Cooking and heating food in the microwave has long ceased to cause a wow effect. But not everyone knows how this kitchen device works. This, in turn, gives rise to many myths and rumors. In this article, we will look at how a microwave oven works and how to use it safely.
Theory of heating in a microwave oven
As is known, polar molecules (otherwise called dipoles) are oriented in space along the magnetic field lines. If such a molecule is placed in an alternating magnetic field, it, having a dipole moment, will begin to rotate, following its magnetic lines. The higher the frequency of direction change by field lines of force, the more often the molecule will change its position.
A striking representative of the dipole is the water molecule, the most common substance on Earth. The application of an alternating magnetic field to water molecules causes them to be in constant motion due to the dipole moment. Due to the friction forces that arise between neighboring molecules, heat is released and, accordingly, the temperature of the material placed in the electromagnetic field rises. Moreover, the faster and more often the direction of the field changes, the faster the internal heating occurs. This behavior of the water molecule is the fundamental principle of microwave cooking .
Any product contains some amount of water, so if you put it under the influence of electromagnetic waves, it is guaranteed to cause it to heat up. Moreover, the product is heated from the inside, and not from the outside, as is the case with traditional cooking methods. Microwave waves penetrate deep into the product by about 2.5-3 cm, and the rest of the heating occurs due to the thermal movement of molecules.
To heat food, they are exposed to electromagnetic waves that change the direction of their electromagnetic fields at a high frequency.
The generally accepted standard for the frequency of the electromagnetic field in microwave ovens is 2.45 GHz.
The main myth about microwave ovens says that the water molecule begins to change its position only at a frequency of 2.45 GHz. Actually it is not. The movement of water molecules is possible in a wider range of frequencies. Moreover, the lower the frequency, the deeper the radio waves penetrate into the thickness of the material. Some industrial designs of microwaves successfully operate at a frequency of 915 MHz.
The frequency of radio waves used is a certain compromise between efficiency, cost and the ubiquitous availability of technology. Firstly, the ability to use a microwave must be everywhere, which means that the frequency of its waves must be in the permitted radio frequency range. Secondly, the final product should not have a high cost in order to be affordable for the majority. Thirdly, the device for cooking should be compact enough and have a small weight.
The device and principle of operation of the microwave
A microwave oven consists of a small number of units and components. It consists of:
high-frequency emitter power supply;
magnetron cooling fan;
microwave control board;
plate rotation mechanism.
The main violin in the operation of the microwave oven is played by the magnetron – a node that generates high-frequency radio waves.
Trying to understand the essence of the magnetron, one wants to exclaim with the words of a famous Internet meme: “Nothing is clear, but very interesting!” In fact, this is so only if you approach the issue, as they say, academically. Simplified operation of the electromagnetic emitter can be described as follows.
Structurally, the magnetron is a vacuum electric lamp, known since the days of grandfather’s TVs and radios. The unit consists of a thick-walled anode, usually made of a copper alloy and having resonator chambers in its design, as well as a cathode with an additional winding made of an alloy of tungsten and thorium. The cathode emits electrons into the vacuum environment of the device. To speed up the process of electron separation from the cathode surface, the winding is heated by applying a small voltage to it. The magnetron bulb from both ends is enclosed in permanent magnets, which create a constant magnetic field inside it.
The occurrence of ultrahigh frequency waves occurs during the interaction of a constant magnetic field perpendicular to each other, formed by permanent magnets, and an alternating electromagnetic field that occurs when a high voltage is applied to the magnetron leads. To power the emitter of microwave waves, a high voltage is used, the value of which is about 4000 V.
In the annular gap between the cathode and the anode, otherwise called the interaction space, a flow of electrons is formed and their circular rotation inside the air gap. During the passage of the electron flow past the cavities of the resonators, the speed of the electrons slows down somewhat. At this moment, energy is taken from the electron beam and microwave waves are formed, which, in turn, are amplified in the resonators and output through a wire loop to the magnetron antenna.
The further path of microwave waves is quite simple. Through the waveguide, they enter the working chamber of the oven and are absorbed by the products placed in it, as a result of which the food is heated.
The magnetron, the waveguide and the working chamber are a closed environment for the propagation of microwave waves, in which they must be absorbed. It is for this reason that the working chamber must contain products that absorb wave energy. If you turn on an empty microwave oven, the waves, reflected from the walls of the chamber, will sooner or later fall back into the waveguide and onto the magnetron antenna, which will cause sparks inside the oven and cause a possible failure of the emitter.
