Differences

This shows you the differences between two versions of the page.

--- faq [2026/05/03 16:45] – created - external edit 127.0.0.1
+++ faq [2026/05/03 19:01] (current) – admin
@@ Line 1: / Line 1: @@
 ====FAQ====
-If you are a first-time user, the best way to start is by completing the FEMM tutorial. You can get to the tutorial via the FEMM entry of your Windows start menu or at the [[magneticstutorial|MagneticsTutorial]] page. Some users prefer the [[https://www.femm.info/Archives/doc/femmtutor.pdf|old tutorial]]  instead.
+If you are a first-time user, the best way to start is by completing the FEMM tutorial. You can get to the tutorial via the FEMM entry of your Windows start menu or at the [[magneticstutorial|MagneticsTutorial]] page.
-Several frequently asked questions about FEMM are addressed below. The answers to many other FEMM-related questions can often be answered by searching the [[https://www.femm.info/list/|old FEMM Mailing List Archives]].
+Several frequently asked questions about FEMM are addressed below.
 There is also an active FEMM user's group at **https://groups.io/g/femm/ **
@@ Line 10: / Line 10: @@
 ===Questions===
-[[https://www.femm.info/wiki/FAQ#anchor1|1  Do I need a special license to use FEMM analyze problems on a commercial job?]]
+[[https://www.femm.info/wiki/FAQ#anchor1|1  Do I need a special license to use FEMM analyze problems on a commercial job?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor2|2  For AC problems, does FEMM use Peak or RMS currents/fluxes?]]
+[[https://www.femm.info/wiki/FAQ#anchor2|2  For AC problems, does FEMM use Peak or RMS currents/fluxes?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor3|3   FEMM returned unexpectedly high flux densities for my problem. What's going on?]]
+[[https://www.femm.info/wiki/FAQ#anchor3|3   FEMM returned unexpectedly high flux densities for my problem. What's going on?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor4|4  Does FEMM take saturation into account?]]
+[[https://www.femm.info/wiki/FAQ#anchor4|4  Does FEMM take saturation into account?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor5|5  Why aren't there any eddy currents in my AC problem?]]
+[[https://www.femm.info/wiki/FAQ#anchor5|5  Why aren't there any eddy currents in my AC problem?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor6|6  Why is H pointing the wrong way inside my permanent magnet?]]
+[[https://www.femm.info/wiki/FAQ#anchor6|6  Why is H pointing the wrong way inside my permanent magnet?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor7|7  Why does the field due permanent magnets disappear in AC problems?]]
+[[https://www.femm.info/wiki/FAQ#anchor7|7  Why does the field due permanent magnets disappear in AC problems?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor8|8  Does FEMM simulate problems with motion-induced eddy currents?]]
+[[https://www.femm.info/wiki/FAQ#anchor8|8  Does FEMM simulate problems with motion-induced eddy currents?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor9|9  The solver doesn't finish running. What's going on?]]
+[[https://www.femm.info/wiki/FAQ#anchor9|9  The solver doesn't finish running. What's going on?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor10|10  Why do the reported losses go down rather than up when I increase the frequency?]]
+[[https://www.femm.info/wiki/FAQ#anchor10|10  Why do the reported losses go down rather than up when I increase the frequency?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor11|11  The eddy current distribution in my device doesn't look right. What's going on?]]
+[[https://www.femm.info/wiki/FAQ#anchor11|11  The eddy current distribution in my device doesn't look right. What's going on?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor12|12  Can I drive coils with a specified voltage rather than a specified current?]]
+[[https://www.femm.info/wiki/FAQ#anchor12|12  Can I drive coils with a specified voltage rather than a specified current?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor13|13  Which version of Lua does FEMM use?]]
+[[https://www.femm.info/wiki/FAQ#anchor13|13  Which version of Lua does FEMM use?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor14|14  How do I put a comment in a Lua script?]]
+[[https://www.femm.info/wiki/FAQ#anchor14|14  How do I put a comment in a Lua script?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor15|15  How do I run a Lua script from the command line?]]
+[[https://www.femm.info/wiki/FAQ#anchor15|15  How do I run a Lua script from the command line?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor16|16  The forces predicted by FEMM are different than I expected. What's going on?]]
+[[https://www.femm.info/wiki/FAQ#anchor16|16  The forces predicted by FEMM are different than I expected. What's going on?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor17|17  The torques predicted by FEMM are different than I expected. What's going on?]]
