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What is a magnetizer?

The magnetizer known also by the magnetic flux concentrator, is used for magnetic control by intensifying the magnetic flux in certain areas of a workpiece. The use of magnetizers is well known in induction heating applications, they are acting as an electromagnetic shield to avoid the undesirable heating of adjacent zones. Their effect is similar to the core function in transformers. They are made by low-power-loss materials with high permeability.

Figure 1 - The effect of magnetizer on the magnetic flux field distribution around the electric conductor [1]

Problem description

To improve the heating performance of a spot inductor, the proposed model uses a U-shaped coil coupled with a Ferrite magnetizer. It is working on increasing the heating rate of the workpiece and improving its temperature uniformity. The CAD design of the whole model is shown in Figure 2.

In this study, a FEM analysis using EMS tool was performed to evaluate the effect of the magnetizer on the induction heating process. The AC Magnetic module of EMS was used with a coupled to the transient thermal analysis. Electromagnetic and temperature distribution were investigated.

Figure 2 - a)3D design of the studied model b)coil and c)the magnetizer

Table 1 - Components' dimensions [1]

Simulation setup

EMS allows for a multi-physical simulation by coupling between magnetic and thermal field; hence it enables the modeling of Induction heating process. The following steps are needed for the simulation setup.

1.Select the appropriate materials

The simulated model is composed of a copper coil, a ferrite magnetizer and a workpiece made of carbon steel. The needed properties are summarized in Table 2.

Part	Material	Density (Kg/)	Magnetic permeability	Electrical conductivity (S/m)	Thermal conductivity (W/m.K)	Specific heat capacity (J/Kg.K)
Workpiece	Carbon Steel (AISI1045)	7870	(T): Figure 3.1	(T) :Figure 3.2	K (T): Figure 3.3	(T): Figure 3.4
Coil	Copper (Cu)	8900	0.99	5.7 E+07	385	390
Magnetizer	Ferrite (Ni Zn)	4900	Initial permeability: 1500 Relative permeability: 1.19 E+08	0	5 E-06	750

The selected material for the workpiece part has a temperature depending properties. They are defined by the figures below.

Figure 3.1 - Electrical conductivity of AISI 1045 Steel

Figure 3.2 - Magnetic permeability of AISI 1045 Steel




Figure 3.3 -Thermal conductivity of AISI 1045 Steel 

Figure 3.4 - Specific heat of AISI 1045 Steel

2.Electromagnetic Inputs

3.Thermal Inputs

The workpiece has an initial temperature of 20°C. A thermal convection is applied on the air body at ambient temperature of 25°C with a coefficient set to 10 W/m²K.

4.Meshing

For each FEM simulation, the results accuracy and the solving time are strongly dependent on the mesh quality. EMS enables to control the mesh size on each region of the model through a Mesh control feature.

In our example, a fine mesh size was applied to the coil and to the top surface of the workpiece. Figure 4 shows the whole meshed model.

Figure 4 - Meshed model

Results

After 7s of induction heating, the simulation revealed the results below. The next figure shows the vector plot of the magnetic flux distribution inside the magnetizer at the end of the heating time. 

The high permeability of the magnetizer's material makes the magnetic flux concentrated around the electric conductor through the core (of the magnetizer). Thus, by preventing the major amount of the field from propagating outside the core, the heated area was subject to an intensified magnetic field. Through this process, the high temperature was restricted to a small area, which means less power will be needed to accomplish the required heating treatment of the workpiece.

Figure 5 - Vector plot of the magnetic flux density after 7s

The figure 6 shows the vector plot of the eddy current distribution in the coil and the workpiece. It achieves a maximum value of 1.19E+08 A/m² for the workpiece top surface and 1.02E+09 A/m²  for the coil. Consequently, highly heated zones are located in the center of workpiece surface. The obtained results are in good agreement with the Reference [1].

Figure 6 - Eddy current density after 7s across a)the workpiece  and b)the coil

The highly heated zone reached a maximum temperature of 850 °C at the center, which confirms the required level of heating. The figure 7 represents the temperature distribution across the top surface of the workpiece.

Figure 7 - Temperature distribution across the workpiece

Conclusion

The high permeability material of the magnetizer provided a selective heating zones through cencentrated magnetic flux arround the subject areas .It allowed to reache high temperature values for lower power consumption.

References