The device that generates the microwave is a magnetron. The step-up transformer is driven by a PLC controlled intermediate solid state relay with a voltage of approximately 6000V. A magnetron is an electrical vacuum device used to generate microwave energy. Essentially a diode placed in a constant magnetic field. Under the control of a constant magnetic field and a constant electric field perpendicular to each other, the electrons in the tube interact with the high-frequency electromagnetic field to convert the energy obtained from the constant electric field into microwave energy, thereby achieving the purpose of generating microwave energy.
The magnetron can be divided into two types: pulse magnetron and continuous wave magnetron due to different working conditions.
The magnetron consists of a die and a magnetic steel (or electromagnet). The structure of the die includes four parts: an anode, a cathode, an energy output device, and a magnetic circuit system. The inside of the tube is kept in a high vacuum state. The structure and function of each part are introduced below.
Anode
The anode is one of the main components of the magnetron, and together with the cathode constitutes a space in which electrons interact with high-frequency electromagnetic fields. Under the action of a constant magnetic field and a constant electric field, electrons complete the task of energy conversion in this space. The anode of the magnetron, in addition to collecting electrons like the anode of a conventional diode, plays a decisive role in the oscillation frequency of the high-frequency electromagnetic field.
The anode is made of a well-conducting metal material (such as oxygen-free copper) and is provided with a plurality of resonant cavities. The number of resonant cavities must be an even number, and the operating frequency of the tube is higher as the number of cavities is higher.
The types of anode resonators are often slotted, sectored and slotted, and each small cavity on the anode is equivalent to a parallel 2C tank. Taking the slot fan cavity as an example, it can be considered that the slot portion of the cavity mainly constitutes the capacitance of the oscillation circuit, and the sector portion thereof mainly constitutes the inductance of the oscillation circuit.
The anode of the magnetron is coupled together by a number of resonant cavities to form a complex resonant system. The resonant cavity frequency of this system is mainly determined by the resonant frequency of each small resonant cavity. We can also estimate the operating frequency band of the magnetron according to the size of the small resonant cavity.
In addition to generating the required electromagnetic oscillations, the magnetron anode resonance system can generate multiple electromagnetic oscillations with different characteristics. In order to stabilize the magnetron in the desired mode, the common isolation mode brings the isolation interference mode. The spacer strip connects the anode vanes one at a time to increase the frequency separation between the operating mode and the adjacent interference mode.
In addition, since the energy exchanged electrons also have a certain amount of energy, these electrons are applied to the anode to raise the anode temperature, the more electrons are collected by the anode (ie, the larger the current), or the higher the energy of the electron (the lower the energy conversion rate) ), the higher the anode temperature, therefore, the anode needs to have good heat dissipation. Under normal circumstances, the power tube is forced air-cooled, and the anode has a heat sink. The high-power tube is mostly water-cooled, and there is a cooling water jacket on the anode.
2. Cathode
The cathode of the cathode and its lead magnetron, the electron emitter, is an integral part of the interaction space. The performance of the cathode has a great influence on the working characteristics and life of the tube and is considered to be the heart of the entire tube. There are many types of cathodes with different properties. In the continuous wave magnetron, a direct-heating cathode is commonly used, which is wound into a spiral shape by a tungsten wire or a pure tungsten wire, and has the ability to emit electrons after being heated to a predetermined temperature by a current. The cathode has the advantages of short heating time and strong anti-electron bombardment capability, and is widely used in continuous wave magnetrons. Such a cathode heating current is large, and the cathode lead is required to be short and thick, and the connecting portion is in good contact. The cathode lead of the high-power tube has a high temperature during operation, and is often forced to cool by air. When the magnetron is working, the cathode is connected to a negative voltage, so the lead portion should have good insulation properties and can meet the requirements of vacuum sealing. In order to prevent the anode from overheating due to electron re-bounce, the cathode current should be reduced according to the regulations after the magnetron is stable to prolong the service life.
