How Induction Cooker Works: a Guide to Induction Cooking

31 May How Induction Cooker Works: a Guide to Induction Cooking

Induction cooker is a new type of kitchen cooker for its high efficiency, fast heating, and environment friendly, thus, understanding how induction cooker works is vital to the user. The ANSYS finite element simulation was used in this work to study the effects of the number, relative permeability, and thickness of magnetic strip as well as the gap between pot and stripe on the magnetic flux density distribution in the pot. The optimized parameters of the magnetic circuit design for high efficiency induction cooker is achieved. According to the simulation results, by adding magnetic stripes, increasing the strip’s relative permeability, and increasing the thickness of magnetic strips, the magnetic flux density in the pot can be improved. Meanwhile, the magnetic flux density decreases as the gap between pot and strips increases.

Introduction

Compared with traditional heating stoves, household induction cookers have high energy efficiency, fast heating, clean energy, easy to use, simple to operate, and easy to adjust. To understand how induction cooker works, this article indicates the effect of structure and material parameters on the magnetic flux density in the pot bottom of induction cooker. The operation process is divided into four steps: (1) The circuit control system connects the household 220V, 50Hz alternating current is converted into high-frequency alternating current; (2) high-frequency alternating current is the surrounding space that induces a high-frequency alternating magnetic field of the same frequency; (3) high-frequency, the alternating magnetic field acts on the bottom of the metal cookware, generating eddy currents that make the cookware heat; (4) heat is conducted from the pot to the food in the pot. Among them, high frequency is the generation of the alternating magnetic field, is a key step in the operation of the induction cooker which very important when conducting an energy efficiency analysis of an induction cooker. According to the principle of electromagnetism, the high-frequency alternating magnetic field is concentrated on the bottom of the pot to generate eddy currents at the bottom of the pot which is the key to generating heat. Therefore, high-frequency alternating magnetic fields is generated, and optimizes the magnetic circuit design. Controlling and optimizing the distribution of magnetic field lines in lifting electromagnetic are extremely important in heating with energy efficiency for there is a lot of energy consumption in this process. Complete reading this article to have a deep understanding of how induction cooker works.

Types: heating of the coil, eddy current heating effect of the magnetic shielding block, electromagnetic energy taken away by the divergent magnetic field lines (not passing through the bottom of the pot), etc. The heating loss of the coil is unavoidable, and only thicker copper wire can be used to reduce the coil resistance and thus reduce the loss. Therefore, it is very important to make the high-frequency alternating magnetic field act on the cookware in a concentrated manner, and the magnetic strip is the key to converging the magnetic field lines, which should be paid attention to. In understanding how induction cooker works, ANSYS finite element simulation is used to optimize the magnetic circuit design of the high-efficiency induction cooker from the perspectives of the magnetic strip, the pot, and the distance between the magnetic strip and the pot.

Magnetic circuit structure and model

Figure 1 shows the principle of how induction cooker works as induction heating. According to Ampere’s law, the alternating current in the coil generates an alternating magnetic field around it, and the soft magnetic strip under the coil gathers the surrounding magnetic lines of force and guides the magnetic lines of force into the pot; according to Faraday’s law of electromagnetic induction, the alternating current entering the pot and the changing magnetic field induces eddy currents in the pot, and the heat generated by the eddy currents is transferred to the food to be heated. The magnetic strips play the role of converging and guiding the magnetic lines of force during the working process of the induction cooker. Under the condition that the current size and frequency in the coil are constant, more magnetic lines of force entering the cooker means stronger eddy currents can be generated in the cooker. Using ANSYS finite element method the analysis software can intuitively study the influence of the size and distribution of the magnetic strips on the magnetic field distribution in the final cookware.

