FEMM: CALCULATION OF INDUCTION AND FORCE OF A MAGNET
I) What is FEMM?
FEMM is a free software that allows you to create magnetic circuits and simulate field lines using the finite element method. FEMM allows you to visualize the induction running through your system, and to quantify it in Gauss or Tesla. The source can be a soft or hard ferrite permanent magnet , N38 to N52 neodymium, or less common grades such as AlniCo and Somarium-Cobalt. The source of magnetic induction can also be an electromagnet with copper, aluminum coil , etc. The environment around the induction source will be air or a steel circuit. Here are a few lines, but the options offered by FEMM are much more numerous: Calculation of heat dissipation by convection, alternating frequency electromagnets, etc. Our goal here is not to write a 200-page manual...
For example, you want to calculate the force of a magnet on a mass of steel , or visualize the lines of the magnetic field generated by your electromagnet ? You can invest a few tens of thousands of euros in a latest generation 3D simulation software or else... download the FEMM software for free here
II) What forms of magnetic circuit with FEMM?
Any shape! If you really have a wacky 3D shape, just simplify it and pull out a section or cut. FEMM works in 2D and not in 3D, but it is possible to give thickness to your magnetic system. For example, you draw and configure the section of a magnetic plate and then define its depth. For cylindrical circuits, you can simulate a revolution of your half-section along an axis.
III) How to draw its magnetic circuit on FEMM?
There are two ways:
1) Import Dxf Geometry...
...from Autocad or your design software. Think carefully before importing, to remove all the superfluous such as texts, dimensions, hatching, and check that there are no superimposed lines. Importing a section is the easiest and fastest way.
2) Create geometry directly in FEMM...
You are back in 1995! The first time we have to draw a geometry, we are lost. To avoid it, first draw your section on a sheet . Take the lower left point as a reference. In a Cartesian coordinate system, this point will have coordinates X=0 and Y=0. Now, like when you were in 9th grade, mark each point of your geometry with (X;Y) coordinates . It is done ? Cheer.
In FEMM , click on the dot icon and then press the TAB key on your keyboard. A window appears and you enter the coordinates of your new point . Repeat the operation for all your other points. The worst is now behind you!
Click the Line icon , then connect the dots . The section of your magnetic circuit is finished!
Solidworks, Inventor or Catia suddenly appear to you as ergonomic and powerful software! Even Autocad suddenly seems easy to use.
IV) Define the parameters of the problem.
1) The environment and its limits
You previously defined your section. Don't forget to also draw a large rectangle around your geometry. Always use the dots and lines method. This large rectangle symbolizes the environment or limits in which the calculations will be carried out and the field lines will be represented. To summarize, you cannot ask for an infinite calculation, you must give the limits. When this rectangle is drawn, it must be indicated that it is the limit and that it is filled with air. For the material, we will see below, but to define a boundary, select the perimeter and click on Properties then Boundary. Below is an example on a cylindrical circuit. Add with Add Property then in the "Boundary Property" window, set everything to 0.
2) Basic assumptions of the problem
When you click on Problem, a very important window opens. You will specify the hypotheses of the problem to be solved:
- plane (with a thickness) or an axis of symmetry (revolution)
- unit: millimeter
- Frequency: 0 Hz if using a permanent magnet or direct current
- Thickness: To be defined according to your needs. This is the "thickness" of your section.
- the other parameters can remain by default.
V) What materials are available?
There is everything: permanent magnets, conductive and non-conductive materials, several types of steel, stainless steel, copper, and aluminum, etc. There is also Air which is essential.
Visit the material library in Properties / Material Library.
On the left the available materials. On the right the materials to be used in our simulation. Take the necessary materials by dragging and dropping from left to right . It's easy !
Then return to the drawing. 
To position each material in each part: first click on the "green circles" icon , then left click to place the material. Then right-click near the newly created "none" point , then Space key to open the properties of this material block. Choose the material of this part, and specify the direction of magnetization if it is a permanent magnet. In the case of a wire coil, specify the direction of the magnetization but also the number of turns of this solenoid. Also specify which circuit this coil is part of. We will not go into this detail, the information is available in the Help.
What if my material is not in the library? You add in the right column a similar material, you edit its properties and you rename it.
VI) Creation of the mesh.
At this point you already have:
- imported or drawn your system (without making duplicate points or lines)
- defined the materials (without forgetting the air)
- set the boundary (Rectangle or Radius)
- indicated the properties of the problem (Plan, mm, depth, ...)
You can now click on this yellow icon whose shape is difficult to perceive... :-)
The processor calculates and creates the mesh. This usually takes less than 30 seconds unless you have defined a fine mesh in the properties of the material block. (Green Point). Indeed, you can do it to be more precise on thin parts. In this case, the calculation time will be longer...even very long.
That's it, the mesh is visible! Start the magnetic calculation by turning the crank and the gear. I'm talking about the icon...
