For DC fans, the intuitive meaning is direct current fans. Why do CPU coolers use direct current fans instead of direct-powered AC fans, that is, alternating current fans? In fact, the answer can be obtained through their differences: DC fan operation principle: According to Ampere’s right-hand rule, when a conductor passes through an electric current, a magnetic field will be generated around it. If this conductor is placed in another fixed magnetic field, an attraction or repulsion will be generated, causing the object to move. Inside the blades of the DC fan, a rubber magnet that is pre-charged with magnetism is attached. Surrounding the silicon steel sheet, two sets of coils are wound around the axis, and a Hall sensor component is used as a synchronous detection device to control a set of circuits that make the two sets of coils wrapped around the axis work in turn. The silicon steel sheet produces different magnetic poles, which produce attraction and repulsion with the rubber magnet. When the attraction and repulsion is greater than the static friction of the fan, the fan blades rotate naturally. Since the Hall sensor component provides a synchronization signal, the fan blades can continue to operate. As for its direction of operation, it can be determined according to Fleming’s right-hand rule.
AC fan operation principle: AC fans work directly through AC power supply. Through the fixed AC power frequency, the silicon steel sheets generate magnetic field switching, which makes the fan blades generate torque and rotate. The higher the power frequency, the faster the magnetic field switching speed, and theoretically the higher the fan speed. However, the frequency cannot be too fast, otherwise it will cause difficulty in activation.
After these two comparisons, I believe you will know why the CPU cooler uses a DC fan. The reason does not need too much explanation. These are all reference answers, because a DC fan is a DC fan, and the computer motherboard can only use a DC fan. 4-2. The composition of a DC fan is roughly divided into four parts: rotor, stator, outer frame, and motor. The following is a brief introduction to the structure of a brushless DC fan and several important components.
The composition of DC fan The principle of cooling fan: The working principle of the fan is realized by energy conversion, that is: electrical energy → electromagnetic energy → mechanical energy → kinetic energy. Its circuit principle is generally divided into many forms, and the performance of the fan will be different depending on the circuit used.

DC fan appearance and air outlet are usually divided into the following three categories :
- AXIAL axial flow: The airflow outlet direction is the same as the axis direction.
- BLOWER centrifugal type: uses centrifugal force to throw the air out along the blades.
- Mixed flow: has the above two airflow modes.
Cooling fans are classified into the following types according to voltage requirements and circuit principles :
- AC cooling fan AC voltage is used (110V 220V 380V other)
- DC cooling fan DC voltage is adopted (5V 12V 24V 48V 110V 220V other)
From the data, we can see that the DC fan used on our commonly used CPU coolers generally has a voltage of around 12V, while the USB fan is 5V. As for other voltage fans, they are rarely used on our CPU coolers.
Cooling fan specifications and dimensions:
It mainly indicates the size of the fan, which is usually in mm. There is a unified set of standards for the size specifications of the fan. As long as this set of standards is followed, the normal installation between the heat sink or other interfaces and brackets can be guaranteed. The size specifications are usually described by a 5-digit number, for example: 01505, 04028, 06015, 08025, 12038, etc. The first three digits of the 5-digit number, 015, 040, etc., represent the side length of the square bottom of the fan, in millimeters; the last two digits, 10, 28, 30, etc., represent the height of the column, that is, the thickness of the fan, in millimeters. Special note: The side length of the 092XX series fan is 092mm, but it is usually called 9cm; the size of the BLOWER centrifugal fan usually adopts the above standards, the difference is that the size is recorded using the side length of the longest diameter of the fan and the actual thickness.
