1. Working principle of cooling fan and fan speed

Cooling fans can be divided into DC cooling fans and AC cooling fans according to the power supply type.

Working principle of DC fan

According to the 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, it will generate suction or repulsion, causing the object to move. A rubber magnet filled with magnetism is attached to the inside of the fan blade of the DC fan. Surrounding the silicon steel sheet, the axis part is wound with two groups of coils, and the hall induction component is used as the synchronous detection device to control a group of circuits, which makes the two groups of coils wound around the axis work in turn. Silicon steel sheet produces different magnetic poles, which generate attraction and repulsion force with rubber magnet. 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 component provides a synchronous signal, the fan blade can continue to operate. As for its operation direction, it can be determined according to Fleming's right-hand rule.

Working principle of AC fan

The difference between AC fan and DC fan. The power supply of the former is AC, and the power supply voltage will be positive and negative alternating. Unlike the fixed power supply voltage of DC fan, it must rely on circuit control to make the two groups of coils work in turn to produce different magnetic fields. Because the power frequency of the AC fan is fixed, the change speed of the magnetic pole generated by the silicon steel sheet is determined by the power frequency. The higher the frequency, the faster the magnetic field switching speed, and the faster the speed in theory, just like the principle that the more poles of the DC fan, the faster the speed. However, the frequency should not be too fast, too fast will cause difficulty in activation.

Fan speed refers to the number of times the fan blades rotate per minute, in RPM, which is generally measured externally by instruments.

2. Cooling fan bearing form

The bearing type refers to the bearing type used by the air-cooled cooling fan. In mechanical engineering, there are many types of bearings, but there are only three types of bearings used in radiator products according to their basic working principles: sleeve bearing using sliding friction and ball bearing using rolling friction, as well as a mixture of the two types of bearings. In recent years, the new technologies introduced by major radiator manufacturers in bearings, such as magnetic levitation bearings, fluid protection system bearings, hydraulic bearings, Laifu bearings, nano ceramic bearings, are also improved from the above basic bearing forms, and the basic working principle has not changed.

Sleeve bearing is a sleeve bearing with sliding friction. It uses lubricating oil as lubricant and drag reducer. In the initial use, it has low operating noise and low manufacturing cost. However, this bearing is seriously worn and its service life is much longer than that of ball bearing. Moreover, this kind of bearing has been used for a long time. Due to the oil seal (it is impossible for computer radiator products to use high-grade oil seal, which is generally an ordinary paper oil seal), the lubricating oil will gradually volatilize, and dust will also enter the bearing, causing problems such as slow fan speed and increased noise. In serious cases, the eccentric cooling fan will cause severe vibration due to bearing wear. In case of these phenomena, either open the oil seal to refuel, or only eliminate and purchase new fans.

Single ball bearing (1 ball+1 sleeve bearing) is an improvement on the traditional oil bearing. It adopts the form of sliding friction and rolling friction. In fact, it uses a ball bearing with an oil bearing to reduce the cost of double ball bearings. Its rotor and stator are lubricated with balls and equipped with lubricating oil. It overcomes the problems of short service life and unstable operation of oil bearing, and the cost rise is extremely limited. The single ball bearing absorbs the advantages of oil bearing and double ball bearing, and improves the service life of the bearing to 40000 hours. The disadvantage is that the operating noise increases after adding the ball, but it is still smaller than the double ball bearing.

Double ball bearing is a relatively high-grade bearing. It adopts the form of rolling friction and adopts two ball bearings. Several tiny steel balls in the bearing revolve around the axis. When the fan leaf or axis rotates, the steel balls rotate with it. Because they are all spheres, the friction is small and there is no problem of oil leakage. The advantage of double ball bearing is its long service life, which is about 50000-100000 hours; Good anti-aging performance, suitable for fans with high speed. The disadvantage of double ball bearings is high manufacturing cost and the highest noise at the same speed level. Double ball air bearings and hydraulic bearings have good sealing performance, especially double ball bearings. The double ball bearing is completely embedded in the fan, and the rotating part is not in direct contact with the outside world. In a sealed environment, the working environment of the bearing is relatively stable. Therefore, almost all 5000 rpm large-diameter fans use double ball bearings. The hydraulic bearing has a unique return oil circuit, so the possibility of lubricating oil leakage is small.

