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Microwaves Cooking - 111128

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Page updated - 06 December 2011



How do Microwave Cookers work.

The term Microwave Cooker may not be correct when compared with the 'conventional' cooker. The 'conventional' cooker uses some form of heat to increase the temperature of the 'cooking' compartment. Microwave cookers use Radio Frequency (RF) energy, which is generated in the cabinet and ducted into the cooking compartment. The food in the cooking compartment 'absorbs' the RF energy and converts it into heat.


The microwave cooking frequency is 2.45GHz (2450MHz) and has been allocated because of the good absorption of the RF energies by water, sugar and fat. Dry foods, without sugar or fat, do not absorb as much wet food or those having a sugar or fat content.


Some believe that the cooking starts from the inside of the food and progresses outwards but this is not the case. The heating effect is proportional to the energy absorbed with the higher energy absorption, and hence heating, being at the surface. The further into the food, the greater the absorbed RF energy, and hence less the heating. Most food should be allowed to 'stand' for a while to allow the heat to penetrate deep into the food and allow it to heat and cook.


The RF is generated by a very crude type of transmitter using a Magnetron valve in a housing that has a natural resonant operating frequency of 2.45GHz. The high voltage for the magnetron is generated by stepping-up the AC mains (line) voltage and converting it to DC by means of a diode and capacitor. The variation of heating effect for different foods, quantities of food or to allow a range from defrosting to rapid cooking is achieved by altering the duty-cycle (mark/space ratio) of the transmitter ON & OFF. Defrosting requires low power and hence a low duty-cycle. Full Power has a high duty-cycle with the magnetron being ON all of the time.


All of the RF energy in the microwave cooker must be dissipated by heating something. If the energy is not absorbed by something in the cooking compartment then the energy will result in heating the magnetron and an expensive result. It could even go BANG. Water content can also be present in ceramic cookware. Try to only use 'microwave-proof' cookware as the high energy associated with microwave cooking can cause rapid pressure inside the material due to the water converting into steam and this can cause the cookware to explode.


Metal in the cooking compartment will reflect the RF energy and a metal tray will deflect (prevent) the RF from heating food close to it. The same applies to aluminium foil although, if a big enough piece is used, it can be used to deflect heat away from the ends of items such as chicken legs. Small pieces of aluminium foil can give a 'pretty' firework display but this could make the food 'taste funny' or even damage the magnetron.


Tony - 06 Dec 2011