What determines the rotational speed of a hard drive?

[CTho9305]

And can this be changed by the user? How much battery life would be gained by slowing down a hard drive? Could something on the PCB be modified, or how does the drive know how fast it is supposed to spin? Are 5400rpm drives truly different from 7200rpm drives, or is a binning-style thing where the drives that aren't reliable at 7200rpm are sold at 5400?

 

[TerryMathews]

Well, there would be two easy ways to do this. Either A) install the correct speed motor at the factory (which is what most companies probably do) or B) say for example, a motor is rated for 7200RPM at 12v. Therefore, it should run at 5400RPM at 9v assuming that it is a linear motor.

B would require the drive controller logic to have a multimeter and electrically-controlled pot.

 

[CTho9305]

Originally posted by: TerryMathews
Well, there would be two easy ways to do this. Either A) install the correct speed motor at the factory (which is what most companies probably do) or B) say for example, a motor is rated for 7200RPM at 12v. Therefore, it should run at 5400RPM at 9v assuming that it is a linear motor.

B would require the drive controller logic to have a multimeter and electrically-controlled pot.

That strikes me as unreliable based on my experience with motors...

 

[Mark R]

B) say for example, a motor is rated for 7200RPM at 12v. Therefore, it should run at 5400RPM at 9v assuming that it is a linear motor

The drive electronics constantly monitor the speed of the motor, and adjust the power supplied, to maintain the rotation speed within an extremely tight margin.

Rotation speed is critical for the operation of the drive - an incorrect rotation speed will cause data read from the platter to be mistimed (making it uninterpretable) and can result in the writing of data in the wrong position (fatal for the drive as the low-level format will be corrupted). The drive controller is programmed for the specific rotation speed, and data density. Conceivably, you could flash the firmware of a 5400 rpm drive with 7200 rpm firmware. It's unlikely to work as the 5400 rpm motor probably won't get to 7200 rpm, and the 5400 and 7200 rpm low-level formats are likely to be different.

If the drive does not achieve correct speed during boot-up, the electronics detect this and register a critical error - usually preventing the BIOS from detecting the drive.

 

[TerryMathews]

Originally posted by: Mark R

B) say for example, a motor is rated for 7200RPM at 12v. Therefore, it should run at 5400RPM at 9v assuming that it is a linear motor

The drive electronics constantly monitor the speed of the motor, and adjust the power supplied, to maintain the rotation speed within an extremely tight margin.

True, but my point was that it is unlikely that a manufacturer builds in the ability for a motor to be run at both 5400RPM and 7200RPM.

I knew that the drive control logic could monitor and adjust the motor. But, I seriously doubt that the controller can adjust the motor voltage down on the order of 25%...

 

[dzt]

Originally posted by: Mark R

B) say for example, a motor is rated for 7200RPM at 12v. Therefore, it should run at 5400RPM at 9v assuming that it is a linear motor

The drive electronics constantly monitor the speed of the motor, and adjust the power supplied, to maintain the rotation speed within an extremely tight margin.

Rotation speed is critical for the operation of the drive - an incorrect rotation speed will cause data read from the platter to be mistimed (making it uninterpretable) and can result in the writing of data in the wrong position (fatal for the drive as the low-level format will be corrupted). The drive controller is programmed for the specific rotation speed, and data density. Conceivably, you could flash the firmware of a 5400 rpm drive with 7200 rpm firmware. It's unlikely to work as the 5400 rpm motor probably won't get to 7200 rpm, and the 5400 and 7200 rpm low-level formats are likely to be different.

If the drive does not achieve correct speed during boot-up, the electronics detect this and register a critical error - usually preventing the BIOS from detecting the drive.

ya, HD motorspeed is controlled constant due to disc surface format (data area). and if you succeeded to get higher speed, without better ATA it will be a small increment.

 

[CTho9305]

So you don't think a lower speed would work either? Overspeeding would be stupid - hard drives are too valuable to screw up. If you blow a CPU, a replacement is not a problem. A hard drive loses data.