A microwawe oven is an apparatus electric household appliances, allowing the fast Cuisson of Aliment S intended for human consumption or animal. It is moreover very practical to heat solid or liquid already prepared food. The Chauffage is carried out by agitation of the Molécule S of Eau which the Aliment S. contain.

This agitation results from the oscillation of the molecule of Eau whose dipole or electric vector of polarization is directed passively in the alternating electric field of the microwave radiation.

It is very important never not to place in this type of apparatus:

• Of ustensils of Kitchen in Metal, or any other metal object: emitted electromagnetic waves, by this type of furnace, inducing important electric currents in metals causing of the short-circuits;
• Nor no Living organism.

History

It is Percy Spencer, when he worked for Raytheon with the construction of Magnétron S for the Radar S, which have the idea to use the microwaves to cook food. One day, whereas it was near a radar in activity, it felt sudden and strange feeling. He also noticed that a bar of Chocolat which was in its pocket had melted.

The first food to be deliberately heated by microwaves is Pop-corn, the second is a egg (which exploded with the face of one of the experimenters).

In 1946, Raytheon Brevet has the process of cooking by microwaves and in 1947 it built the first microwawe oven which it marketed, the Radarange . It measured 1,8 meter in height for a weight of 340 kilograms. It was cooled by a system containing Eau, had a power of 3.000 Watt S and produced three times more parasitic radiations than a current microwawe oven.

General operation

The electrical energy, in the form of a alternating voltage (high and low tension) is transformed into tension continues via a transformer lifting, diodes and of capacities.

The arriving current of the step-up transformer is used to feed the Magnétron.

Magnetron

See also: Magnetron

The magnetron consists of a cylindrical Anode, composed of cavities, those are in the axis of a heating Cathode. It should be known that more there are cavities plus the output is high.

The anode and cathode are separated by a space which one calls the space of interaction which is vacuum. These cavities known as “cavity resonators” can have different forms according to the magnetron considered. One finds also two Aimant S which is fixed perpendicularly compared to the axis of the tube.

A continuous electric field is applied between the anode and cathode. This field has a tension about several kilovolt S for a space of interaction of a few millimetres. The electron S released by cathode are accelerated by the continuous electric field. In the absence of magnets, the electrons would go directly on the anode, the combination of the two fields creates a cloud of electrons turning between the anode and cathode. These loads enter in interaction with the cavity resonators of the anodic block which become the support of electromagnetic oscillations. Dimensions of these cavities are calculated so that the waves have a frequency of 2.450 MHz. Part of these waves are forwarded to the guide of wave thanks to various connections.

The guide of wave transmits those in the cavity of the furnace and they will make it possible to cook food.

Power supply

A magnetron of 1000W (puissanse provided) request approximately 2500V with 0,6A in power supply. The diagram running is an assembly " Anode with the masse" , simplifying the insulation. The food is very simple, with a single tranformator of food for the filament and the VHV and a rectification monoalternance with a diode and a condenser of filtering. It should be noted that the parts under high voltage present a mortal danger and that the breakdown service of a microwave under tension with the opening cap is extrémement dangerous.

Power control

The magnetron does not comprise a control electrode and functions only in " all or rien". To vary the power of cooking, the magnetron functions intermittently, the food is ordered in repeating spring at relatively low intervals and with a cyclic report/ratio from ~0 to 100% according to the power required by the user.

For example, for a furnace of 800 Watts, if the control knob is placed on 400 Watts the magnetron will be fed during 7,5 seconds, crossed during 7,5 the second following ones, whereas if one places the button on the maximum capacity, the magnetron is fed permanently. On the mechanical timer apparatuses, it is a simple system of cam with a contact, related to the timer which makes it possible to vary the power, and this in a linear way. On the modern apparatuses, it is a logic with a microcontrollor who orders a relai modulation. this logic can also comprise functions of clock and shifted starting, keys preset for defrosting, etc.

Cooking of food

Action of the microwaves on the water molecule

The Molécule of Eau is made of a Atome of Oxygène and two atoms of Hydrogène (H₂O). It is dipolar, i.e. the Barycentre of the negative charges and that of the positive loads are not confused, that is due to the fact that the oxygen atom is more electronegative than that of hydrogen, and with the bent geometry of the molecule.

