HDD Spindle Motor

Hard disk drive is a compact mechatronic component and there is no lot available space for placing its motor. On the other hand, the motors of the hard disks (called "spindle motors") must be able to generate enough torque so the magnetic platters could rotate at the desired speed. This means that the crucial requirement for spindle motors is the large ratio between their power and size.


Hard drive motors are designed for operation at high speeds over a long period of time. This principle is, as such, problematic in the start and can easily lead to defect. Some time ago, in hard drive industry, leading trend was continuous increase of motor speed, so some devices were working on speed over 15,000 RPM


Speed control, i.e. precise achieving and maintaining the desired RPM, is very important to perform for realizing the basic functions of the hard disk drives - reading and writing data. With the increasing capacity of hard drives, i.e. increasing density of data written on the platters, the speed control becomes more and more difficult task.


Additional conditions set in front of spindle motors are low noise, low cost, low power consumption and less heating during operation. All the above mentioned conditions were satisfied by using brushless DC electric motors. These motors have a rotor with permanent magnet, while current is passed through the stator windings, which leads to a rotation of the rotor. 


Inverter, which replaced the classic commutator, is controlled by the current which passes through the stator coils. Rotor position sensor controls the operation of the inverter itself.


Photo: Spindle motor configuration (DiamondMax 10): Stator core, platter carrier, rotor axle, platter tier


Rotating parts of the spindle motors are rotor (spindle) and platter carrier (hat) with a permanent magnet on itself. Stator core is rigidly tied to the hard drive casing. Before, ball bearings were used, while now, almost exclusively fluid bearings secure rotation of HDD spindle motors. The platter carrier, along with the platter tier is rigidly attached to the rotor shaft. In this way, hard drive platter(s) rotates together with the spindle rotor.


Оn older hard drive models, main spindle motor load was emerging in the moment of start, because of stiction between read/write heads and surface of landing zones on the platters. Implementing ramp parking system solved this issue completely. Nowadays, main spindle motor load is coming from windage friction, which is result of high rotational speed of platters. 


Engineering teams of leading hard drive manufacturers are actively working on solving this issue. The most frequently mentioned solution is development of hard drives with sealed casing filled with helium. Idea is that if platters would rotate in controlled area with less windage friction, spindle motor load would be significantly reduced, which can lead to larger RPM and better performances of hard disk drives.

Žarko Damjanović


There is no physical "rotor position sensor" in modern hard drives. "SMOOTH" motor controllers control the spin speed by sequentially driving each pair of three windings via an inverter running in PWM mode, and then sensing the back-EMF of the non-driven winding. There is a detailed explanation in the SMOOTH L7250 datasheet: http://pdf.datasheetcatalog.com/datasheet/SGSThomsonMicroelectronics/mXyuswx.pdf

Franc Zabkar

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Francisco Maya

@Francisco Hi, thank you for your kind words, you may translate and publish it, with linking to our website as original source. Looking forward to see your translation :) Kind regards.

Žarko Damjanović