VII) Calculation and visualization of the magnetic flux.
When the calculation is finished, a new tab appears at the bottom. The one on the left is your system with its mesh, and the one on the right is...the result of your hard work. Cheer !
To make the result more visible , 3 tips:
- Adjust the number of field lines to your needs. To do this, simply click on the " Black hatched square " icon. A dialog box opens and increase or decrease the "number of contours" as you wish.
- The following icon, which looks like a rainbow , will color your vision of the flow and the areas where its density is important . Show density plot! Also adjust the Low point and High point values of your flow density scale. From 0 to 1.6 Tesla for example. By keeping the same scale on your different calculations, this allows you to compare the configurations of your systems.
- Add the vectors to visualize the importance and direction of the fluxes traversing your magnet or electromagnet solution. Simply click on the Black Arrow icon.
You now have graphical support to interpret:
- the converging, or useful, field lines. Is this the result I was hoping for?
- the diverging field lines, which are leaks . Are these leaks insignificant or do they represent an unacceptably large loss?
- unnecessary geometries in which the flux is very low. I reduce the size of my part, I chamfer it, I decrease its thickness, I delete a part, etc.
- overstressed geometries which are magnetically saturated . I change their shape, I increase the thickness or why not, I decrease the shade of my magnet or the intensity of my coil.
- Directions and intensity of flows at a given location .
It's a lot of work to interpret, correct your hypotheses, compare the solutions . Prefer a "Save As" to create a copy of your system and then modify it.
VIII) Calculate the force of attraction in Newtons.
You have noticed that our example is composed of a thin sheet located at a distance from our magnets. The air gap is important, and you want to know the force of attraction or lifting? Simply click on the inner area of the sheet after activating the " Green circle " icon. The "S" integral icon offers you a large number of available calculations. Have fun. Here we will only take here Force .
The result corresponds to the forces in Newtons along X and Y. Important detail, if you have previously indicated a depth to your Plan problem, the result takes this into account. It's proportional.
Congratulations, you have your result! You know the force that your system deploys! So, the capacity of your magnet is oversized or did you come close to disaster?
IX) Some additional functions
1) Make a section and analyze the magnetic flux density.
To analyze the flux densities in the different parts and get a nice curve out of it in a marker (X:Y), you have to go back to your geometry tab and create 2 additional points . These points will be the ends of the segment of your cut. Restart the calculations , otherwise these points will not be visible on the result tab. Next time you will create these points along with your magnetic system geometry. We call it experience!
Back on your flow visualization screen, connect the 2 points using the Segment function . The Curve icon opens a menu in which you are free to choose what you want to visualize . Start with... Bn or B to visualize the evolution of the density as we go through the section
.
2) Retrieve information from an electromagnetic circuit.
If you have created an electromagnetic system, equipped for example with a copper coil delivering I amps in N turns, you can know the electrical information such as the total current, the voltage, the flux leaks, the power. To do this, use the "turns" icon located just after the integrals icon. Our example was based on permanent magnets but it is easy to replace this ferrite magnet with a mass of copper for which you enter the characteristics of your coil (IxN). Useful little tip: Half of your coil will be +NI and the other half will be -NI. Dig, you will get there and then you can recover the data from the electromagnetic circuit.
3) Heat flow and heat dissipation
If you are familiar with magnetic simulations relating to an electromagnet, it will be quite easy for you to tackle heat fluxes. Moreover, it is quite complementary: an electromagnet is equipped with a coil that generates heat. It is interesting to know the temperature of this coil but also to study the best way to dissipate it through in particular resins or materials that conduct heat. Coil life and performance can be adversely affected by excessive temperatures. An insulating resin ages much faster when it undergoes a temperature beyond reasonable, resulting in the degradation of its dielectric properties and therefore the appearance of problems such as short-circuit turns or a clean ground.
The method will not be developed here. That's another topic.
X) CONCLUSION ON FEMM.
A free tool, and it is important to specify it because the software of 3D simulations by volume finite elements have a price with 5 digits.
FEMM comes from a project initiated in 1993 but whose last update is in 2019 ! Little things are added regularly, like the latest generations of N55 neodymium.
Admittedly, FEMM is not very ergonomic and it is tedious if you do not have a geometry in dxf, initially carried out on a modern and fast CAD. This is its main flaw, for many advantages. We can blame it for being in 2D, except that we can simulate a depth or a revolution. We can also blame him for the lack of French-language literature about him.
We hovered over its capabilities relative to our need for force calculation. But there are much broader possibilities of calculations , which it is possible to discover through the instructions but especially through the dedicated English-speaking forum .
Remember that all software is simulation tools giving approximations and that the reality is slightly different. Simulate, calculate, rough but think about making a prototype to check your hypotheses with in particular... a gaussmeter or a teslameter!
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