Finite element analysis can be divided into three parts: pre-processing, analytical solution and post- processing. The pre-processing includes defining the analysis unit type, establishing the solid model, and discretizing the solid model through mesh division, so as to transform the practical problem into a finite element model that can be numerically calculated. In the pre-processing, the geometry, material properties, loads, constraints, etc. of the analysis object are defined to prepare for the next solution. The quality of the finite element mesh will directly affect the accuracy of the solution, so the solid model must be meshed reasonably according to the actual situation. The first step of finite element analysis is to establish a suitable model. This article is based on a certain induction cooker from an induction cooker manufacturer. The modelling process is shown in Figure 2. Figure 2a is the magnetic strip model of the induction cooker , 15 mm wide , 10 mm high, 4 mm thick , two adjacent long magnetic strips with an included angle of 60° and six magnetic strips 50 mm long and 50 mm wide.

It is composed of short magnetic strips with a height of 15 mm, a height of 10 mm and a thickness of 4 mm. Figure 1b shows the model after adding a coil disk based on the magnetic stripe model. The coil disk consists of two concentric cylinders, wherein a large coil disk with an outer diameter of 85 mm and a thickness of 5 mm is located on the outside. The upper surface of the coil model is on the same plane as the upper surface of the magnetic strip, and there is a 1 mm gap between the lower surface and the magnetic strip. Figure 1c shows the model after adding the pot. The outer diameter of the pot is 100 mm, the height is 50 mm, and the thickness is 2 mm. The distance between the lower surface of the pot and the upper surface of the coil is 5 mm. Figure 1d shows the model of the surrounding air when the induction cooker is working. It is a cylinder with a radius of 150 mm and a height of 100 mm. The finite element model is generated after meshing the solid model with ANSYS. The analysis and solution stage is to numerically calculate the model through reasonable finite element equations. This step can be automatically realized in the commonly used finite element analysis software, which greatly reduces the workload. In the process of solving, it is necessary to calculate the finite element equation that meets the conditions, analyze the finite element equation according to the boundary conditions satisfied by the equation to obtain the characteristic equation, and solve the final characteristic equation and output the data result, etc. Post-processing is the viewing, analysis and manipulation of the results obtained from the solution, including the relevant analysis and processing of the data output from the solution process, and outputting the results according to the needs in order to analyze the structural design and performance impact of the model.

By analyzing and evaluating the model, the entity model can be optimized and improved accordingly. For electromagnetic field problems encountered in practical engineering, such as linear induction motors that use eddy current effects to generate force and heat, and design of induction heating equipment, it is usually necessary to analyze the distribution of eddy currents in heat-generating components. Use the symmetry of the model to simplify it.

Results and Discussion

In understanding how induction cooker works, it is important to further discuss the result. Generally speaking, the heat generated by the eddy current is determined by the strength of the eddy current, and the strength of the eddy current is determined by the frequency and size of the alternating magnetic field. When the frequency is constant, the greater the magnetic flux inside the pot, the stronger the eddy current is. Therefore, the more magnetic lines of force passing through the bottom of the cookware, the greater the eddy current and the more heat is generated. Starting from a simple static magnetic field, we can analyze the magnetic flux density distribution at the bottom of the cookware under different parameters of the model, so as to infer the high electromagnetic field. This requires heating energy efficiency.

Influence of the number of magnetic strips on the magnetic flux density at the

bottom of the pot

In order to comprehend how induction cooker works, this part discusses the Influence of the number of magnetic strips on the magnetic flux density at the bottom of the pot. Magnetic strips are usually added under the coil of the induction cooker to converge the magnetic lines of force. The model in Figure 2 is solved to obtain the magnetic flux density distribution at the bottom of the cooker. Comparing the solution results of the model after removing 6 small U-shaped magnetic strips, the solution results of the two models are shown in Figure 3. As shown, it can be found that the magnetic flux distribution at the bottom of the 12 magnetic strip model cookware is relatively uniform, and there are two areas with high magnetic flux density; while the magnetic flux density distribution at the bottom of the 6 magnetic strip model cookware is uneven, and the center of the cookware is close to Magnetic flux at the port position in the U -shaped magnetic strip.

