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About induction heating

Induction heating is a process of heating an electrical conductor (usually ferromagnetic materials and metals) by electromagnetic induction. The heat generated in the conductor is due to the eddy currents induced in the conductor. In the simplest form, an induction heater consists of a coil through which a high frequency AC current is passed. This high frequency AC current causes rapidly alternating magnetic field which then causes eddy currents in the conductor. The eddy currents are responsible for heating the conductor, the higher the resistance to the flow of current, the more the heating. This phenomenon of eddy currents heating the conductor is called Joule effect.

Figure 1- A conducting rod is heated using induction
 

One needs to contrast between conduction and induction. In the case of induction heating, the heat is generated inside the object and the object does not need to be in contact with the heat source. Hence induction facilitates rapid heating. There are many applications where induction is used such as induction furnace, induction welding, induction cooking appliances etc. The rest of this article is about induction cooking application.

What is induction cooking?

In the case of induction cooker, a cooking vessel usually made of a ferromagnetic material is heated by induction. Contrast this to the same vessel heated by flame or an electric coil. Induction heating brings about a rapid increase in temperature of the vessel. As shown in figure 2, a coil of copper is placed under the vessel. There is also a layer of ceramic between the coil and the vessel. This is commonly referred to as top plate.

Figure 2 - Elements of induction cooking appliance

When high frequency AC current is passed through the copper coil, large eddy currents are induced in the vessel. The surface resistance of the vessel heats it rapidly which enables cooking. Now there are choices of the material used for the cooking vessel but it is highly recommended that the vessel be made of a ferromagnetic material like cast iron or some specific grades of stainless steels. It is not recommended to use Aluminum or Copper vessels (you can use Aluminum or Copper with modification to the cooking appliance by including a ferromagnetic disk which functions as a hot plate). The use of ferromagnetic material has 2 advantages –

1. The electrical resistance is higher than pure conductors and hence the heat produced is more.
2. The skin depth (more about this in a later blog post) of ferromagnetic material is lower than pure conductors and hence there is more surface resistance resulting in higher joule heating.

Why induction based cooking is attractive?

1. It is energy efficient. It provides faster and more consistent heating with higher thermal efficiency. According to a technical document from S. Department of Energy (DOE) in 2001, the efficiency of energy transfer for an induction cooker is 84%, versus 74% for a smooth-top non-induction electrical unit.
2. The heating performance is uniform and compares to a gas burner.
3. A control system usually shuts down the heating element if the cooking vessel is not present or is not large enough.
4. They are easy to clean and maintain because the cooking surface is flat and doesn’t get too hot to burn and stick spilled food. Figure 3 shows that heat is produced only in the vessel.

Figure 3 - Heat is produced only in the vessel and not in the top plate

Simulation using a standard induction cooker coil arrangement

Figure 4 shows a CAD model of a coil and iron core arrangement which can be used for induction cooking. A simulation was performed using EMS for SolidWorks using AC excitation at 24 KHz. The inductance of the coil was computed and the magnetic flux density was visualized.

Figure 4 - CAD model of a typical coil using for induction cooking
 

The inductance value calculated by the software was 92.67 micro Henry and compared very well with the laboratory measurement result (93.8 micro Henry). Figure 5 shows the plot of magnetic flux density in the coil and the iron cores.

Figure 5 - The plot of magnetic flux density in the coil and the iron cores
 

Conclusion

It is engineers who gave chefs a perfect solution to an energy efficient cooking appliance. EMS for SolidWorks can help engineers design and simulate various types of induction coil arrangements for cooking application. As it is completely embedded inside SolidWorks, EMS can directly simulate SolidWorks designs thereby avoiding loss of CAD data due to translation.