The representative manufacturer of rifle bearing technology is CoolerMaster. At present, CoolerMaster has upgraded most of its traditional oil bearing fans to rifle bearings. As an improvement of the traditional oil bearing, Laifu bearing is made of wear-resistant materials into a high oil content hollow bearing, which reduces the friction between the bearing and the shaft core. Laifu bearing also has a shaft core with reverse spiral groove and oil retaining groove. When the fan is running, the oil will form a reverse backflow, so as to avoid the loss of oil, so as to improve the service life of the bearing. By adopting the above structures and parts, the oil containing and oil retaining capacity of Laifu bearing fan is greatly improved, and the noise is reduced.

The name of hydro bearing comes from hy (hydrodynamic wave) pro (oil protection system). It is a patented product of ADDA, a well-known radiator and fan design and manufacturer. It is also made by many improvements on the basis of traditional oil bearing. The bearing of fluid protection system and hydraulic bearing can be said to have the same goal by different ways. The two designs have adopted some unique improvement measures respectively, but the essence is the circulating oil circuit system, and the performance in all aspects is basically the same. Generally, the service life of cooling fan products can reach more than 50000 hours.

Hydraulic bearing is a technology initiated by AVC and improved on the basis of oil bearing. The hydraulic bearing has larger oil storage space than the oil bearing, and has a unique loopback oil supply circuit. The working noise of the hydraulic bearing fan has been significantly reduced, and the service life is also very long, which can reach 40000 hours. The hydraulic bearing is still essentially an oil bearing. However, after improvement, the service life of this kind of bearing is much longer than that of ordinary oil seal bearing, and it inherits the advantage of oil bearing - low running noise. At present, hydraulic bearings have been used in AVC radiators, but not all AVC cooling fans use hydraulic bearing fans.

Vapor bearing (vapo bearing) is improved by Sunon's magnetic levitation technology, which is to strengthen the hardness of the shaft sleeve of the oil bearing, and use special materials, and its inner surface is also specially processed, so as to overcome the shortcomings of the oil bearing that it is not resistant to high temperature, and then cooperate with the magnetic levitation technology, it will greatly extend the service life of the cooling fan.

The motor of magnetic bearing has magnetic system (MS) design, and its magnetic induction line is perpendicular to the magnetic levitation line, so the shaft core and the magnetic levitation line are parallel, so the weight of the rotor is fixed on the running track, and the almost unloaded shaft core is used to support in the direction of the anti magnetic levitation line, forming the whole rotor suspended on the fixed running track. Therefore, magnetic levitation is actually only an auxiliary function, not an independent bearing form. The specific application has to cooperate with other bearing forms, such as magnetic levitation + ball bearing, magnetic levitation + oil bearing, magnetic levitation + vaporization bearing, etc. This technology has not been recognized by European and American countries.

Nano ceramic bearing (NCB) is still an oil bearing in essence, which was first introduced by Foxconn in its products. The traditional oil bearing fan is seriously worn in the process of use, and its reliability is low when it is used for a long time. Nano bearing effectively overcomes this problem: ceramic bearing technology adopts special polymer materials and special additives to fully integrate, the bearing core is fully made of special white oxide materials, and is made by stamping and sintering process. The crystal particles are reduced from the past 60um to 0.3um, which has the characteristics of firmness, smoothness, wear resistance and so on. Nano ceramic bearings have strong high temperature resistance and are not easy to volatilize, which greatly extends the service life of fans. The properties of nano bearings are similar to ceramics, and the more they are ground, the smoother they become. According to the test, the average service life of fans with nano ceramic bearings is more than 150000 hours. In fact, this technology is not a real nanotechnology, and the materials used are not real nanoscale materials, but the words like nano are used to attract eyeballs.

3. How to achieve low noise value of cooling fan

1. System impedance

The range between the air inlet and air outlet of a shell accounts for 60% to 80% of the total system impedance. In addition, the greater the air flow, the higher the noise. The higher the system impedance, the greater the air flow required for cooling. Therefore, in order to minimize noise, the system impedance must be reduced to a minimum.

2. Airflow disturbance

Noise is generated by turbulence caused by obstacles encountered along the air flow path. Therefore, any obstruction, especially in the range of key air inlets and outlets, must be avoided to reduce noise.

3. Fan speed and size

Since high-speed fans produce more noise than low-speed fans, try and choose low-speed fans as much as possible. A fan with larger size and lower speed is usually quieter than a fan with lower size and higher speed when delivering the same air volume.