Therefore, the farther away from this position, the smaller the magnetic flux density, the same distance from the center of the pan, the closer to the magnetic strip port, the lower the magnetic flux density, the bigger it is.

Whether it is a 12 -stripe model or a 6 -stripe model, it can be seen from the magnetic flux density distribution at the bottom of the cookware that the magnetic flux density around the cookware is relatively small, indicating that the magnetic field lines around the cookware have serious divergence and should be improved.

Influence of the magnetic permeability of the magnetic strip on the magnetic flux density at the bottom of the cookware

When understanding how induction cooker works and studying the influence of the magnetic permeability of the magnetic strip on the magnetic flux density at the bottom of the cookware, two cases are considered. Changes in density, figure 4 shows the magnetic flux density distribution at the bottom of the pot when the magnetic permeability of the pot is 3000 and the magnetic permeability of the magnetic strip is 500, 1000 and 2000 respectively. It can be found that the magnetic permeability of the magnetic strip is different, the size and distribution of the magnetic flux at the bottom of the pot change little, the magnetic flux density is the largest at the center of the pot bottom near the port of the magnetic strip, and the magnetic flux distribution at the entire bottom of the pot is very uneven. This shows that when the magnetic permeability of the pot is much greater than that of the magnetic strip, the magnetic strip can hardly control the magnetic flux density distribution at the bottom of the pot. At this time, the function of the magnetic strip can only be to prevent the magnetic field lines under the heating coil from divergent

There is another situation, which is more realistic, that is, the magnetic permeability of the magnetic strip is greater than the magnetic permeability of the cookware. Assuming that the magnetic permeability of the cookware is 500, change the magnetic permeability of the magnetic strip and analyze the magnetic flux density at the bottom of the cookware.

Figure 5 shows the magnetic flux density distribution at the bottom of the pot when the magnetic permeability of the magnetic strip is 1000, 2000, 3000 and 5000 respectively. It can be found that as the magnetic permeability of the magnetic strip increases, the magnetic flux at the bottom of the pot becomes more concentrated, and the overall magnetic flux at the bottom of the pot also becomes larger. This shows that when the magnetic permeability of the pot is smaller than the magnetic permeability of the magnetic strip, the magnetic flux density at the bottom of the pot is greatly affected by the magnetic strip, and the magnetic flux density distribution at the bottom of the pot can be adjusted by changing the magnetic permeability of the magnetic strip. Increasing the magnetic permeability of the magnetic strip within a certain range can increase the magnetic flux density at the bottom of the pot, but if the magnetic permeability of the magnetic strip is too large, it is not conducive to the uniform distribution of the magnetic flux at the bottom of the pot, and it is also not conducive to the uniform distribution of the eddy current heat at the bottom of the pot.

Influence of the thickness of the magnetic strip on the magnetic flux density at the

bottom of the cookware

Influence of magnetic flux density at the bottom of the cookware by the magnetic permeability of the magnetic strip simulation, when it is found that when the magnetic permeability of the magnetic strip is greater than the magnetic permeability of the pot material, the properties of the magnetic strip have a great effect on the magnetic flux distribution at the bottom of the pot. We speculate that the size of the magnetic strip should also have a great influence on the magnetic flux at the bottom of the pot. Figure 6 shows the change of the magnetic flux density at the bottom of the pan when the thickness of the magnetic strips changes. It can be found that the thickness of the magnetic strip has a great influence on the magnetic flux at the bottom of the pot. As the thickness of the magnetic strip increases, the magnetic flux density at the bottom of the pot increases, and the increase in the thickness of the magnetic strip does not make the magnetic flux distribution at the bottom of the pot uneven. It shows that increasing the thickness of the magnetic strip is beneficial to increase the magnetic flux at the bottom of the pot, thereby enhancing the eddy current at the bottom of the pot.

