Debate about Thermal Dissipation

[hotvette]

My colleague and I are having a debate and would appreciate your opinion. The question is:

How much of the power consumed by a disk drive is thermally dissipated to the surroundings?

My answer is very nearly 100%. The only energy that isn't thermally dissipated is a small amount of electrical energy of the interface signals plus whatever vibration energy is absorbed by the mounting structure. I came to this conclusion by considering a control volume surrounding the disk drive. Inputs are electrical power and outputs are electrical signals, mechanical vibration, and thermal energy.

My colleague insists that a substantial amount of the energy is used to spin the disks and move the actuator, the rest being thermal. I think the motion of the internal mechanical components is irrelevant except the extent to which it contributes to vibration that is absorbed by the surrounding structure.

 

[chroot]

According to Newton's laws, if the spindle were frictionless, energy would only be required to spin the disks up; once spinning, they would spin forever with no additional energy required.

In reality, spindles are not frictionless, and the drive motor has to constantly supply energy to overcome friction. The energy lost to friction is indeed dissipated as heat.

The drive accepts energy from the power supply. The only ways for energy to leave the drive are as heat, as vibration, or as electrical energy in the form of signalling. Each time the drive's control circuitry changes from signalling a low to a high (or high to low) state, the capacitance of the wire and receiving circuitry has to be charged and discharged. The energy needed to do this is taken in from the power supply. Compared to the energy used in spinning the disks, it is quite small.

You are correct, and your colleague is wrong. Nearly all of the energy delivered from the power supply to the drive is ultimately converted to heat within the drive itself. A very small amount of it ends up lost in signalling, and another, even smaller amount is used to actually flip the magnetic domains on the disks' surfaces.

- Warren