As known, the use of microwaves for cooking was invented by Percy Lebaron Spencer in 1945. Already in 1947, the first microwave oven was created which weighed more than 660 pounds. But, in the second half of the 20th century, experts repeatedly discussed the safety of microwave cooking, which, of course, limited the distribution of these devices. Impossibility of using traditional metal utensil was also a limiting factor in that period. But today, these two problems are completely resolved. Companies offer a huge range of special plastic and glass microwave cookware at a reasonable price, and numerous studies have repeatedly proved the complete safety of modern models. Moreover, reliable door screening completely eliminates the penetration of high-energy microwave radiation into the room. However, there is an easy way to independently verify the quality of the door screening. It is enough to put a mobile phone into the chamber, close the door and dial its number. The lack of communication will be convincing proof of the door screening reliability. Nevertheless, knowledge of the microwave oven operation principle can additionally remove suspicions about possible harm from its use. Technical values of modern models vary in a wide range, affecting their functionality, price and, accordingly, the choice of the optimal microwave.

Operation principle

As known, microwave oven uses non-contact heating by converting the electromagnetic energy into heat. Traditional cooking methods suggest heat input to the food surface. Further heat is distributed inside due to heat conduction. The heating food process in a microwave oven differs fundamentally. In this case, heat is generated inside the food using water molecules.

Therefore, the rate of volumetric heating by microwaves is significantly higher. The microwave radiation does not cause chemical changes in food and preserves from 75 to 98 % of the vitamins in food. For comparison, conventional heat treatment ensures the preservation of only 35-60%.

Main components

Microwave contains: - operating chamber with screened door; - high-voltage transformer for power supply of magnetron; - switching circuit and control; - magnetron to generate microwave radiation; - waveguide for transmitting radiation from magnetron to chamber. This scheme demonstrates their traditional placement.

Auxiliary elements: - turntable increases the heating uniformity; - control circuit; - fan for cooling the magnetron and ventilation of chamber.

Operation process

The microwave generator is called magnetron and is the main element of device. Special transformer - stabilizer is the most expensive element.

It provides power to the magnetron. Its nominal operating voltage on high-voltage winding is 2100-2300 V, rated voltage of primary winding is 3-3.2 V. But it's absent in modern inverter microwave ovens, which use a different principle of magnetron power control. High leakage inductance on the high-voltage winding and special design of the magnetic circuit with magnetic shunts is a hallmark of such transformers. They provide stability high voltage. This value is changed by only 1.2 % at fluctuation of supply voltage on 10 %. Individual elements of the magnetic circuit are welded together for ensuring the quiet transformer operation. Accumulating a high-voltage capacitor with a capacitance from 0.8 to 1.2 uF is calculated to operate at a voltage up to 10 kV. A rectangular waveguide is used as a line for transmission of power from a magnetron to the emitter. The emitter provides RF power in the working chamber. Design of the emitter and waveguide provides the matching between a working chamber and magnetron. Emitter excites a wide range of wave types in chamber for ensuring a heating uniformity. Working chamber is a hollow rectangular resonator. The dimensions of the inner walls are much larger than the wavelength. Electromagnetic waves are repeatedly reflected in the camera from its walls and form numerous standing waves of the electromagnetic field with the nodes and tufts.

Food heat treatment

Intensity of the electromagnetic field in chamber is increased to a level of absorption of microwave power by food. Spot food heating is proportional to the square of effective value for electric field strength at a given point. Engineers provides optimal superposition of standing waves for maximum uniformity of the heating food. But, unfortunately, the perfect heating uniformity is impossible due to significant variations of dielectric properties and form of food and cookware. Therefore, this problem is solved by additional methods. These include: - microwave turntable for foods; - superposition of the antenna rotation axis with the axis symmetry of working chamber; - asymmetrical shape of the radiation pattern; - rotating impeller with metal blades, which performs the function of a microwave flow stirrer; - the use of two emitters from branched waveguide.

Microwave door

The door of the working chamber is very important part, because it prevents the spread of the microwave energy in kitchen. Therefore, the door design is quite complicated.

The door has a high-frequency throttle shutter along the entire perimeter that reduces the microwave power to a safe value. Open slit of the throttle shutter is filled with a special plastic that efficiently absorbs microwave energy. The door design provides a very snug fit to plane of front surface of working chamber. Established official requirements permit the gap not more than 0.5 mm. In this case, the energy flux density outside of microwave oven does not exceed the permissible level that is 2.1 mW / cm.

Features

Power control is usually carried out by changing the ratio durations of pauses and periods of generation magnetron. The absence of pauses corresponds to the maximum power. The equality between the pause duration and generation periods corresponds to a power level of 50%, etc. Magnetron operation is controlled through primary winding of the transformer. All microwave ovens have a common feature. They are not designed for using the cookware with metal content due to risk forming the current induction. Induction current is accompanied by a sincere and can cause an arc discharge. Control panel of modern models provide their programming and turns off device at violation of blocking door or at temperature increase of magnetron, transformer or in the working chamber. Controls include electromechanical dials, electronic buttons and touch panel.