Composition and working principle of magnetic couplings

The magnetic couplings are mainly composed of two parts: a magnetic conductor connected to the shaft end of the motor and a permanent magnet connected to the load end. In operation, according to the principle of eddy current induction, the relative motion of the above two parts generates a magnetic field, and in this way, an eddy current will be generated in the disk-shaped conductor, and the magnetic field generated by the eddy current and the magnet attract each other, so that the two parts of the rotor and the conductor pass through the air. The gap transmits torque, so that the motor and the load are changed from the original hard connection to the soft connection, as shown in Figure 1 and Figure 2.

According to the above principles, in recent years, China has developed different types of magnetic couplings such as delay type, torque-limiting type, and speed-regulating type. In recent years, with the development of technological innovations such as energy saving and consumption reduction in cement enterprises and internal potential tapping, such as magnetic couplings, dynamic harmonic energy saving devices, etc., they have gradually been applied and promoted in the cement industry.


The advantages of magnetic couplings are mainly reflected in the following aspects:

1) The current of the driving motor is reduced, and the energy saving effect is remarkable. After using the magnetic couplings, whether it is the energy efficiency of a single device or the total energy efficiency of the system, the efficiency of the magnetic coupling is the highest. Therefore, the use of magnetic couplings will reduce energy consumption and save operating and maintenance costs for cement production line equipment.


2) After using the magnetic couplings, the vibration of the equipment can be greatly reduced and the service life of the motor and its bearings can be extended. The magnetic couplings transmit torque through the air gap and are a real non-contact connection device. This connection method can make the equipment connection stress more uniform, the centering performance is better, and the bearing capacity is greatly enhanced. Through testing, the use of magnetic couplings can reduce vibration by more than 80%.


3) After using the magnetic couplings, the flexible start of the equipment (that is, soft start) can be well realized, and the motor and load can be well protected.


4) The use of magnetic couplings can reduce the failure rate. Because the magnetic couplings transmit torque through the air gap, there are no wearing parts, and there is basically no failure, which will reduce the failure rate and greatly reduce the downtime.


5) The magnetic couplings have an overload protection function, which improves the safety and reliability of the system operation. The hydraulic couplings commonly used in cement enterprises are protected against overload through oil injection and pressure relief. This overload protection method not only pollutes the environment, but also increases repair time and maintenance costs.


6) The magnetic couplings have a simple structure, do not require lubrication, and have no pollution to the environment. It is a green product.


7) For equipment with a narrow range of speed regulation, such as high-temperature fans, the stepless speed regulation of the load can also be realized by adjusting the air gap between the two parts of the magnetic coupling.


Selection and attention of magnetic couplings

1)The magnetic couplings are a part of standardized production products. At the initial stage of selecting the coupling, you can know whether there is a type that meets the needs of the use in China's national standards and machinery industry standards. If the matching type of coupling cannot be found, you can contact the manufacturer to design the couplings product by yourself.


2)The torque of the magnetic couplings should meet the requirements of the power machine of the transmission system. According to the power and speed of the power machine and the working machine, the reasonable torque value of the couplings can be calculated, so as to initially select the power model of the couplings.


3)After the primary selection of the magnetic couplings, the size of the couplings should be determined according to the shaft diameter and the length of the shaft hole of the driving shaft and the driven shaft, and the specification of the couplings should be adjusted. The adjustment of the coupling model should also consider whether the speed of the driving shaft and the driven shaft connected by the couplings are consistent and whether the shaft diameters are the same.


4)For the selection of magnetic couplings, finally, all factors should be considered comprehensively again to ensure that the size, torque, shaft diameter, and shaft hole of the couplings are matched with the motor and reducer, and can adapt to the corresponding working environment. After the model of the couplings is selected, it is necessary to check and calculate the strength of the shaft key, and finally determine the model of the couplings.


5)According to the load characteristics driven by the equipment and the soft start characteristics of the magnetic couplings, the selection of the magnetic couplings should be cautious. Such as hoist load, limestone conveyor belt conveyor, clinker zipper machine and other heavy-duty equipment, especially in order to reduce the installed capacity of the design department, the power margin when selecting the drive equipment is small, and it often fails to start normally with materials after stopping due to failure. For this situation, try to choose a torque-limiting magnetic couplings when selecting magnetic couplings, and the power should be one gear larger.


6) When using frequency conversion and speed regulation equipment, try not to use magnetic couplings. This is because the mechanical characteristics of frequency conversion equipment are relatively soft at low frequencies, coupled with the flexible start characteristics of magnetic couplings, it is easy to cause motor stalls, resulting in failure shutdowns The device cannot be started again.


Matters needing attention in practical application of magnetic couplings

1)The magnetic couplings support the rotation of the motor in any direction. Whether it is the first commissioning of equipment for a new enterprise, or the modification of couplings such as hydraulic couplings during use, in order to avoid damage to the equipment, the motor must be jogged first during the test run, and the test run of the equipment can only be carried out after confirming that its steering is correct. Under normal circumstances, the low-voltage motor is allowed to start twice in the cold state, and the interval between each time shall not be less than 5 minutes, and it is allowed to start once in the hot state; the high-voltage motor is allowed to start once in the hot state, and the time interval shall not be less than 240 minutes, and it is allowed to start 2 times in the cold state, and the interval between each time should not be less than 60 minutes.


2)When using magnetic couplings, it is necessary to properly consider that the ambient temperature cannot be too high, because the magnetic couplings use the principle of eddy current induction. Under normal circumstances, the loss caused by eddy current is released from the magnetic conductor in the form of heat, which causes the temperature of the working environment of the permanent magnet to rise. Therefore, the protective cover of the magnetic coupling at the production site should be well ventilated to avoid heat accumulation.


3)When using magnetic couplings, it is necessary to ensure the safety and reliability of various interlocking signals of the host equipment, such as the interlocking protection between the tail wheel speed measurement signal of the hoist and the drive motor, and the interlocking tripping protection between the equipment and the upper process equipment. After the overload protection of the magnetic coupling, the host equipment installed with the magnetic coupling will stop running. If the upper-level process equipment cannot be chained to stop, it will cause the elevator to press the material and the feeding slide to block the material.


4)If the load end stops due to overload or equipment failure, and the tail wheel speed detection of the hoist and other equipment happens to be damaged, in this case the motor will run normally, and all or most of the power generated by the eddy current will act on the magnetizer, causing the working temperature of the permanent magnet rises rapidly. When the temperature rises beyond the allowable temperature of the permanent magnet, the magnetic performance will decrease with the increase of this temperature. When the temperature rises to the critical point of permanent magnet failure, the magnetic coupling will "lost magnetism" and stop working. Therefore, in production, one is to avoid overloading the equipment as much as possible; the other is to install speed measurement and detection protection devices on the head and tail wheels of the equipment to prevent incidents of equipment damage.


5)From another point of view, in order to avoid the "loss of magnetism" phenomenon of magnetic couplings, it should be considered to increase the wind guide vane or the air guide hole from the beginning of the design of the equipment. In addition, when the magnetic coupling is overloaded and protected, the gap between the two parts can be designed to be larger, so that the conductor cutting the magnetic field lines will be greatly reduced, and the heat generation will be relatively reduced a lot. In this way, even if the speed measurement detection protection is not in place, it will not damage the equipment and affect normal production.


With the development of technology and the maturity of products, it is believed that the energy-saving and environmental protection equipment of magnetic coupling will be widely promoted and applied in many cement enterprises in the near future.

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