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I noticed the other day that the rotational frequency of typical consumer grade HDD are either 5400rpm or 7200rpm. These correspond to 90Hz and 120Hz respectively.

Given that in parts of the world the grid frequency is 60Hz, and 120Hz is the first harmonic. What effects does this have on the platters.

It would seem to me that this choice in rotational speed (7200 rpm) would be problematic in these certain countries.

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Never realized that. Anyway, modern harddrives are designed to operate under conditions of both mechanical and electrical noise. What exactly do you expect to happen? A resonance condition perhaps? –  lionelbrits Nov 4 '13 at 13:33
I was expecting that time dependent magnetic fields would be setup by the electrical system. Given that the first harmonic is equal to the period of the platter, the platter should experience some form of constant magnetic field wrt the rotating frame. –  Aron Nov 4 '13 at 13:40
Disk drives are on the other side of well regulated DC power supplies from the AC line power. Just like you don't hear 50 or 60 Hz hum in a properly designed and set up audio system, disk drives don't see the power line signal either. –  Olin Lathrop Nov 4 '13 at 14:06

3 Answers 3

The power supply to a hard disk is DC not AC (5V and 12V) and the disks themselves are heavily shielded from external magnetic fields - for obvious reasons! So there is no interaction, adverse or otherwise, with the mains frequency.

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My experience is that no DC/AC converter is perfect and GMR is pretty sensitive. –  Aron Nov 4 '13 at 13:06

The ideal power supply that outputs a perfect DC is ...just ideal. This coupled to the fact that the engine is mounted axially without any weird transmission ratio that averages out the initial pulses may have some side effects.

Let's take a deeper look: a fluctuation in the torque that drives the plates influences a bit the rotational velocity of the plates. However fluctuations of rotational velocity would be bad independently from their frequency, but also they tend to zero as the torque applied to the spinning disks is very low and there is some inertia.

What a fluctuation of the torque coherent with the rotation speed actually means, is that any transversal force related to the production of the torque applied to the bearings would happen again and again at the same position putting some stress on them. But again the torque is very low and due to the axial mounting no transversal force can be generated (leaving out the ones coming from small unbalance which are regardless of the input AC).

So I would say that there are no concerns in driving an HDD at whatever frequency, but there may be in other (more intense) applications especially if you cannot have an axial mounting of the engine.

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I would expect the mechanical aspect of the system to have little effect. –  Aron Nov 4 '13 at 13:38
@Aron I agree, but that's all. –  DarioP Nov 4 '13 at 13:46

Well the moment of inertia of a hard disk platter is quite large compared to that of the drive motor, and the rotational inertia is huge for such a mass. The drive motor, is designed to have a quite small torque, barely enough to overcome the frictional drag of the heads on the parking track. Once the disk is rotating, and the heads are flying, the residual rotational friction is miniscule, so very little drive power is required, and not much heat is generated, which keeps thermal coefficient problems in check. So the link between the weak motor, and the huge rotating inertia, is a guarantee, that very little torsional transient effects can get to the platters in the form of "wow".

The state of the art of magnetic disk drives, and their engineering, is beyond trivial chatter question, and answer conversations.

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