+[[https://www.femm.info/wiki/FAQ#anchor17|17  The torques predicted by FEMM are different than I expected. What's going on?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor18|18  How do I simulate an induction motor?]]
+[[https://www.femm.info/wiki/FAQ#anchor18|18  How do I simulate an induction motor?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor19|19  There are weird errors and/or crashes when I try to run FEMM. How do I fix this?]]
+[[https://www.femm.info/wiki/FAQ#anchor19|19  There are weird errors and/or crashes when I try to run FEMM. How do I fix this?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor20|20  Is there a 3D version of FEMM?]]
+[[https://www.femm.info/wiki/FAQ#anchor20|20  Is there a 3D version of FEMM?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor21|21  What are some good reference books about finite elements?]]
+[[https://www.femm.info/wiki/FAQ#anchor21|21  What are some good reference books about finite elements?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor22|22  Why does the Triangle mesh generator fail on some geometries?]]
+[[https://www.femm.info/wiki/FAQ#anchor22|22  Why does the Triangle mesh generator fail on some geometries?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor23|23  Is there a native Linux version of FEMM?]]
+[[https://www.femm.info/wiki/FAQ#anchor23|23  Is there a native Linux version of FEMM?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor24|24  How do I reference FEMM in an academic publication?]]
+[[https://www.femm.info/wiki/FAQ#anchor24|24  How do I reference FEMM in an academic publication?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor25|25  Why is the AC force for my problem about half of the force reported in a DC simulation?]]
+[[https://www.femm.info/wiki/FAQ#anchor25|25  Why is the AC force for my problem about half of the force reported in a DC simulation?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor26|26  Why are are DC and 2X frequency force results reported when I compute force on an AC problem?]]
+[[https://www.femm.info/wiki/FAQ#anchor26|26  Why are are DC and 2X frequency force results reported when I compute force on an AC problem?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor27|27  Why isn’t the flux in phase with the current in an AC analysis and/or why is the inductance complex-valued?]]
+[[https://www.femm.info/wiki/FAQ#anchor27|27  Why isn’t the flux in phase with the current in an AC analysis and/or why is the inductance complex-valued?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor28|28  How do I separate the real and imaginary parts of a complex number in Lua?]]
+[[https://www.femm.info/wiki/FAQ#anchor28|28  How do I separate the real and imaginary parts of a complex number in Lua?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor29|29  How do I build FEMM with Visual Studio Community?]]
+[[https://www.femm.info/wiki/FAQ#anchor29|29  How do I build FEMM with Visual Studio Community?]] \\
-[[https://www.femm.info/wiki/FAQ#anchor30|30  How do I minimize the FEMM window and/or solver during Matlab script execution?]]
+[[https://www.femm.info/wiki/FAQ#anchor30|30  How do I minimize the FEMM window and/or solver during Matlab script execution?]] \\
-===<html><a name="anchor1" id="anchor1">1</a></html>  Do I need a special license to use FEMM analyze problems on a commercial job?===
+<html><a name="anchor1" id="anchor1">1</a></html>  Do I need a special license to use FEMM analyze problems on a commercial job?===
 There's no fee or special license required for simply using the results of the codes that are up on the website as part of an analysis done for some commercial purpose. Like it says in the [[license|license]]:
@@ Line 55: / Line 55: @@
 ----
-===<html><a name="anchor2" id="anchor2">2</a></html>  For AC problems, does FEMM use Peak or RMS currents/fluxes?===
+<html><a name="anchor2" id="anchor2">2</a></html> For AC problems, does FEMM use Peak or RMS currents/fluxes?
 In FEMM, everything is Peak, not RMS.  More specifically, for AC problems, all quantities (currents, fluxes, etc.) are represented as complex numbers where the absolute value of the number represents the amplitude, and the argument of the numbers represents phase shift.  For example, if //a// was a complex number representing the magnetic vector potential at some point, we could explicitly represent vector potential as a function of time, //A//(//t//), using the equation:
-<html><center>A(t) = Re[a (cos &#969;t + j sin &#969;t) ] = Re(a) cos &#969;t - Im(a) sin &#969;t = Re [a e<html><sup>j&#969;t</sup></html>]</center></html>
+<html><center>A(t) = Re[a (cos &#969;t + j sin &#969;t) ] = Re(a) cos &#969;t - Im(a) sin &#969;t = Re [a e<sup>j&#969;t</sup>]</center></html>
 ----
-===<html><a name="anchor3" id="anchor3">3</a></html>  FEMM returned unexpectedly high flux densities for my problem.  What's going on?===
+<html><a name="anchor3" id="anchor3">3</a></html>  FEMM returned unexpectedly high flux densities for my problem.  What's going on?
