The radiator needs to speed up the heat dissipation through the forced convection of the fan, so the quality of a fan plays a decisive role in the overall cooling effect. Equipped with a high-performance CPU fan is also one of the key factors to ensure the smooth operation of the entire computer. DC fan operation principle: According to Ampere's right-hand rule, a conductor passes through a current, and a magnetic field will be generated around it. If the conductor is placed in another fixed magnetic field, suction or repulsion will be generated, causing the object to move. Inside the fan blade of the DC fan, a rubber magnet pre-filled with magnetism is attached. Surrounding the silicon steel sheet, the shaft is wound with two sets of coils, and the Hall induction component is used as a synchronous detection device to control a set of circuits, which make the two sets of coils wound around the shaft work in turn. The silicon steel sheet produces different magnetic poles, and the magnetic poles and the rubber magnets produce attraction and repulsion. When the suction and repulsion force is greater than the static friction force of the lice fan, the fan blade rotates naturally. Since the Hall sensing element provides a synchronizing signal, the fan blade can continue to run, and its running direction can be determined according to Fleming's right-hand rule.
AC fan operation principle: The difference between AC fan and DC fan. In the former, the power supply is AC, and the power supply voltage will alternate between positive and negative. Unlike the DC fan, the power supply voltage is fixed, and it must rely on circuit control to make the two sets of coils work in turn to generate different magnetic fields. The AC fan has a fixed power frequency, so the changing speed of the magnetic pole generated by the silicon steel sheet is determined by the power frequency. Same. However, the frequency can not be too fast, too fast will cause activation difficulties. All our computer radiators are DC fans. In general, a good fan mainly examines the air volume, speed, noise, service life, and what kind of fan blade bearings are used.
These parameters will be explained separately below.
Air volume refers to the total volume of air discharged or incorporated by the air-cooled radiator fan per minute. If it is calculated in cubic feet, the unit is CFM; if it is calculated in cubic meters, it is CMM. The air volume unit often used for radiator products is CFM (about 0.028 cubic meters per minute). A 50×50×10mm CPU fan will generally reach 10 CFM, and a 60×60×25mm fan will usually reach a CFM of 20-30. When the material of the heat sink is the same, the air volume is the most important indicator to measure the heat dissipation capacity of the air-cooled radiator. Obviously, the larger the air volume, the higher the heat dissipation capacity of the radiator. This is because the heat capacity ratio of the air is constant, and a larger air volume, that is, more air per unit time, can take away more heat. Of course, in the case of the same air volume, the heat dissipation effect is related to the flow of the wind. Air volume and wind pressure Air volume and wind pressure are two relative concepts. Generally speaking, to design a fan with a large air volume, it is necessary to sacrifice some air pressure. If the fan can move a lot of air, but the wind pressure is low, the wind will not reach the bottom of the radiator (this is why some fans have high speed and high air volume, but the cooling effect is not good). On the contrary, if the wind pressure is large, the air volume is small, and there is not enough cold air to exchange heat with the heat sink, which will also cause poor heat dissipation. Generally, the heat sink of aluminum fins requires the fan's wind pressure to be large enough, while the heat sink of copper fins requires the fan's air volume to be large enough; A fan with a larger wind pressure, otherwise the air will not flow smoothly between the fins, and the heat dissipation effect will be greatly reduced. Therefore, for different radiators, manufacturers will match fans with appropriate air volume and air pressure according to their needs, rather than a single fan that pursues large air volume or high air pressure.
Fan speed refers to the number of times the fan blades rotate per minute, and the unit is rpm. The fan speed is determined by the number of turns of the coil in the motor, the working voltage, the number of fan blades, the inclination angle, the height, the diameter and the bearing system. There is no necessary connection between speed and fan quality. The speed of the fan can be measured by the internal speed signal or externally (the external measurement is to use other instruments to see how fast the fan rotates, and the internal measurement can be directly viewed in the BIOS or through the software. The measurement error is relatively large). ? Because with the change of ambient temperature, different speed fans are sometimes required to meet the demand. Some manufacturers have specially designed radiators with adjustable fan speed, which are divided into manual and automatic. The main purpose of manual is to allow users to use low speed in winter to obtain low noise, and use high speed in summer to obtain good cooling effect. The automatic temperature-adjusting radiator generally has a temperature control sensor, which can automatically control the speed of the fan according to the current working temperature (such as the temperature of the heat sink). balance, so as to maintain an optimal combination of wind noise and heat dissipation.
Fan noise In addition to the cooling effect, the working noise of the fan is also a common concern. Fan noise is the size of the noise generated by the fan when it is working, which is affected by many factors, and the unit is decibel (dB). When measuring the noise of the fan, it needs to be carried out in an anechoic chamber with a noise of less than 17dB, one meter away from the fan, and aligned with the air inlet of the fan along the direction of the fan shaft, and the A-weighted method is used for measurement. The spectral characteristics of fan noise are also very important, so it is also necessary to use a spectrum analyzer to record the noise frequency distribution of the fan. Generally, the noise of the fan should be as small as possible, and there should be no abnormal sound. Fan noise is related to friction and air flow. The higher the fan speed and the larger the air volume, the louder the noise will be. In addition, the vibration of the fan itself is also a factor that cannot be ignored. Of course, the vibration of a high-quality fan will be very small, but the first two are difficult to overcome. To solve this problem, we can try to use a larger size fan. In the case of the same air volume, the working noise of the large fan at low speed should be smaller than that of the small fan at high speed.
