Hard disk drives store and retrieve digital data by using rapidly rotating rigid disks coated with magnetic material. A spindle motor is used to spin the disks at high speeds, allowing the read/write heads to access data on the disks. The key components that make up a hard disk drive include the platters, spindle, actuator arm, read/write heads, and motor. The motor plays a vital role in hard drive operation by precisely rotating the disk platters so data can be reliably written and read.
Spindle Motor
The spindle motor is located under the platters in a hard drive and spins them at a constant speed. Its purpose is to rotate the disks inside the hard drive enclosure so that the read-write heads can access data on the platter surfaces. The motor spins the disks between 3,600 RPM on slower drives and 15,000 RPM on high performance drives.
The spindle motor needs to spin the platters at a constant speed to enable accurate positioning of the read-write heads over data tracks. Any variation in speed can cause errors and impact performance. Hard drives use high precision spindle motors, with speed fluctuations less than 0.1%.
Some key specifications for HDD spindle motors include speed, torque, power consumption, noise level and bearing system. Manufacturers like Nidec and Mabuchi supply custom designed spindle motors for major hard drive brands including Western Digital, Seagate and Toshiba.
Newer hard drive designs are transitioning to a single platter motor design versus traditional multi-platter motors. This helps reduce power consumption and cost.
Sources:
https://hddsurgery.com/blog/hdd-spindle-motor
Function
The spindle motor in a hard disk drive is responsible for providing rotational motion to the platters inside the drive enclosure. The platters consist of the magnetic surfaces that are used to store data. They are stacked on top of a spindle hub which is connected to the spindle motor. When powered, the spindle motor spins the platters at a constant speed typically between 5,400 and 15,000 RPM depending on the drive model. The high-speed rotation of the platters enables the read/write heads to access data quickly as they fly just above the platter surfaces.
Motor Types
There are three main types of motors used in hard drives:
Brushless DC
Brushless DC (BLDC) motors are the most common type used in modern hard drives. They contain a rotor with permanent magnets and a stator with windings. BLDC motors operate without brushes by using a rotating magnetic field generated electronically. Advantages of BLDC motors include high efficiency, high torque, low noise, and low maintenance due to the lack of brushes.
According to MinebeaMitsumi, BLDC motors allow for high precision and speed in hard drives.
Brushed DC
Earlier hard drives used brushed DC motors which contain brushes that transfer power between the stationary and rotating parts of the motor. They are cheaper and simpler to manufacture than brushless motors but have higher friction and require more maintenance due to wear on the brushes.
Stepper
Stepper motors move in discrete steps allowing for precise positioning control. They contain multiple windings that are energized in a sequence to rotate the motor. Stepper motors have been used in floppy disk drives and older hard drives but modern high capacity drives require higher speeds than stepper motors can provide.
Brushless DC
Brushless DC (BLDC) motors are the most common type of motor used in modern hard drives today (Source). BLDC motors have the rotor portion containing permanent magnets and the stator containing windings. The key benefits of brushless DC motors over brushed DC motors include:
- Higher efficiency due to lack of brushes and ability for higher speeds
- Low maintenance as there are no brushes to replace
- Longer lifespan due to no brush/commutator erosion
- Better speed versus torque characteristics
- Low noise operation
BLDC motors offer performance advantages over brushed DC motors while also being simple to control. This makes them well-suited for use in computer hard drives where precision and reliability are critical. Their efficiency, low-maintenance, and long lifespan also reduce costs over the long run.
Brushed DC Motors
Brushed DC motors are an older type of motor commonly found in hard drives before the mid-2000s. They consist of an armature containing the winding coils and a permanent magnet stator. The rotor has a mechanical commutator that periodically switches the direction of current in the windings to produce continuous rotation.
Brushed DC motors have two issues that limit their reliability in hard drive applications. First is the presence of brushes that physically make contact with the spinning commutator. These brushes wear down over time from friction which can eventually lead to failure. Second is the generation of electromagnetic interference caused by the commutator that can introduce noise into sensitive electronics like the read/write heads.
Newer hard drives have transitioned to using brushless DC motors that eliminate the brush commutator. According to research on brushed DC motor drivers from Pololu, brushed motors are still used in some applications where low cost is more important than reliability.
Stepper
Stepper motors were commonly found in older hard drives from the 1970s to early 1980s (1). These drives had very low capacities, usually around 10MB. The stepper motor was used to move the actuator arm and heads across the platters (2). However, they were not suitable for high performance drives due to their low torque and inability to move the arm quickly and precisely enough. Stepper motors have been entirely replaced by voice coil actuators in modern hard drives.
(1) https://electronics.stackexchange.com/questions/263135/are-hard-drive-motors-stepper-motors
(2) https://hddsurgery.com/blog/hdd-actuator
Specifications
The specifications for hard drive spindle motors are focused on three key areas – speed, power, and precision.
Spindle motors in hard drives typically operate at speeds between 5,400 RPM and 15,000 RPM. Higher RPM ratings allow the drive to access data faster. Consumer hard drives often use 5,400 or 7,200 RPM motors, while high performance drives may use 10,000 or 15,000 RPM motors (https://hddsurgery.com/blog/hdd-spindle-motor).
These motors require very little power, usually under 12V DC. But they need to generate enough torque to spin up the disks from a standstill to operating speed quickly. More platters and higher RPMs require higher torque (https://www.electroschematics.com/hdd-bldc-motor/).
Precision is critical – any wobble, vibration, or variation in speed can affect performance. Spindle motors must rotate with minimal runout (less than a few microns). They use fluid dynamic bearings and carefully balanced rotors to achieve this (https://www.minebeamitsumi.com/english/product/rotary/1181998_6224.html).
Manufacturers
The major companies producing hard drive spindle motors include:
MinebeaMitsumi is one of the biggest manufacturers of spindle motors for hard drives. They utilize high quality bearings, contributing to high precision and reliability (source).
Nidec Corporation is another leading producer of brushless DC motors for HDDs. They have a large market share in this area (source).
Other major manufacturers are Magnecomp Precision Technology, ABB, Allied Motion Technologies, NMB Technologies Corporation, Wolong Electric Group, Jiangsu Changzheng Electric, and Zhejiang Founder Motor (source). These companies supply HDD spindle motors to top hard drive brands.
Future Trends
Motor technology for hard drives continues to advance. One emerging technology called OptiNAND combines HDDs with embedded flash drives (EFDs). According to The Future of Hard Drives: New Technologies on the Horizon, OptiNAND was announced in 2021 by Western Digital and aims to boost HDD performance. The flash memory acts as a cache to store frequently accessed data.
Manufacturers are also working on motors and actuators that are more precise and reliable. For example, Modern HDD technologies that are bringing hard disks back notes that Seagate has introduced Mach.2 dual-actuator technology to optimize data transfers. The goal is to improve I/O performance and reliability as HDD capacities continue to increase.
In general, hard drive companies continue innovating with the motor and actuator technology to maximize speed, precision, efficiency and data density. Advanced motors will be critical for next-generation hard drives with capacities over 50TB.