Thursday, June 9, 2016
Microwave Working principle – Sharp Microwave oven - How magnetron works – Microwave oven Repair and Service
Category: Microwave Oven Repair and Servcie
Contents of this article
- How Magnetron works
- Magnetron Construction
- Energy and Standing waves
Sharp Microwave oven
Distribution of Energy within the Oven Cavity
The microwave energy produced by the magnetron is fed to the oven cavity through a waveguide. The waveguide shape and size is designed to enable the energy to be transferred with very little loss. When entering the cavity, the energy will, if left unmodified set up 'Standing Waves'. These standing waves are produced by reflected energy from the wall, floor and ceiling of the cavity. A regular pattern is established, with energy present along the standing waves. Some areas within the cavity will have no microwave energy p
The microwave energy produced by the magnetron is fed to the oven cavity through a waveguide. The waveguide shape and size is designed to enable the energy to be transferred with very little loss. When entering the cavity, the energy will, if left unmodified set up 'Standing Waves'. These standing waves are produced by reflected energy from the wall, floor and ceiling of the cavity. A regular pattern is established, with energy present along the standing waves. Some areas within the cavity will have no microwave energy p
The difference in energy levels throughout the cavity, caused by the standing waves, would produce uneven heating of the food during the cooking process. However, there are two main ways in which this problem can be alleviated. That is by using either a mode stirrer system, or by the use of a turntable.
The Turntable
This method leaves the heating pattern produced by the cavity design unaltered. The food is placed on a turntable during cooking, the turntable is rotated so that all parts of the food pass through several standing waves of energy during each rotation of the turntable.
The most effective way of using a turntable system is to place the food close to the outside of the turntable allowing the maximum travel through the standing wave energy field. Note that if food is placed too near to the edge of the turntable, it may become unbalanced and not rotate correctly
This method leaves the heating pattern produced by the cavity design unaltered. The food is placed on a turntable during cooking, the turntable is rotated so that all parts of the food pass through several standing waves of energy during each rotation of the turntable.
The most effective way of using a turntable system is to place the food close to the outside of the turntable allowing the maximum travel through the standing wave energy field. Note that if food is placed too near to the edge of the turntable, it may become unbalanced and not rotate correctly
Mode Stirrers
Mode stirrers, stirrer fans and devices known as rotating antennae, work in such a way as to constantly change the energy wave pattern within the cavity during the cooking process; These units are made of materials that will reflect microwave energy. Therefore when they are placed in the energy field and made to spin, they will randomly affect the pattern. This sets up a continuously changing energy field in the cavity. These devices are usually motor driven, but can be air driven, usually the food remains static within the cavity.
Mode stirrers, stirrer fans and devices known as rotating antennae, work in such a way as to constantly change the energy wave pattern within the cavity during the cooking process; These units are made of materials that will reflect microwave energy. Therefore when they are placed in the energy field and made to spin, they will randomly affect the pattern. This sets up a continuously changing energy field in the cavity. These devices are usually motor driven, but can be air driven, usually the food remains static within the cavity.
Microwave ovens for domestic use employ the turntable method to produce even cooking, whereas microwave ovens designed for commercial use generally incorporate mode stirrers. Commercial ovens also use two magnetrons, which gives a double advantage of more power with two heating patterns. A detailed description of differences of a commercial oven compared with the domestic models is included in the Sharp Commercial Microwave Ovens section at the end of this book. The magnetron, waveguide and cavity can be thought of as a 'matched' or 'tuned' circuit. The oven cavity is a multi-mode cavity resonator, designed to resonate at the frequency generated by the magnetron. The whole system requires a load to work into. So when running a microwave oven in the cook condition, there should always be at least a small load in the cavity. The most convenient load is probably a glass of water, this will enable the oven to be run long enough to carry out most tests.
If an oven is run for any length of time without a load, then the magnetron will be stressed. This is caused by a back heating effect, and if left too long, eventual damage causing low output will result. Modern magnetrons are fairly tolerant of the no load condition, so a sudden catastrophic failure is unlikely, however the effective life expectancy could be greatly reduced.
Another way in which a magnetron may be damaged is by the use of an excessive amount of metal foil or large metal utensils within the oven cavity. The effect can be that energy is reflected back to the magnetron where it will be dissipated as heat. Small amounts of foil can be tolerated, as called for in certain recipe books, and devices such as temperature probes, which are part metal in their construction. It should be remembered though, when using metal in an oven to keep it well away from the cavity walls. if metal objects are placed in the energy field and then come close to an earthed surface, arcing will occur and the surface of the metal could be marked. In the case of the temperature probe, it could be rendered inoperative. Metal racks and turntables, designed for use in the cavity, have good insulation or make good contact each other, Therefore it is expected that most magnetrons will last for the lifetime of the oven.
If an oven is run for any length of time without a load, then the magnetron will be stressed. This is caused by a back heating effect, and if left too long, eventual damage causing low output will result. Modern magnetrons are fairly tolerant of the no load condition, so a sudden catastrophic failure is unlikely, however the effective life expectancy could be greatly reduced.