Another factor that we tend to ignore is the bearing of the fan. When the fan rotates at high speed, there is friction and collision between the shaft and the bearing, so it is also a major source of fan noise.
The source of fan noise is because:
1. Vibration If the physical center of mass of the rotor and the center of inertia of the rotating shaft are not on the same axis when the fan rotor rotates, it will cause the unbalance of the rotor. The closest distance between the physical center of mass of the rotor and the center of inertia of the rotating shaft is called the eccentric distance. The unbalance of the rotor causes the eccentric distance. When the rotor rotates, the centrifugal force generates a force on the rotating shaft bracket to form vibration, and the vibration is transmitted to the rotating shaft through the base path. Mechanical parts.
2. When the wind noise fan is working, the blades are periodically subjected to the pulsating force of the uneven airflow at the outlet, resulting in noise; Rotation noise is formed; in addition, vortex noise is generated due to the turbulent surface layer, vortex and vortex separation when the gas flows through the blade, which causes the pulsation of the pressure distribution on the blade. The noise caused by these three reasons can be collectively called "wind cutting noise". Generally, fans with large air volume and pressure have large cutting wind noise.
3. The abnormal sound and wind noise only sounds like a simple wind sound, but the abnormal sound is different. When the fan is running, if there are other sounds in addition to the wind noise, it can be judged that the fan has abnormal sound. Abnormal noises may occur due to foreign objects or deformations in the bearings, as well as collisions caused by improper assembly, or uneven winding of the motor windings, resulting in looseness, which may cause abnormal noises. The service life of the fan The service life of the fan refers to the trouble-free working time of the radiator product, and the service life of the high-quality product can generally reach tens of thousands of hours. In the case of similar price and performance, choosing a product with a long service life is obviously more protective of our investment.
The life of the fan is composed of various factors such as the life of the motor, the use environment, and the power supply. The most widely used form of air supply is downward blowing with an axial fan (that is, the most common type of fan), which is so popular because of its good overall effect and low cost. If the direction of the axial flow fan is reversed, it becomes an upward draft, which is used in some special models of radiators. The difference between the two types of air supply lies in the different air flow forms. When blowing, the turbulent flow is generated, and the wind pressure is large but it is easy to suffer resistance loss; when the air is exhausted, the laminar flow is generated, and the wind pressure is small but the air flow is stable. In theory, the heat transfer efficiency of turbulent flow is much greater than that of laminar flow, so it has become the mainstream design form. But the movement of the airflow is also directly related to the heat sink. In some heatsink designs (such as fins that are too tight), the airflow is very hindered by the heatsink, and it may be better to use exhaust in this case. As for the design of the side blower, there is usually no difference in the effect of the top blower. A more effective improvement method is to establish a dedicated cooling air duct for the CPU, so that it will not be affected by the hot air near the CPU, which is equivalent to reducing the ambient temperature.
Although axial fans are widely used, they also have inherent defects. The axial flow fan is blocked by the position of the motor, and the air flow cannot smoothly pass through the middle of the blowing area, which is called "dead zone". On a typical heat sink, it is precisely the middle fin that has the highest temperature. Due to this contradiction, the heat dissipation effect of the heat sink is not sufficient when the axial flow fan is used.
Centrifugal fans are completely different from axial fans, and are also gradually used in CPU cooling. They are usually called "turbo fans" by computer users. The advantage of this fan is that it solves the "dead zone" problem very well. The difference between a centrifugal fan and a traditional fan is that the rotation of the blades is carried out in a vertical plane, and the air inlet is located on the side of the fan. The airflow received by the underside of the radiator is more evenly distributed. There are no obstacles in the blowing direction of the centrifugal fan, so there is the same airflow in every position. At the same time, its air pressure and air volume adjustment range is also larger, and the speed control effect is better. The negative effects are the same as high-power axial fans - high price and loud noise. Improve the air duct design Another way to solve the blind spot of wind is to change the wind direction of the fan. The traditional way to mount a heatsink is with the airflow facing down, i.e. perpendicular to the CPU. After improving the air duct design, the fan is changed to blow sideways, so that the direction of the airflow is parallel to the CPU. The primary benefit of lateral blowing is to completely solve the wind blind spot, because the airflow passes through the heat dissipation fins in parallel, and the airflow speed is the fastest on the four sides of the airflow section, and the heat point of the CPU is located on just one side. In this way, the heat absorbed by the CPU cooling base can be taken away in time. Another advantage is that there is no rebound wind pressure (usually when blowing downward, a part of the airflow rushes to the bottom surface of the heat sink and bounces, which will affect the direction of airflow movement in the radiator, and the efficiency of heat exchange will be lost). The heat exchange efficiency is higher than that of downward blowing
Classification of micro cooling fans:
1. According to the working voltage of the cooling fan: AC cooling fan (AC FAN); DC cooling fan (DC FAN)
2. According to the drive motor of the cooling fan: brushless DC cooling fan (DC BRUSHLESS FAN); brushed DC cooling fan (DC BRUSH FAN); brushless AC cooling fan (AC BRUSHLESS FAN).
3. According to the fan motor bearing system: oil bearing type (SLEEVE BEARING); ball bearing type (BALL BEARING); ceramic nano-bearing type (CERAMIC NANOMETER BEARING).
4. According to the direction of steam flow: axial flow fan (AXAL FAN); centrifugal fan (BLOWER FAN); cross flow fan (CROSS FAN).
With the development of technology, waterproof fans used in water have also been produced, which can be regarded as a milestone in the history of fans!