 FEMM will only give reasonable results if you are interpolating between defined points on the B-H curve for the material. Depending on your particular problem, you may need to add points to your B-H curves so that you are always interpolating rather than extrapolating.
@@ Line 67: / Line 67: @@
 ----
-===<html><a name="anchor4" id="anchor4">4</a></html>  Does FEMM take saturation into account?===
+<html><a name="anchor4" id="anchor4">4</a></html>  Does FEMM take saturation into account?
 Yes.  See Question [[https://www.femm.info/wiki/FAQ#anchor3|3]].
 ----
-===<html><a name="anchor5" id="anchor5">5</a></html>  Why aren't there any eddy currents in my AC problem?===
+<html><a name="anchor5" id="anchor5">5</a></html>  Why aren't there any eddy currents in my AC problem?
 Make sure that the materials that you are using have reasonable values for conductivity (//i.e.// not zero).
 ----
-===<html><a name="anchor6" id="anchor6">6</a></html>  Why is H pointing the wrong way inside my permanent magnet?===
+<html><a name="anchor6" id="anchor6">6</a></html>  Why is H pointing the wrong way inside my permanent magnet?
  By default, FEMM takes the view that permanent magnets are essentially the same as solenoids. That is, the magnet is modeled as a chunk of low-permeability material.  The "source" of the PM's flux is a current sheet of density //H//<html><sub>c</sub></html>*(//m// ×//n//) on the edges of the permanent magnet, where //H//<html><sub>c</sub></html> is the magnet's coercivity, //m// is a unit vector in the direction of magnetization, and //n// is an outward unit normal to the edge of the PM. This is a fairly typical way to model permanent magnets in the context of a finite element formulation (because it is any easy way to include PMs), and it is also equivalent to a magnetic charge-based formulation.  This issue is also discussed in one of the appendices of the FEMM manual.
@@ Line 84: / Line 84: @@
 ----
-===<html><a name="anchor7" id="anchor7">7</a></html>  Why does the field due permanent magnets disappear in AC problems?===
+<html><a name="anchor7" id="anchor7">7</a></html>  Why does the field due permanent magnets disappear in AC problems?
 The AC simulations in FEMM simulate only one frequency at a time.  Since permanent magnets produce a DC flux, their contribution to the field is only visible in DC simulations.  At frequencies other than DC, permanent magnets look like "plain" low permeability (and possibly conductive) materials.
 ----
-===<html><a name="anchor8" id="anchor8">8</a></html>  Does FEMM simulate problems with motion-induced eddy currents?===
+<html><a name="anchor8" id="anchor8">8</a></html>  Does FEMM simulate problems with motion-induced eddy currents?
 No.  FEMM does not model eddy currents induced by motion.
 ----
-===<html><a name="anchor9" id="anchor9">9</a></html>  The solver doesn't finish running.  What's going on?===
+<html><a name="anchor9" id="anchor9">9</a></html>  The solver doesn't finish running.  What's going on?
 You probably didn't apply any boundary conditions to the problem.  Although the solver will sometimes find a solution in cases in which boundary conditions haven't been specified (//e.g.// in cases with only permanent magnets), it's not guaranteed to finish unless you have defined boundary conditions on the edges of your problem.
@@ Line 101: / Line 101: @@
 ----
-===<html><a name="anchor10" id="anchor10">10</a></html>  Why do the reported losses go down rather than up when I increase the frequency?===
+<html><a name="anchor10" id="anchor10">10</a></html>  Why do the reported losses go down rather than up when I increase the frequency?
 This seemingly odd behavior can happen in AC problems in which you specify the currents by imposing a "source current density" (//J//) via the definition of a material, rather than creating a "circuit" to supply the current. The source current density that is being specified as part of the material properties is the current density that would be there in the absence of induced currents. Setting the source current is, in some sense, like setting the applied voltage. When the problem is evaluated at a non-zero frequency, there is non-negligible inductive impedance so that extra voltage is required to get the same net current as would be obtained at DC. Since the current is attenuated by inductance, the total losses are less.