Technical Specifications of DC Fan Speed (RPM):
In physics, speed refers to the number of times an object makes circular motion per unit time. It is represented by the symbol “n” and the unit is rpm (r/s) or (r/min), that is, frequency f = 1/T (T is the period of circular motion). It is one of the important performance indicators of fans and is closely related to the air volume, one of the two most important performance indicators. Voltage (V), also known as potential difference or electric potential difference, is a physical quantity that measures the energy difference generated by the unit charge in the electrostatic field due to the different electric potential. This concept is similar to the “water pressure” caused by the height of the water level. It should be pointed out that the term “voltage” is generally only used in circuits, while “potential difference” and “potential difference” are generally used in all electrical phenomena. The international unit of voltage is volt (V). 1 volt is equal to 1 joule of work done for every 1 coulomb of charge, that is, 1 V = 1 J/C. The minimum voltage that can start the fan when the power is suddenly turned on. The smaller the starting voltage, the wider the operating range of the voltage. Generally 3V, 5V, 9V, 12V, 24V, 48V, 110V, 220V, and others. Current (A) refers to the flow of a group of charges. The magnitude of the current is called the current intensity, which refers to the amount of charge passing through a certain cross-section of the wire per unit time. The amount of electricity passing through one coulomb per second is called an “ampere”. Ampere is a basic unit in the International System of Units. The current output when the motor is working. The smaller the working current, the smaller the heat generated by the motor and the more reliable it is. Power: the work done per unit time, or the energy transferred or converted per unit time. The work done and consumed per unit time. The unit is “watt”, abbreviated as “W”, and the symbol is “w”. The product of the working current and the rated voltage. That is, the consumption of the fan (power/power consumption) Air volume (CFM): Air volume is one of the two most important performance indicators of the fan. Air volume is the volume of air passing through the fan outlet (or inlet) section per unit time, and the unit is generally cfm, cubic feet per minute, or cmm, cubic metres per minute, or m3/min. Air volume is an overall measure of fan performance, and is not limited to size, structure, or method, nor is it limited to DC brushless fans, and can be applied to any air flow guidance equipment.
Technical indicators of DC fans: Wind pressure:
Wind pressure is one of the two most important performance indicators of a fan. Wind pressure is the pressure difference between the air outlet and the air inlet that the fan can generate. The unit is generally mm (cm) water column, that is, millimeter (centimeter) water column (similar to the millimeter mercury column that measures atmospheric pressure, but because the pressure difference is smaller, it is generally measured in water column) mmH2O/mmAq/Pa/mmHg. In order to carry out normal ventilation, it is necessary to overcome the resistance within the fan’s ventilation stroke. The fan must generate pressure to overcome the air supply resistance. The measured pressure change is called static pressure, that is, the difference between the maximum static pressure and atmospheric pressure. It is the pressure of the gas acting on the surface parallel to the object. The static pressure is measured through a hole perpendicular to its surface. The kinetic energy required for gas flow is converted into a pressure form called dynamic pressure. Wind pressure is an important indicator to measure the “power” of the fan. If the air volume is compared to the swinging force of a weapon, then the wind pressure is the sharpness of the weapon. Noise: Noise is an indicator that is increasingly concerned by various equipment. Noise is the “non-musical” sound generated during the operation of the fan. The most common measurement standard at present is the weighted sound level measurement, which usually adopts the A sound level weighting, and the common units are decibel (A) or dBA. Lifespan (MTBF): Lifespan is an indicator that must be paid attention to in the long-term use of fans. Lifespan refers to the time that the fan can be used without failure, and the unit is generally a number of hours. We spend money to buy a cooling fan, so naturally we hope that it can work normally for as long as possible, at least within the service life of the radiator.
The common fan blade cross-sectional curve of the fan blade is generally designed based on the Joukowski (Zhukovsky, a famous Russian aerodynamicist and a pioneer of contemporary aviation science. He proposed the Zhukovsky function and laid the theoretical foundation for the calculation of wing aerodynamics) wing cross-sectional curve. Then, according to the design requirements, the angle between the root and end of the blade and the rotation axis, the root and end width and other data are rotated and stacked to form a three-dimensional fan blade surface, and curve fitting is performed to finally complete the design of the entire fan blade model. The performance of the fan blade is affected by many parameters, such as stacking height, blade curvature, blade inclination, blade spacing, blade thickness, number of blades, blade angle of attack, blade tip clearance, blade width, main shaft diameter, etc., and the parameters are mutually restricted and the relationship is complex. Constantly adjusting various parameters and finding the combination that best suits the target work requirements is the work of researchers who design fan blades. If all these parameters are explained and analyzed clearly, it is enough to write several doctoral dissertations, but this article is just a brief introduction to the relevant technologies of air-cooled radiators. Therefore, only a few parameters that users are more concerned about are briefly introduced: Blade curvature: Within a certain range, the greater the blade curvature, the greater the gas kinetic energy at the same speed, that is, the greater the air volume and wind pressure; at the same time, the greater the resistance of the blade, the greater the torque of the motor is required. When the ratio of arc height/chord length exceeds 0.1, the lift coefficient no longer increases linearly, so “a certain range” is 0.05<arc height/chord length<0.1. Blade inclination: The greater the inclination angle, the greater the pressure difference between the upper and lower surfaces of the blade, and the greater the wind pressure at the same speed; but if the pressure on the upper surface is too high, backflow may occur, which will reduce the fan performance. Therefore, the blade inclination angle should only be increased within a certain limit. Blade spacing: If the distance between the blades is too small, it will cause airflow disturbance, increase the friction on the blade surface, and reduce the fan efficiency; if the distance between the blades is too large, it will increase the pressure loss and insufficient wind pressure. Number of blades: The cross-sectional curves and inclination angles of fan blades of various specifications are basically similar, and the width of each blade often depends on the height of the blade. In order to ensure that the distance between the blades is not too large and affect the wind pressure, fans with a relatively small diameter-to-height ratio (i.e., relatively thin) often use the method of increasing the number of blades to compensate. Regardless of the number of blades, the number of blades of axial flow fans is often an odd number such as 3, 7, or 11. This is because if an even number of symmetrical blades are used and the balance is not adjusted well, it is easy to cause the system to resonate. If the blade material cannot resist the fatigue caused by vibration, the blade or the spindle will break. Therefore, it is often designed with an odd number of blades that are asymmetric about the axis. This principle is widely used in various fan blade designs, including some helicopter propellers.