3. Energy output
The energy output device is a device that delivers the microwave energy generated in the interaction space to the load. The function of the energy output device is to wear the microwave without breakdown, to ensure the vacuum sealing of the tube, and at the same time to facilitate the connection with the external system. Low-power continuous-wave magnetrons mostly use a coaxial output where the high-frequency magnetic field of the anode cavity is the strongest. A coupling loop is placed which, when the magnetic flux passing through the annulus changes, generates a high frequency induced current on the loop, thereby directing the high frequency power out of the loop. The larger the coupling loop area, the stronger the coupling. High-power continuous wave magnetrons are commonly used as axial energy output devices, and the output antenna is connected to the anode fin through the pole hole. The antenna is generally formed as a strip or a round rod or a cone. The entire antenna is sealed by the output window. The output window is usually made of glass or ceramic with low loss characteristics. It must ensure the loss of microwave energy without loss and have a good vacuum tightness. The output window of a high power tube is often forced air cooled to reduce the heat generated by the loss of the medium.
4. Magnetic circuit system magnetron
A strong constant magnetic field is required for normal operation, and the magnetic field induction intensity is generally several thousand Gauss. The higher the operating frequency, the stronger the applied magnetic field. The magnetic circuit system of a magnetron is a device that generates a constant magnetic field. The magnetic circuit system is divided into two major categories: permanent magnet and electromagnetic. The permanent magnet system is generally used for a small power tube, and the magnetic steel and the die are firmly combined to form a so-called package type. The high-power tube uses a magnet to generate a magnetic field, and the die and the electromagnet are used together. The upper and lower pole pieces are provided in the die to fix the distance of the magnetic gap. When the magnetron is working, it is convenient to adjust the output power and operating frequency by changing the strength of the magnetic field. In addition, the anode current can be fed into the magnet wire package to improve the stability of the pipe operation.
two. Precautions
Since the heating part of the device is heated by microwaves, it is necessary to take into account the interference source caused by the microwave. Yonghong PLC is a self-developed product. All the controls are integrated on a chip called “SOCâ€, namely: system integrated chip, so the anti-interference ability is strong. Since the device requires heating control, the analog module must be used. The heating sensor of this device uses a far-infrared sensor with a signal of 4-20MA. The accuracy of the analog board of Yonghong PLC is 12 bits. And the analog value is read every scan cycle, so it is updated in real time, and the price of the analog board is moderate, the customer will accept it.
three. Process flow
four. Introduction of Yonghong Motor
Yonghong Electric Co., Ltd. was founded in 1992 by a group of engineers who have worked in PLC design for many years. In 1993, FBE and FBN series products were launched. Through years of efforts, the new FBS series of products was highly praised by the industry. All along, the company has been focusing on the high-performance small and medium-sized PLC market, and its own brand “FATEK†has enjoyed a high reputation in the industry. Providing customers with perfect and cost-effective products has always been Yonghong's ambition, and many years of quiet cultivation and practice can finally bear fruit.
Fives. HMI
The man-machine interface adopts the new product of Yonghong Motor: 5.7-inch touch screen.
Figure 1 is the parameter setting of the microwave vacuum drying and sterilizing machine.
Figure 2 is the motion control of the microwave vacuum drying and sterilizing machine
Figure 3 is a microwave vacuum drying and sterilizing machine to manually control the operation of each magnetron
Figure 4 is the automatic state control of the microwave vacuum drying and sterilizing machine. The temperature and time of each gear are displayed. The first temperature control ends automatically jumps to the second gear and is controlled downwards.
six. Analog board description
The temperature is controlled by Yonghong's cost-effective analog panel FBS-B4AD. The FBS-B4AD is 12-bit precision. The analog board is unipolar with a measurement range of 0-20MA or 0-10V. Yonghong PLC design is expressed in 14-bit mode, that is: 0-16380 corresponds to 0-20MA or 0-10V, and the maximum resolution is: 2.44mV (voltage), 4.88μA (current). The method of directly reading the register is convenient for a large number of programmers, and the fixed address is: D4072~D4075.
Seven. Conclusion
Yonghong's superior quality, superior application command function, and diverse communication methods have made us play a key role in production design. With the richness and development of Yonghong mechanical and electrical products, the continuous improvement of functions, and the customer's demand for overall service, the products have broad and long-term market expansion space.
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