So far we have discussed microwave energy and its characteristics.If you haven't seen this click here to see article about microwave energy. In this section we will look at how the microwave energy is generated. The component used to generate microwave energy in a microwave oven is called a Magnetron. It is a thermionic device similar in some respects to a thermionic diode. To understand the basic operation of the magnetron, the operation of a thermionic diode valve is discussed below.
THERMIONIC DIODE OPERATION:
A diode consists of two electrodes which are
THERMIONIC DIODE OPERATION:
A diode consists of two electrodes which are
- the Anode
- the Cathode
These electrodes are enclosed in an evacuated glass or metal envelope. The cathode is coated with a material that, when heated, will emit electrons (sub-atomic particles). The cathode has to be heated in order to free these electrons. In a magnetron the cathode is directly heated and is usually referred to as the filament. The anode is used to collect electrons that are produced by the filament. To do this, the anode has to be positive with respect to filament. Electrons are negatively charged particles, therefore they are attracted towards the positive anode. Electrons will flow constantly as long as the potential difference is maintained, providing current flow through the device.
MAGNETRON OPERATION:
The magnetron is a specially designed type of thermionic diode, which is made to self oscillate. The major differences being the shape and structure of the anode and the addition of two strong external magnets, one above and one below the anode chamber. The resultant magnetic field is very high, as together with the anode voltage, it determines the path the electrons will take.
Without the magnets in position the electrons would travel directly to the anode in the normal way in a straight line. With the magnets in position, the strong magnetic field exerted across the magnetron envelope will cause the electrons emitted by the filament to take a spiral path as they move towards the anode structure.
The shape of the anode forms an even number of structures called cavity resonators, which form individual tuned circuits. These tuned circuits will oscillate as the passing of the electrons induces charges into them. All the tuned circuits are connected together in phase and the resultant power is transmitted via the antenna, which is connected to the anode structure, into the cavity. A more detailed explanation of this concept follows.
MAGNETRON CONSTRUCTION:
The diagram in below is the diagram of a typical magnetron used in a Sharp microwave oven. The left-hand side shows the outside appearance while the right hand side shows a ‘cut away’ views
When examining a magnetron it would appear that there are only two terminals for connection. These are in fact for the filament and cathode. However, it should be noted that the anode structure is electrically connected to the outer case of the magnetron, this therefore comprises a third connection.
As discussed in the Basic Thermionic Diode Operation section, the anode is at a positive potential with respect to the filament. The anode of a magnetron is connected to its outer metal case, which is in turn connected to ground. It therefore becomes necessary to apply a negative potential to the filament.
Whilst the magnetron is operating, it runs quite hot at approximately 96 degrees Celsius. For this reason it has to be cooled, air is continually being blown over it by a fan. Cooling fins are fitted to the magnetron to allow the free flow of air around the anode structure, maximising the dissipation of excess heat.
PRINCIPLES OF MAGNETRON OPERATION:
The magnetron has a specially shaped Anode cavity resonator structure, as can be seen from the diagram below, which creates twelve cavity resonators formed by the anode vanes.
Each cavity resonator forms a conventional parallel tuned circuit, which consists of a capacitor connected in parallel with an inductor. In the case of the cavity resonator the capacitance is created by the vanes, which are seen as the two plates of the capacitor and the gap between the vanes is the dielectric. The length of each vane forms the inductance. The diagram below shows the magnetron anode as conventional components for ease of understanding
A conventional parallel tuned circuit required to oscillate at 2450MHz would require very small values of inductance and capacitance. These can be calculated by using the following equation.
F=1➗2ℼ√(LC)
Therefore possible values could be:
C (Capacitance) 64.95 x 10-12Farad (64.95pF)
L (Inductance) 64.95 x 10-12 Henry (64.95pH)
The above examples are not practical values, but they do illustrate that the values of capacitance and inductance created within a magnetron by the cavity resonators are very small.
By inter-connecting every other anode vane, using mode or strap rings, it is possible to ensure that adjacent cavity resonators oscillate 180 degrees out of phase when the magnetron is active. This configuration is shown in the diagram below.