Influence of the distance between the magnetic strip and the pot on the

magnetic flux density at the bottom of the pot

Fig. 7 shows the change of the magnetic flux density at the bottom of the cookware when the magnetic permeability of the magnetic strip is smaller than that of the material of the cookware and the distance between the magnetic strip and the cookware changes. It can be clearly found that as the distance between the magnetic strip and the pot increases, the magnetic flux at the bottom of the pot decreases significantly, indicating that the smaller the distance between the magnetic strip and the pot, the greater the magnetic flux at the bottom of the pot, and the greater the eddy current heat at the bottom of the pot. However, in the actual induction cooker, since a thermal insulation panel needs to be placed between the magnetic strip and the pot, the thicker the hot panel, the more heat the pan will lose towards the insulated panel. Therefore, in practice, it is necessary to reduce the thickness of the thermal insulation panel as much as possible under the premise of considering the thermal insulation effect of the panel.

Longitudinal magnetic flux density at the bottom of the cookware

Figure 8 is a model with 12 U-shaped magnetic strips. The magnetic flux density distribution on the upper surface of the bottom of the cookware, the bottom of the cookware is 0, and the upward direction is the positive direction. Figure 7 shows the magnetic flux density distribution of different sections in the cookware. It can be clearly found that from the bottom surface of the cookware to the pot on the upper surface of the bottom of the pot, the magnetic flux density gradually decreased, and the magnitude of the decrease of the magnetic flux density at different positions of the bottom of the pot was different, the middle part was larger, and the surrounding area was smaller. According to the magnetic flux density of the interface at different heights at the bottom of the pot, it can be known that the eddy current heating on the lower surface of the pot bottom will be greater than the eddy current heating on the upper surface of the pot bottom. This shows that more eddy current heat comes from the lower surface of the cooker. Therefore, in order to improve the energy efficiency of the induction cooker, it is necessary to transfer the eddy current heat from the lower surface of the cooker to the upper side as much as possible, square delivery, high-quality insulation panels are essential, and the material of the bottom of the pot should have excellent thermal conductivity.

Conclusion

As part of understanding how induction cooker works, this article mainly uses the ANSYS electromagnetic module, starting from the induction cooker heating system model, to simulate the electromagnetic heating system of the induction cooker magnetic field distribution, focusing on the distribution, properties and material properties of the magnetic strips of the induction cooker, and put forward some conditions for optimizing the energy efficiency of the induction cooker.

• The number of magnetic strips has a great influence. The more magnetic strips, the greater the magnetic flux density at the bottom of the pot, and the magnetic flux density are divided, eddy current heat, but the increase in the magnetic strip will increase the losses in the magnetic strip and increase the cost, therefore, the loss of the magnetic stripe and the overall cost should be considered comprehensively, and a reasonable increase should be made to increase the number of magnetic strips to improve heating energy efficiency.
• When the magnetic permeability of the pot material is greater than the magnetic permeability of the magnetic strip material, the magnetic permeability of the magnetic strip has a limited effect on the magnetic flux at the bottom of the cookware; when the magnetic permeability of the material is less than the magnetic permeability of the magnetic strip material, the magnetic permeability of the magnetic strip increases. It can increase the magnetic flux at the bottom of the pot, thereby increasing the eddy current heat at the bottom of the pot, thus conducive to improving energy efficiency.
• The closer the pot is to the magnetic strip, the greater the magnetic flux density at the bottom of the pot. However, in order to reduce the vertical downward dispersion of the eddy current heat at the bottom of the pot, a certain amount of the thickness of the thermal insulation panel, considering the magnetic flux at the bottom of the pot and the thermal insulation of the panel, a reasonable spacing between the magnetic strips of the pot is required.
• When the thickness of the magnetic strip changes, the magnetic flux density distribution at the bottom of the cookware changes, the thicker the magnetic strip, the greater the magnetic flux density at the bottom of the pot, and the eddy current heat is. The larger the value is, the thicker the magnetic strip will increase the loss of the magnetic strip in the actual situation, so the thickness of the magnetic strip should be selected reasonably to improve the energy efficiency of electromagnetic heating. Consequently, comprehending this article is important in understanding how induction cooker works.