Another way in which a magnetron may be damaged is by the use of an excessive amount of metal foil or large metal utensils within the oven cavity. The effect can be that energy is reflected back to the magnetron where it will be dissipated as heat. Small amounts of foil can be tolerated, as called for in certain recipe books, and devices such as temperature probes, which are part metal in their construction. It should be remembered though, when using metal in an oven to keep it well away from the cavity walls. if metal objects are placed in the energy field and then come close to an earthed surface, arcing will occur and the surface of the metal could be marked. In the case of the temperature probe, it could be rendered inoperative. Metal racks and turntables, designed for use in the cavity, have good insulation or make good contact each other, Therefore it is expected that most magnetrons will last for the lifetime of the oven.
Cavity and Waveguide Practical Problems
A motor usually rotates mode stirrers and turntables and therefore occasional motor failure is possible. It is important that the turntable couplings are kept clean otherwise food debris may find their way down to the motor below and this may cause premature failure because of seizure. It is also possible for turntable couplings to become damaged due to the repeated heating and subsequent carbonising of food debris beneath the turntable. This may cause the plastic coupling and 'spider' to melt and the excessive heat may crack the glass turntable. It should be noted that the turntable motor is asymmetric, which means that it is possible for the motor to turn in either a clockwise or anticlockwise direction.
Another item that may give problems from time to time, if it is not kept clean, is the waveguide cover, or the stirrer cover. Waveguide and stirrer covers are made of materials that are inert to microwave energy. However, if the cover is not kept clean, food debris will build up and be repeatedly cooked. This will then carbonise, and arcing will occur, finally the waveguide cover will have holes burnt in it. The cure is a new waveguide cover and a tactful reminder to the customer to keep the oven clean. Although this problem is simple to cure, and the customer can replace most waveguide covers, when the fault occurs it can nevertheless be disconcerting.
The oven should not be operated with a damaged waveguide cover or with the cover removed. In these situations food splashes could enter the waveguide, causing arcing and eventually corrosion. The result is that a new cavity is required to remedy the problem.
Very occasionally an engineer may encounter an oven that has a broken glass turntable. Apart from the obvious possibility of its having been dropped, the damage could have been caused by the use of a browning dish. To safeguard against possible damage when using a browning dish, use an upturned oven proof plate on the turntable, so that the turntable is insulated from the heat source generated by the browning dish. Periodically an engineer may come into contact with an oven that has been damaged because of severe overcooking of food. The amount of damage can vary between smudging that can be cleaned off, to extensive damage requiring the replacement of the cavity and other components. Sharp ovens incorporate temperature fuses, which will operate and stop further magnetron output in the event of severe overheating within the cavity.
Magnetron Theory
So far we have discussed microwave energy and its characteristics. 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; this 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 will be discussed.
Thermionic Diode Operation
A diode consists of two electrodes, the Anode and the Cathode, which are contained within 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 given off 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.
A motor usually rotates mode stirrers and turntables and therefore occasional motor failure is possible. It is important that the turntable couplings are kept clean otherwise food debris may find their way down to the motor below and this may cause premature failure because of seizure. It is also possible for turntable couplings to become damaged due to the repeated heating and subsequent carbonising of food debris beneath the turntable. This may cause the plastic coupling and 'spider' to melt and the excessive heat may crack the glass turntable. It should be noted that the turntable motor is asymmetric, which means that it is possible for the motor to turn in either a clockwise or anticlockwise direction.
Another item that may give problems from time to time, if it is not kept clean, is the waveguide cover, or the stirrer cover. Waveguide and stirrer covers are made of materials that are inert to microwave energy. However, if the cover is not kept clean, food debris will build up and be repeatedly cooked. This will then carbonise, and arcing will occur, finally the waveguide cover will have holes burnt in it. The cure is a new waveguide cover and a tactful reminder to the customer to keep the oven clean. Although this problem is simple to cure, and the customer can replace most waveguide covers, when the fault occurs it can nevertheless be disconcerting.
The oven should not be operated with a damaged waveguide cover or with the cover removed. In these situations food splashes could enter the waveguide, causing arcing and eventually corrosion. The result is that a new cavity is required to remedy the problem.
Very occasionally an engineer may encounter an oven that has a broken glass turntable. Apart from the obvious possibility of its having been dropped, the damage could have been caused by the use of a browning dish. To safeguard against possible damage when using a browning dish, use an upturned oven proof plate on the turntable, so that the turntable is insulated from the heat source generated by the browning dish. Periodically an engineer may come into contact with an oven that has been damaged because of severe overcooking of food. The amount of damage can vary between smudging that can be cleaned off, to extensive damage requiring the replacement of the cavity and other components. Sharp ovens incorporate temperature fuses, which will operate and stop further magnetron output in the event of severe overheating within the cavity.
Magnetron Theory
So far we have discussed microwave energy and its characteristics. 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; this 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 will be discussed.
Thermionic Diode Operation
A diode consists of two electrodes, the Anode and the Cathode, which are contained within 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 given off 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.
Overview of 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 critical, 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.
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 critical, 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
Shown below is a 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
Shown below is a 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.
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.
Resonant frequency = 1/2π Root LC
Therefore possible values could be:
C (Capacitance) 64.95 x 10-12 Farad (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.
C (Capacitance) 64.95 x 10-12 Farad (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.
The two diagrams 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.
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.
# 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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