Basic knowledge of household microwave ovens Microwave heating principle
Microwave cooking has the advantages of fast speed, high thermal efficiency, cleanliness and so on. In recent years, it has been gradually recognized by people and entered the public life, but because most people lack understanding of it, it is still a fashionable consumer product and is far from being popularized. Now we will do some analysis and introduction on the heating principle, performance characteristics, structure, function and future development of microwave oven.
First, the principle of microwave heating
When the microwave acts on the food, the polar molecules of the heated food oscillate at a very high frequency, and the molecules interact with each other. friction and collision, resulting in frictional heat. Microwave ovens use microwave generators to convert electric field energy into heat energy to achieve the purpose of heating the medium. Since this energy conversion process is carried out at the same time inside and on the surface of the heated object, the inside and outside of the object are heated uniformly at the same time. The stronger the alternating electric field emitted by the microwave generator, the greater the swinging amplitude of the polar molecules of the heated medium, and the higher the frequency, the more frequent the friction and collision between molecules, both of which aggravate the heat generation. In the same electric field, the molecular polarization ability of different media, that is, the ability to absorb microwaves, is different, so the degree of heat generation is also different. Generally, the molecular polarization of water-containing substances is strong, so heating food with microwaves is very effective.
The frequency of the microwave electric field of a domestic microwave oven is 2450 MHz, the direction of the electric field changes 2.45 billion times per minute, and the polar molecules swing 2.45 billion times along the direction of the electric field. The oscillation of the species molecules is interfered and hindered by the intermolecular force, which produces similar frictional heat, which becomes macroscopic microwave heating.
Second, performance and characteristics
Compared with traditional electric stoves and gas stoves, the performance and characteristics of microwave ovens are as follows:
1. Fast heating speed and high thermal efficiency – Since the thermal conductivity of food is usually relatively low, it takes a long time to use the general heating method to make the temperature inside and outside the object become consistent. Using microwave heating can directly heat the inside and outside of the object at the same time. It heats food 4-10 times faster than ordinary electric stoves, and can save 30%-80% of electric energy at the same time.
2. The heating is uniform and the control is convenient – the microwave has strong penetrating ability and can penetrate deep into the interior of the object, so that it is heated evenly, and the situation of outer coke and inner generation will not occur. The magnetron that emits microwaves can generate an alternating electric field and heat immediately after the power is turned on, and stop heating immediately when the power is turned off, so it can be easily controlled.
3. It can maintain the original color, shape and nutrition of food, and also has bactericidal effect.
4. Clean and convenient – in the process of cooking food in the microwave oven, there will be no splash of juice, no smoke and burnt smell of the food, and no increase in the temperature of the kitchen. The food placed in the bowl is directly heated, saving the trouble of transferring the food in the general method.
5. Disadvantages of microwave heating—the surface of food heated by microwave cannot form a golden charred layer, lacking the flavor of barbecue; it is not suitable for cooking thick food, because microwave only It can penetrate the food to a depth of 50-80 mm, that is, the surface absorbs more microwaves, and the more the microwaves are absorbed, the smaller the microwaves are; the microwave radiation in the oven cavity is not very uniform, and the cooking food is heated unevenly; The time required for the food is very short, and if the time is exceeded, the food will be overcooked, so be careful.
Three. Structure
A household microwave oven is mainly composed of a power transformer, a magnetron, a microwave generator, a cooling fan, a metal structure, and an electrical control system. composition.
1. Power transformer – The power transformer of microwave oven generally has three windings: primary winding, filament winding and high-voltage winding, and some have no power adjustment winding. When working, add 220V AC voltage to the primary winding to generate 3.2V or 3.4V voltage on the filament winding to supply the magnetron filament. 1900V or 2000V is generated on the high-voltage winding, and after going through the voltage-doubling rectifier circuit, a negative high-voltage is provided for the cathode of the magnetron.
2. Magnetron – The magnetron of a household microwave oven is actually a vacuum tube with a cylindrical cathode in the tube, which is the heart part of the magnetron to emit electrons, and the cathode is surrounded by an anode. An axial magnetic field is established between the cathode and the anode by means of permanent magnets. When a voltage is applied to the magnetron, after the cathode is preheated, a large number of electrons are generated, and they fly around the circular rolling line to the anode under the action of the formed electric field and the external magnetic field. There are many resonant cavities evenly distributed on the anode. When the electrons hit the anode, they will oscillate in these resonant cavities, and the oscillation frequency will continue to be as high as 2450 MHz, forming high-frequency electromagnetic waves to be emitted.