Blade tip clearance: How to adjust the gap between the fan blades and the outer frame is a major problem in fan design. If the gap is too small, the airflow here will rub against the blades and the outer frame, increasing the noise; if the gap is increased, the fan efficiency will be reduced due to the impact of the reverse airflow, etc. – if the gap increases by 1%, the full pressure power will drop by about 2%.
Blade curvature: In addition to having a certain curvature in the cross section, the fan blades do not extend straight along the radial direction in the top view, but are slightly curved in the direction of rotation, forming a certain curvature. If the blades extend straight along the radial direction, the airflow driven by the rotation of the fan will be scattered on the side of the air outlet, the air supply distance will be short, and the “power” will not be concentrated; if the current product version has a slight curvature, it can ensure that the blown airflow is concentrated in the columnar space directly in front of the air outlet, increasing the air supply distance and wind pressure.
Spindle diameter: Due to the presence of the motor and bearings, the central part of the axial fan spindle is inevitably a blind spot where there is no airflow passing through, and the spindle diameter determines the size of this blind spot. The size of the spindle diameter mainly depends on the power of the fan motor – a high-power motor requires a larger stator winding coil, which will inevitably take up more space. If it cannot be expanded vertically (increase the height), it has to expand horizontally (increase the area).
Blade smoothness : This is an indicator that is not affected by design factors and basically depends on the mold forming and post-processing process of the manufacturer. Outside the design curve, the unevenness on the blade will produce turbulence during rotation, increase friction, reduce fan efficiency, compromise fan performance, and increase operating noise. Therefore, the smoothness of the blade surface should be strictly controlled. If the purchased product is not well processed, subsequent measures such as manual polishing should be considered to make up for it. Although the fan blade cross-sectional curves are mostly derived from the same basis, and the various adjustments that can be made are also limited to a certain range, some manufacturers have made major improvements and proposed very distinctive designs.
The outer frame of the fan is the home of all the parts of the fan. Ensuring that all parts are firmly installed on it is its most basic function. At the same time, it should also ensure that the materials and structures used have a certain strength, can withstand physical impact to a certain extent, and protect the more fragile components such as the fan blades and motors. The outer frame is the ultimate embodiment of the fan size specifications, and also determines the specific installation method and difficulty. A unique design on the fan frame may bring convenience and consideration to the user’s installation and use process without violating the prevailing standards. In addition to being the basis for assembly and the bracket for installation, the outer frame of the fan also plays an important role in guiding the air passing through the fan. The outer frame can “constrain” the airflow driven by the fan blades, control its outflow direction, suppress back-excitation and scattering, and concentrate it in the desired air supply direction. In addition, some special designs can also be used to achieve special effects such as increasing wind pressure.