4. Temperature rise

In a system, the air volume required for cooling is inversely proportional to the allowable temperature rise. If the temperature rise is allowed to increase slightly, the required air volume can be greatly reduced. Therefore, if the restrictions imposed on the allowable temperature rise are slightly relaxed, the required air volume and noise will be reduced.

5. Vibration

In some cases, when the weight of the whole system is very light, or the system must operate in a specified way, it is particularly recommended to use soft isolation equipment to avoid the transmission of fan vibration.

6. Voltage variation

Voltage variation will affect the noise level. The higher the voltage added to the fan, the greater the vibration and noise due to the increase of rotating speed.

7. Design considerations

The design of each part constituting the fan will affect the noise level. The following design considerations can achieve noise reduction: the size of winding core, the design of fan blade and outer frame, and accurate manufacturing and balance.

4. How to measure the noise value of cooling fan

The noise of the hxht fan is measured in a non reverberation room with a background noise of less than 15dba. The fan to be tested operates in free air, and a noise meter is disposed one meter away from the air inlet.

The sound pressure level depends on the background factors, and the sound power level is expressed by the following formula:

SPL=20logP/Pref

And swl=10logw/wref

P= sound pressure

Pref= reference sound pressure

W= noise energy of sound source

Wref= noise energy of sound source

The noise value of the fan is usually plotted in the octave band of sound pressure level (SPL). The effect caused by the change of decibel (DBA) is shown in the following symptoms:

3 DBA almost no feeling

5 DBA feel it

10 DBA feels twice as loud

Noise level:

0 ~ 20 DBA is weak

20~40 DBA weak

40~60 DBA moderate

60~80dba loud

80-100 DBA very loud

100`140 DBA deafening

5. Parallel and series operation of cooling fans

Parallel operation

Operation curve of parallel fan and separate fan

Parallel operation is to use two or more fans in parallel. The volume of air volume generated by two fans in parallel is only twice that of a single fan in free space. When parallel fans are applied to the case of higher system impedance, the higher the system impedance, the lower the air volume that can be increased by parallel fans. Therefore, the application of parallel connection is only used under the condition of low system impedance, that is, the fan operates under the condition of almost complete free air supply.

Series operation

Operation curve of series fan and individual fan

Series operation is to use two or more fans in series. The static pressure generated by two fans in series can be twice under the condition of zero air volume, but in the case of free space, it cannot increase the air volume. An additional fan in series can increase the air volume in a system with high static pressure. Therefore, series operation can achieve the highest effect for systems with high system impedance.

6. Third wire signal wire of cooling fan

The switching drive circuit design of the fan provides the measurement of speed: the fan motor has three wires (red wire: +, black wire: -, yellow wire: the third signal transmission wire)

The company provides the following two signal types

Type R (operation detector)

Type F (square wave generator)

The relationship between rotation and the output pulses of the third wire is as follows according to different products:

(a)1Rotation=2 Pulses(4 poles' motor)

(b)1Rotation=3 Pulses(6 poles' motor)

(c)1Rotation=4 Pulses(8 poles' motor)

remarks:

The normal 8-pole motor should be 1rotation=4 pulses; However, if there is a frequency division circuit design, it will be 1rotation=2pulses after frequency division

7. How to choose the right fan or blower fan

The design engineer of all motors and electronic products that need to use fans for heat dissipation must decide the air volume required for heat dissipation of a specific system, and the required air volume depends on understanding the power consumption of the system and whether it can take away enough heat to prevent the system from overheating. Facts show that the service life of the system will be reduced due to the lack of cooling system, so the design engineer should also understand that the sales volume and price of the system may decline because the service life of the system does not meet the expectations of users.

To select the correct ventilation components, the following objectives must be considered:

● best air flow efficiency

● minimum suitable size

● minimum noise

● minimum power consumption

● maximum reliability service life

● reasonable total cost

Step 1: total cooling demand

First, three key factors must be understood to obtain the total cooling demand:

● heat that must be converted (i.e. temperature difference DT)

● wattage to offset converted heat (W)

● air volume required to remove heat (CFM)

The total cooling demand is very important for the effective operation of the system. Efficient system operation must provide ideal operating conditions, so that all components in the system can play the maximum function and the longest time