The water molecules of one food in a normal state are in the disorder: they do not respect any particular order of orientation. But when they are subjected to a continuous electric field the negative poles of the water molecules tend to be directed in direction of this last.

When they are submitted to the microwaves, the water molecules of food, are directed in direction of the electric field which composes these waves. This field being alternate the poles are directed successively in a direction then in the other, which results from several changes of orientation approximately 2.450.000 000 times in one second at the same rate/rhythm as the wave which oscillates 2.450.000 000 times a second.

If the furnace emitted in lower frequency, it would make as much oscillate the water molecules but there would be no energy absorption of the waves in food and thus of heat emission. Indeed, it is only beyond the frequency of 1 Gigahertz (1 000.000.000 times a second) approximately that the oscillation of water " evil with suivre" has; the oscillation of the electric field of the microwaves. It follows that for frequencies equal or higher than this one, a dephasing appears between the respective orientations of this field and the water molecule. The consequence is what one calls a loss dielectric, generating of heat, and due to a phenomenon that one calls " relaxation" water molecules. It is not thus about any phenomenon of resonance.

The choice of the frequency of the microwaves comes out from a right compromise between warming of food and penetration in this one. Indeed, if a weaker frequency had been chosen, the wave " traverserait" the food without heating it, since the molecules would oscillate freely, allowing a conservation of the electric field in the matter, and thus without causing dielectric loss. On the other hand, if more raised frequency had been chosen, the wave would be completely absorbed on the surface of food, from the impossibility of water of oscillating in phase, and thus the localization of the totality of the dielectric losses on the surface.

Following the heat emission, the temperature is transmitted to the various layers of food by conduction and thus heats part of food. The quantity of water not being distributed in the same way in food does not make that certain parts of food are more or less hot that others.

Moreover when there is heat emission the water molecules tend to pass from the liquid state in a gas state, the volume of vapor thus produced cannot inevitably be contained in food therefore certain food explode.

At the frequency of the furnace microwaves, the water molecule is almost the only one to vibrate, from its small size. But it is sufficient to heat the food from its relative abundance, whatever the food.

Penetration of the waves inside food

The penetration of the waves inside food differs according to this last with knowing its concentration and its composition. When a food is subjected to a radiation of microwaves it tends to reject part of the wave and to store the other. The absorptive part is called calorific energy and it is thanks to it that the food heats. The rejected part is called the considered wave.

To prevent that certain parts of food are burned or the other cold ones it is necessary that the distribution of the waves is the same one at all the places of food. With this intention the walls of the cavity make rebound the waves and a rotary table allows a homogeneous distribution of the waves food.

Metal objects in a microwawe oven

One disadvises putting metal objects in a microwawe oven because of the produced electric field. With the angular and pointed corners of the metal which is an electric conducting , Gradient S of the field are created, which gives place to electric arcs. There exist however special metal objects with corners rounded, which do not cause electric shocks. Packing low height out of steel or aluminum, with a broad opening, allows an effective reheating and without production of arcs in the current furnaces.

The radiation and the Man

The microwave radiation is not ionizing. It is thus much less dangerous than the x-rays or gamma. There are however standards concerning the level of escape of the furnaces, and one should not use a microwawe oven whose door would be damaged.

The maximum level of escape tolerated by the standards of 5mW/cm ² is measured has 5 cm, that is to say the same level as a telephone GSM of 2W.

A Swiss study gives some elements: worn furnace 0,41 mW/cm ² is 10 times less than one portable GSM, for the new furnaces and the majority of the worn furnaces the radiation is lower than that of a Wifi connection.

If the furnace is not damaged, being given the distances and periods utilization the exposure to the electromagnetic waves is negligible compared to the use of a cellphone.

noise of the microwawe ovens

The bodies of a microwawe oven are mainly optimized to lower its cost and to improve ergonomics and esthetics, secondarily to decrease its noise. Apart from the noises of cooking, there are three sources of noises:

• the transformer vibrating at the frequency of the sector, because of the extreme saving made on its technology (not coated rollings up, " losses fer" important).

• the ventilator, which must be enough powerful to evacuate approximately 40% of the power consumption.
• the rotary table, its engine and its bearings.

These noises are possibly amplified by the very simplifée sheet-iron works.

Notes and references of the article

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