Unofficial
FEMM
documentation
by Dr Stan Zurek

This tutorial shows a very simple problem for HEAT FLOW coupled with MAGNETIC simulation.

Open FEMM and start new problem, menu > File > New .

Select Magnetics problem from the pop-up window, and set up magnetic problem as shown in Fig. 1 (example file can be downloaded at the bottom of this page). In this example there is single copper conductor next to a single magnetic block, both surrounded by a layer of non-magnetic (lossless) resin, and all of the geometry is placed in air.

Fig. 1. Magnetic problem

Once the magnetic problem is solved it is necessary to extract the power loss data from the regions of interest. Fig. 2 shows the sequence in which a given region can be selected, and the integral for Total Loss Density in W/m^3 can be extracted. This value will be used for the corresponding region in the Heat/Thermal model.

The values of loss density should be extracted for each region which contributes to power loss.

But all the lossless regions (such as air) will have zero loss by definition, so there is no need to process them.

In this example there are just two regions with loss:

• Copper bar = 706971 Watts/meter^3
• Magnetic bar = 77860.5 Watts/meter^3

Fig. 2. Use magnetic solution

Open FEMM and start new problem, menu > File > New .

Select Heat flow problem from the pop-up window, and set up the geometry as shown in Fig. 3 (example file can be downloaded at the bottom of this page).

NOTE: The geometry in the heat flow solver should correspond exactly to the geometry from the magnetic solver. To help with this process it is best to export the DXF file from the magnetic geometry by menu > File > Export DXF, and then importing this file into the heat solver by menu > File > Import DXF.

In the heat flow problem it is not necessary to create the “air box” so this can be removed, and the appropriate thermal boundary conditions can be assigned to all the edges of the geometry, as shown in Fig. 3 and as defined in Fig. 4.

Fig. 3. Heat flow geometry

Fig. 4. Heat flow boundary conditions

The heat sources can be assigned in several ways, also by forcing heat through boundary conditions. However, in this case we can use the values of Loss Density from the magnetic solver (as shown above in Fig. 2) and assigned them as Volume Heat Generation as shown in Fig. 5. If the value is set to zero in this field then it means that there is no heat produced within that material. Obviously also the thermal conductivity has to be set up correctly.

The Volumetric Heat Capacity is only important for transient problems, but it is a good practice to set it to correct values too.

Fig. 5. Heat sources in materials