3. Cooling fan – Since the magnetron needs to generate high heat when it is working, it must be cooled. A miniature motor is used to drive a small blower, blowing along the direction of the parallel fins of the magnetron, cooling the magnetron, and introducing the hot air into the furnace cavity to improve the utilization rate of heat energy.
4. Metal structural parts – mainly including furnace cavity, furnace door, waveguide, etc.
The oven cavity is the place where food is placed for cooking. It is a rectangular resonant cavity. The inner side of the cavity is processed into a microwave reflective surface by stretching and other methods, so that the microwave energy is continuously reflected between the inner walls of the oven cavity. Each reflection propagates through the food. When the microwave is input from the outside, if the frequency is equal to the resonant frequency of the resonant cavity, a violent oscillation will occur in the cavity. There are dehumidification holes on the side or top of the oven cavity.
The oven door is a main barrier for the microwave oven to prevent the micro oven from leaking out. The choke structure is used to ensure electrical sealing even if the oven door is not very tightly mechanically contacted, so that the leakage of the microwave is extremely high. so small that it does not affect the surrounding environment. There is a glass observation window on the furnace door, and there is a layer of extremely fine metal mesh in the glass interlayer, which can not only clearly observe the cooked food, but also play the role of preventing microwave leakage.
Waveguides are mostly rectangular hollow metal tubes, which are components that transmit microwaves to the furnace cavity. In order to reduce the loss of microwave in the transmission process, a layer of metal with high conductivity is often plated on the inner wall. Many microwave ovens produced today dispense with the waveguide, and microwaves enter the cavity directly through the launch port.
5. The electrical system consists of timer, double lock switch, oven door switch, cooking relay, thermal circuit breaker and so on.
Four. Functions
Combining the microwave oven products currently on the market, its functions include:
1. Set the automatic nutrition menu. According to the characteristics of the food, the “Automatic Nutrition Menu” is preset. After selecting a menu, you only need to adjust the amount or weight of the food to automatically complete the nutrition mode grilling.
2. Microwave cooking.
3. Microwave cooking and coloring function. The coloring function is a function of supplementing microwave cooking by heating methods such as radiating elements, quartz tubes, etc., which can make the surface of the cooked food appear golden yellow.
4. Microwave/grill combined cooking function.
5. Clock. Set the 24-hour clock for easy time setting.
6. Memory cooking. Set the user menu memory function, which can memorize the cooking program prepared by the user for future cooking.
7. Deodorant function. Working in the lower microwave section can sterilize and remove odors in the furnace cavity.
8. Delay function. After cooking, the fans and oven lights are turned off with a delay to remove gas and facilitate food removal.
9. Voice prompt. There is a clear and pleasant sound prompt when the key is valid or when the cooking is completed.
10. Pause function. Press the pause button to enter the standby state.
11. Furnace door switch. When the oven door is closed, the cooking work can be carried out. When the oven door is opened during the cooking process, the work stops.
12. Thaw function. The function of automatically defrosting by portion or weight is preset. Just enter the amount or weight value, and the defrosting process will be started and the defrosting work will be completed automatically.
13. Child safety lock. After setting the security lock, it enters the locked state, and any operation is invalid.
V. Development direction of microwave ovens
According to its development and product grade, household microwave ovens can be divided into: the first generation – pure microwave cooking, no traditional The second generation – microwave cooking plus coloring function; the third generation – microwave/grill combination cooking. The fast roasting method of microwave/grill combination cooking not only improves the cooking speed, but also makes the cooked food have the flavor of barbecue, which is more suitable for people’s traditional taste requirements.
Household microwave ovens are divided into control modes, including mechanical control, computer control, and frequency conversion control. The price of mechanically controlled microwave ovens is relatively low, and there are some markets, but they are about to be phased out because they are not suitable for the development trend of consumption; computer microwave ovens are much favored by consumers because of their powerful functions, easy operation, and friendly human-machine interface, and they are also in the current market. Mainstream; frequency conversion microwave ovens use frequency conversion to control the microwave output power, provide a variety of microwave heating power, and enable the microwave oven to uniformly heat food when working in each power section, which overcomes the need for ordinary microwave ovens to work at 100% power. The disadvantage that the rest of the power segments cannot evenly heat the food.
It can be said that the microwave/grill combined cooking microwave oven controlled by frequency conversion will be the ideal target for the development of household microwave ovens.