Bearings
Bearings are the bottleneck factor of fan life, and also have an important impact on the fan’s operating noise and manufacturing cost. They have attracted much attention from users. Fan designers and manufacturers have also invested a lot of technology and manpower in them. A little understanding of various bearing technologies is also one of the prerequisites for choosing the right fan. Currently, many brands on the market have their own unique bearing designs, and each design has its own unique advantages in terms of life, noise, cost, etc. In this section, we will briefly introduce several common or distinctive bearing technologies:
1. Oil-retaining bearings Traditional bearing technology
The shaft core is nested in the sleeve and rotates, and the gap between the shaft core and the sleeve is filled with lubricating oil to reduce friction and vibration. In the early stage of use, the lubricating oil evenly fills the gap between the shaft core and the sleeve, making the rotation smooth and stable, so the working noise is very low and the bearing wear is less. However, as the use time increases and the dust adsorption increases, the lubricating oil will evaporate due to friction and heat, the oil volume will gradually decrease, and the friction and vibration of the bearing will increase. As a result, the bearing noise increases, the wear is aggravated, and the life is shortened. Therefore, it is not suitable for high-speed “high-speed enhanced” fans and large-blade fans with large fan blades, and it cannot achieve the goal of “longevity”. The product life is usually 5000 to 8000 hours. As the most basic bearing technology, many special bearing technologies are improved on this basis.
2. Hydraulic bearings are an upgraded version of oil-containing bearings
- Adding a magnetic suspension baffle reduces the chance of the shaft core contacting the sleeve during rotation;
- The fixed end of the bearing bottom is sealed to form a large oil storage space, which can store more lubricating oil;
- Using lubricating grease with low heat loss, the bearing can still ensure the lubricating properties of the grease under high temperature conditions;
- The hydraulic automatic oil circulation system connects the bearing and the oil storage tank through a special oil circuit, so that the lubricating oil forms a circulation loop; 2. 5. The shaft core uses metal powder metallurgy technology to improve the surface smoothness and rigidity, high-precision sleeve/precision clearance, and precision combination to ensure high lubricity of the bearing. Hydraulic bearings have been widely recognized through more than 20 years of continuous updating and application! There are too many types of similar structures in Japanese/Taiwanese/NBR/NDB/Hypro/Rifle/EBR/ceramic/magnetic suspension, etc. Through many improvements, hydraulic bearings can achieve the speed, operating temperature and life of double ball bearings while maintaining or even lower than the noise level of oil-containing bearings. Usually the product life can reach more than 50,000 hours .
3. Ball bearings
Anyone with a certain knowledge of physics knows that when the pressure and the roughness of the contact surface are the same, the rolling friction is less than the sliding friction. The wheel is one of the greatest inventions of mankind based on this theory, and the ball bearing is also a great example of the practical application of this theory. The ball bearing no longer relies solely on lubricants to fill the gap between the shaft core and the sleeve, but slightly expands the space and places several metal balls in it. When the shaft core and the sleeve move relative to each other, the balls also roll with them, and there is no sliding friction with the two, which reduces the rotational resistance and energy loss. Of course, in order to fill the space and play a lubricating role, ball bearings also need to use lubricants, but the working space is better sealed than oil-containing bearings, and the friction is smaller and the life is very long. If only one ball is used for support, it is difficult to ensure the stability of the rotating axis and the friction deviation of the blades due to their own weight, so springs are usually required to separate the two balls for constraint. The manufacturing cost of the double ball bearing structure is relatively high. In order to control the product cost, two types of ball bearings, double ball and single ball, are produced: double ball bearings should be regarded as a typical representative of ball bearings. In addition to the characteristics of low resistance, uniform friction and long life, due to the small gap between the shaft core, ball and sleeve, there is always rolling friction, so there is a disadvantage of high working noise. Usually the product life can reach 50,000 to 100,000 hours. Personal recommendation: the most traditional, the most stable, the most reliable! Trustworthy! Single ball bearings are just a compromise product to make up for the high cost and general life value of double ball bearings. Oil-containing bearings (or other) are used in conjunction with a ball bearing to support the shaft core. There is a wide range of bearings that can be matched with ball bearings. From traditional oil-containing bearings to magnetic suspension bearings, there are products available at present, and the most used is still the combination of oil-containing bearings and single-track balls. In this combination, in theory, the oil-containing bearing only plays a small supporting role, and the wear is greatly reduced compared to working alone; at the same time, it can also reduce the vibration generated by the double ball bearing and reduce the working noise.

Side air intake fan: In order to solve the problem of blade tip gap, the blade design was improved and the outer frame was worked on. The blade tip gap was slightly reduced, and 4 wind slots were opened on the outer frame, so that the outer frame not only plays a guiding role, but also increases the air intake path and reduces the reverse airflow. While reducing the working noise, the performance (air volume) is further improved. Personal recommendation: This fan design has a low wind pressure and is suitable for environments with smooth channel design and low heat dissipation requirements.