The diagram below shows the anode structure of the magnetron and the position of the magnets. A strong magnetic field is present around the chamber. The effect of the magnetic field causes the electrons to take a spiral path as they travel towards the anode.
For the magnetron to operate correctly, a very high potential difference between the filament and anode is needed, the anode being positive with respect to the filament. In practice this is achieved by connecting the anode to ground and applying a high negative voltage to the filament.
When the filament is heated, the electrons become excited and begin to jump from the filament. These free electrons form a cloud or 'space charge' around the filament. The electrons are then attracted towards the anode due to its positive polarity. However they are forced into taking a spiral path due to the influence of the external magnetic field that is created by the magnets above and below the anode chamber (Lorentz's law). As the electrons move closer to the cavity resonators they induce a charge within the resonator and this sets up the initial oscillation. Their movement over the gaps of the vanes creates a positive feedback effect, which causes the oscillation to continue.
As the oscillation develops some resonators will be in a negative state and some positive state, each cavity resonator being 180 degrees out of phase with its neighbour. These conditions reverse as the cycle of oscillation is completed, that is the resonators that were positive become negative and those that were negative become positive. This has a further effect on the paths taken by the electrons.
Any electron in the area of the negatively charged resonator vane is repelled because of their 'like charges', negative electrons and negatively charged resonator. The velocity of these electrons causes them to return to the filament, where they impact upon it, causing 'back heating' and 'secondary emission'. Conversely electrons in the vicinity of a positively charged resonator are attracted further towards the anode where they will finally land.
As shown in the diagrams below, these two conditions create a pattern of electrons within the magnetron chamber. This pattern is usually referred to as the ‘spoked wheel effect’; the 'spokes' are formed because of the positively charged cavity resonators attracting electrons towards the anode. The spaces between the spokes are caused by electrons being repelled due to the negatively charged resonators.
It is important to remember that the polarity of charge is constantly changing within the cavity resonators. As the oscillation continues, during one half cycle of operation electrons are attracted by alternate resonators and repelled by the others. On the next half cycle the polarities will change. This effect together with the magnetic field causes the 'spoked wheel' to rotate so that the 'spokes' are always pointing to the positively charged cavity resonators, and therefore the gaps are aligned with the negatively-going cavity resonators. As the oscillation continues the 'spoked wheel' will progressively turn.
Diagram above show the 'Spoked wheel' pattern formed by the electron cloud in the two maximum states of oscillation.
It can be seen from the diagram below, all twelve cavity resonators are effectively connected in parallel, therefore the power available from each one is added together. As the cavity resonators are in parallel, it is possible to connect an antenna (aerial) to any of the anode vanes, enabling the total amount of microwave energy produced to be transmitted through the waveguide into the oven cavity.
When replacing a magnetron care should be taken on the following points:
• There is a RF gasket fitted around the antenna to prevent microwave energy escaping from the seal between the magnetron and the waveguide. Always ensure the gasket is not distorted when fixing the magnetron in place.
• When handling, take care not to leave greasy deposits either around or on the antenna, which may carbonise, causing arcing at a later date.
• Ensure that the connections to the magnetron terminals are tight. If they are loose, overheating and damage will occur.
• Always remember the 3D checks when working around the magnetron and high voltage circuit.
Several Sharp microwaves may use the same type of magnetron, but have different output RF powers. This is due to the RF output power being directly proportional to the anode current, which can be controlled in the HIGH VOLTAGE circuit design. The filament potential is altered to give the required power for individual models.
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There is another step to the process. The spokes of the cloud need to be prevented from going to the next vane as soon as it becomes more positive, or they would all be held midway at the same potential. When a capacitor is charging or discharging, there is a charge current between the plates, which has a magnetic field like a real current. As the vanes are being pulled from positive to negative, the magnetic field from the charge current confines them there until the vanes are almost fully negative & the current decreases. The cloud is then allowed to switch to the now positive vanes, pulling them down until they are almost fully negative. I posted a description & diagram in the stackexchange site under "magnetron working principle".
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