@@ Line 108: / Line 108: @@
 ----
-===<html><a name="anchor11" id="anchor11">11</a></html>  The eddy current distribution in my device doesn't look right.  What's going on?===
+<html><a name="anchor11" id="anchor11">11</a></html>  The eddy current distribution in my device doesn't look right.  What's going on?
 In 2D planar problems, all parts of the problem are, by default "shorted together at infinity." If there is a specific connectivity of the regions in your problem, you have to //impose// that connectivity by the way that you define the problem.  This is usually done by defining a parallel-type circuit in which the total current is zero.  This circuit property is then applied to all regions that are supposed to be connected together (that is, all the regions over which the current is to be conserved).
@@ Line 115: / Line 115: @@
 ----
-===<html><a name="anchor12" id="anchor12">12</a></html>  Can I drive coils with a specified voltage rather than a specified current?===
+<html><a name="anchor12" id="anchor12">12</a></html>  Can I drive coils with a specified voltage rather than a specified current?
 No.  In FEMM, you can only specify the current in a coil. The post-processor does, however, report the voltage across a coil in which the current in the coil has been defined via "circuit properties."
 ----
-===<html><a name="anchor13" id="anchor13">13</a></html>  Which version of Lua does FEMM use?===
+<html><a name="anchor13" id="anchor13">13</a></html>  Which version of Lua does FEMM use?
 FEMM has the Lua 4.0 scripting language embedded within it.  The reference manual for Lua 4.0 is [[http://www.lua.org/manual/4.0/|here]].
 ----
-===<html><a name="anchor14" id="anchor14">14</a></html>  How do I put a comment in a Lua script?===
+<html><a name="anchor14" id="anchor14">14</a></html>  How do I put a comment in a Lua script?
 Precede the comment by two hypens, //e.g.//:
@@ Line 132: / Line 132: @@
 ----
-===<html><a name="anchor15" id="anchor15">15</a></html>  How do I start a Lua script from the command line?===
+<html><a name="anchor15" id="anchor15">15</a></html>  How do I start a Lua script from the command line?
 You can specify the Lua script that you want to run via the syntax:
@@ Line 145: / Line 145: @@
 ----
-===<html><a name="anchor16" id="anchor16">16</a></html> The forces predicted by FEMM are different than I expected.  What's going on?===
+<html><a name="anchor16" id="anchor16">16</a></html> The forces predicted by FEMM are different than I expected.  What's going on?
 It's possible to get bad force results if you don't set up your problems carefully and if you don't compute the force in the right way.
@@ Line 162: / Line 162: @@
 ----
-===<html><a name="anchor17" id="anchor17">17</a></html>  The torques predicted by FEMM are different than I expected.  What's going on?===
+<html><a name="anchor17" id="anchor17">17</a></html>  The torques predicted by FEMM are different than I expected.  What's going on?
 See above Question [[https://www.femm.info/wiki/FAQ#anchor16|16]] - the same issues that influence force calculation also apply to torque calculation. Also remember that the torque computed by FEMM is typically the torque about the point (0,0).  Misleading results can often occur if you are attempting to compute the torque on a rotor that is not centered at (0,0).
 ----
-===<html><a name="anchor18" id="anchor18">18</a></html>  How do I simulate an induction motor?===
+<html><a name="anchor18" id="anchor18">18</a></html>  How do I simulate an induction motor?
 Look [[https://www.femm.info/examples/imx/imx.htm|here]] for an example analysis of an induction motor.
 ----
-===<html><a name="anchor19" id="anchor19">19</a></html>  There are weird errors and/or crashes when I try to run FEMM.  How do I fix this?===
+<html><a name="anchor19" id="anchor19">19</a></html>  There are weird errors and/or crashes when I try to run FEMM.  How do I fix this?
 You may need the [[http://www.microsoft.com/download/en/details.aspx?id=5582|VC<del> 2008 (x86) Redistributable Package]] or the [[http://www.microsoft.com/download/en/details.aspx?id=2092|VC</del> 2008 (x64) Redistributable Package]], depending on whether you have installed a 32- or 64-bit version of FEMM.
@@ Line 179: / Line 179: @@
 ----
-===<html><a name="anchor20" id="anchor20">20</a></html>  Is there a 3D version of FEMM?===
+<html><a name="anchor20" id="anchor20">20</a></html>  Is there a 3D version of FEMM?