Three-blade fan: It is not surprising that the fan has only three blades. This is the most common electric fan, but it is rare to use this design on small DC brushless fans. As mentioned above, too many blades and too close distances will increase airflow disturbance and blade surface friction, reduce fan efficiency, and increase working noise. This 4cm three-blade fan is specially matched with some heat sinks that do not require too much wind pressure. The number of blades is reduced. In order to increase the blade area, the inclination angle is also reduced, so the wind pressure is weakened to a certain extent. Although the wind pressure is insufficient, the distance between the blades is large, the fan efficiency is improved, and the maximum wind volume is guaranteed while reducing the working wind pressure noise. Personal recommendation: This fan is designed with low wind pressure and is suitable for environments with smooth channel design and low heat dissipation requirements.

Fans with enhanced impeller diversion: Some fans have a guide vane that is similar in shape to the fan blades but curved in the opposite direction on the outlet side of the outer frame to add another “constraint” to the airflow. It can make the air flow more concentrated, reduce the blind area, and further increase the wind pressure; however, the interference of the air flow and more friction will undoubtedly increase the working noise. A typical representative is the 4028/6038/12038 series axial flow fan. Take the “high-speed enhanced” fan with enhanced impeller diversion as an example: 4028 specification, ball bearing, 16000rpm, rated voltage 12V, rated current 0.69A, maximum air volume 0.66cmmm, maximum static pressure 33.50mmAq. It is actually based on the 4020 specification axial flow fan and adds 7 reversely curved guide blades about 8mm high. Personal recommendation: The wind pressure is strong and the noise is high; it is necessary to obtain heat dissipation and exchange heat from the heat sink in time, and it is suitable for environments with relatively tight space; the noise is relatively high. Double-blade reinforced fan: The picture shows a 4056 double-blade reinforced axial flow fan for the server cooling chip. What’s more, not satisfied with the increased wind pressure caused by the reversely curved air outlet guide vanes, a more extreme multi-blade design is adopted, mainly two-blade fans. The two blades have opposite inclination angles, opposite bending directions, and opposite rotation directions. They are driven by two motors respectively. When the maximum air volume remains unchanged, greater static pressure can be obtained.
The typical representative is the 4056/4048 series double-blade reinforced axial flow fan. (In fact, obtaining static pressure is not a multiple increase, it only increases 20-40%. The difference in wind pressure/air volume is not big) For example: 4056 specifications, ball bearings, 16000rpm/12000rpm, rated voltage 12V, rated current 1.40A, maximum air volume 0.76cmm , Maximum static pressure 45.70mmAq Personal recommendation: The noise is too high; the cost is also high; if the air duct can be reasonably designed and the structure improved, try to use a “high-speed enhanced” fan with enhanced impeller diversion.

Round frame fan: In order to adapt to some special installation requirements, such as round air ducts, round heat sinks, etc., it is not easy to use “square” fans, but the guide function of the outer frame is needed, so the round frame fan is born. Personal recommendation: The wind pressure is not high, and it can be used for smooth air ducts; the pipe is too long/the air duct is not smooth; the fan application effect will be greatly affected!
Bracket fan: In some cases, the guide function of the outer frame is no longer necessary, and even has a certain negative impact, or there is an obstacle to installation, so it is “abandoned”^_^. It is mainly determined by the installation and applicable environment! The heat dissipation effect determines… At present, it is more common in some round radial radiators, especially the built-in fan type. The typical Intel “sunflower” shape combination heat dissipation graphics card with boxed radiators is the most commonly used. Personal recommendation: The air duct in the environment is smooth and rich, and the use of this model is conducive to heat dissipation and noise reduction.

Suspended fan: In response to the changes in CPU cooling equipment and environment, better coordination with heat dissipation conduction – the original fan air outlet structure is purely obstructive; the entire structure needs to be reversed. In this way, a fan with an air outlet structure opposite to that of an ordinary fan is launched, namely: an inverted fan! The motor of this series of fans is different from that of ordinary axial flow fans. It is installed on one side of the air inlet, and the main shaft bracket is also moved to this side accordingly, clearing the way for the air flow to be delivered. The fan’s blade shape, inclination direction, bending direction, and rotation direction are still the same as traditional axial flow fans. There are also air supply blind spots, and the price is more expensive. Personal recommendation: Most suitable for CPU cooling applications! High comprehensive ratio of air volume and air pressure!
