WAVE DISTRIBUTION:
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 present.
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.
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.
Sharp 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
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, e.g. the metal rack and metal turntable tray used in convection ovens. Therefore arcing does not occur.
The life expectancy of a magnetron is approximately 2000 hours. Which equates to about seven years of use by the average customer. Therefore it is expected that most magnetrons will last for the lifetime of the oven.
- A motor usually rotates mode stirrers and turntables and therefore occasional motor failure is possible.
- 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
- 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.
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