased indefinitely. Centrifugal force tends to tear apart anything that spins at high rates, and hard disks are no exception. Disk designers must balance achieving better performance with the self-destructive tendencies of rapidly spinning mechanisms. Moreover, overhead in PC disk systems tends to overwhelm the speed increases won by quickening disk spin. Raising speed results in diminishing returns. According to some developers, the optimum rotation rate (the best trade-off between cost and performance) for hard disks is between 4500 and 5400 RPM.
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