The operation of the magnetron is provided by a powerful transformer that increases the mains voltage up to 2 kV. By means of a pass-through capacitor installed in the secondary winding, the voltage form is converted into a pulsating one, doubled in magnitude. Thus, a voltage of about 4 kV is supplied to the magnetron contacts, which is necessary for the normal operation of the emitter. Additionally, the secondary winding of the transformer has leads for powering the cathode heating filament. The supply voltage of the heating circuit is in the range of 2.5-4.5 V.
During operation, the magnetron generates a large amount of heat, which is removed by the flow of air blown by the fan. Air passes through the radiator fins and then enters the working chamber. Its excess exits through special ventilation holes.
For uniform exposure of microwaves to the entire surface of the products, a rotating plate is installed in the working chamber.
According to the type of magnetron operation control, microwave ovens can be transformer (the device of which is discussed above) or inverter . The difference between the designs of both concepts lies only in the power source of the microwave emitter, but the logic of operation differs significantly.
If in the classical design the power of microwave radiation is regulated by periodically turning on the magnetron at full power, then with inverter control, the emitter remains on all the time, and the power of its radiation is regulated by an inverter.
What does this give the end user? First of all – a large working chamber. The physical dimensions of the inverter are much smaller than the dimensions of the transformer, so the freed up interior space of the furnace can be used to increase the useful volume of the chamber. Second, weight loss. The high-voltage microwave transformer is a fairly powerful device, weighing from 3 to 5 kg, which cannot be compared with the weight of the inverter. Thirdly, improving the taste of ready meals. Since the emitter is switched on constantly, “shock” loads on water molecules are excluded at the moments when the magnetron is turned on at full power. As a result, excessive drying of products by excessive evaporation of moisture is eliminated. This has a positive effect on the final taste of finished dishes. Fourth,
Many sites and ignorant home appliance sellers claim that inverter models do not have a turntable mechanism, and uniform cooking occurs by adjusting the length and intensity of the inverter controlled waves. This is not true! There are simply models in which the magnetron is installed above or below the working chamber. Such a constructive solution makes it possible to win several more tens of cubic centimeters of the useful volume of the furnace.
Safe Use of the Microwave Oven
In order for the kitchen appliance to serve faithfully for many years and not require the intervention of service departments in its “inner world”, a few simple rules should be followed:
1. Do not turn on the oven with an empty chamber, without food. Microwaves are not absorbed by anything, so pops and sparks are guaranteed. In addition, this does not have the best effect on the resource of the magnetron.
2. The surface of the working chamber should be kept clean. Food residues on the walls are not only unaesthetic, but also contain water, so microwaves will be absorbed during operation of the oven. Ultimately, this will lead to damage to the coating of the chamber walls.
3. Before cooking products enclosed in any kind of casing with a hint of tightness (sausages, sausages, etc.), the casing of the product must be pierced in several places. This simple action will create channels for the steam to escape, which will inevitably form inside the product during cooking. This will allow the product to retain its original appearance and remain on the plate, and not “decorate” the chamber walls with its parts.
It is highly recommended not to cook eggs in a microwave oven. Due to the increase in pressure inside the shell, an explosion is guaranteed. If you really want to boil an egg with microwaves, then you need to at least pierce the shell or cut off the top. And it is best to use a special microwave egg cooker.
4. Do not use the microwave oven with a defective door lock or intentionally turn it off. The walls of the chamber and the shielded glass of the door reliably protect against the penetration of microwaves outside the working space of the oven. After all, we remember that exposure to microwave radiation causes heating of tissues, so you can get deep burns. How not to use a magnetron can be seen in this short video.
Attention! Such experiments are dangerous to health!
5. It is not necessary to place metal objects and utensils with a metallized border into the working chamber. The fact is that although microwaves are not absorbed by the metal, they lead to the appearance of large eddy currents in it. It is not a good idea to create a kind of welding machine inside the microwave and ruin the internal surfaces of the oven.
6. It must always be remembered that the microwave emitter is powered by a voltage of about 4 kV. It is not necessary to carry out self-repair. The risk of electrical injury is very high!
Additional equipment for the microwave
In modern microwave ovens, food is cooked not only using microwave radiation. Almost all models are equipped with a grill, and more “advanced” models – with a convection mode .
There are two types of heating elements that are used to create a ruddy crust on the finished dish or full cooking in this mode. It can be a conventional heating element or a quartz emitter.
Cooking comes from exposure to thermal (or infrared) radiation generated by the heating element. Heaters have low power, so cooking only with them is rather doubtful and takes a lot of time. But as an aid to microwaves, it is quite a working option. Culinary masterpieces cooked in the combined Microwave + Grill mode are obtained with a well-baked crust on the outside and juicy inside.
For the “Convection” mode , a fan is additionally installed in the chamber of the microwave oven, which provides forced ventilation of hot air. In this mode, the dish is guaranteed to be baked on all sides.