 No.  For intrinsically 3D magnetostatic problems, I sometimes use [[http://www.esrf.eu/Accelerators/Groups/InsertionDevices/Software/Radia|Radia]].
 ----
-===<html><a name="anchor21" id="anchor21">21</a></html>  What are some good reference books about finite elements?===
+<html><a name="anchor21" id="anchor21">21</a></html>  What are some good reference books about finite elements?
 P. R. Silvester and R. L. Ferrari, [[http://www.amazon.com/exec/obidos/ASIN/0521449537|Finite elements for electrical engineers, third edition]], Cambridge University Press, 1996.
@@ Line 192: / Line 192: @@
 ----
-===<html><a name="anchor22" id="anchor22">22</a></html>  Why does the Triangle mesh generator fail on some geometries?===
+<html><a name="anchor22" id="anchor22">22</a></html>  Why does the Triangle mesh generator fail on some geometries?
 If your geometry contains intersecting lines that form very small angles, triangle can run into problems meshing in the region of the intersection. The same issue can occur if you have a geometry with a line tangent to an arc, because very small angles can occur at the point of tangency. Sometimes, it is sufficient to change the ''Min Angle'' parameter in the Problem Definition dialog (reached by the ''Problem'' main menu selection in the preprocessor) so that it is a smaller value than the default 30 degrees (//e.g.// 20 degrees).  In other cases, it may be necessary to redraw the geometry to get it to mesh correctly, //e.g.// put a small radius instead of a sharp corner; put a small gap between a tangent circle and line, etc.
 ----
-===<html><a name="anchor23" id="anchor23">23</a></html>  Is there a native Linux version of FEMM?===
+<html><a name="anchor23" id="anchor23">23</a></html>  Is there a native Linux version of FEMM?
 No. However, FEMM has been tuned so that it runs properly on Linux machines via Wine.  See the LinuxSupport page for more details.
 ----
-===<html><a name="anchor24" id="anchor24">24</a></html>  How do I reference FEMM in an academic publication?===
+<html><a name="anchor24" id="anchor24">24</a></html>  How do I reference FEMM in an academic publication?
 Reasonable ways to reference computer programs in journal articles, dissertations, etc., are described on the web, //e.g.//:
@@ Line 213: / Line 213: @@
 ----
-===<html><a name="anchor25" id="anchor25">25</a></html>  Why is the AC force for my problem about half of the force reported in a DC simulation?===
+<html><a name="anchor25" id="anchor25">25</a></html>  Why is the AC force for my problem about half of the force reported in a DC simulation?
 Although this behavior might seem strange, it is actually correct. The trick here is that in a harmonic problem, the field varies sinusoidally, but the force is related to the //square// of the field, producing a DC component, and a 2-times frequency component, but no force at the drive frequency.
@@ Line 235: / Line 235: @@
 So there are 2 components of the force: a DC part of the force, and a part at //twice// the drive frequency. Now, the DC part is 1/2 of what it would be if the problem were evaluated at 0 Hz, so that the //peak// force is the same as in the DC case (//e.g.// when t=0, &#960;/&#969;, etc.) when the cosine part adds to the DC component. Conversely, the minimum force is zero when the cosine part cancels the DC part (at the instants where the current driving the fields passes through zero).
-----===<html><a name="anchor26" id="anchor26">26</a></html>  Why are are DC and 2X frequency force results reported when I compute force on an AC problem?===
+----<html><a name="anchor26" id="anchor26">26</a></html>  Why are are DC and 2X frequency force results reported when I compute force on an AC problem?
 See Question [[https://www.femm.info/wiki/FAQ#anchor25|25]].
-----===<html><a name="anchor27" id="anchor27">27</a></html>  Why isn’t the flux in phase with the current in an AC analysis and/or why is the inductance complex-valued?===
+----<html><a name="anchor27" id="anchor27">27</a></html>  Why isn’t the flux in phase with the current in an AC analysis and/or why is the inductance complex-valued?
 Eddy currents and hysteresis make the flux lag the current and make the inductance complex-valued. The imaginary part of the resulting inductance is associated with eddy current and hysteresis losses.
@@ Line 249: / Line 249: @@
 The imaginary part of the inductance contributes a real part to the impedance, associated with losses.  You could interpret the (&#969; L<html><sub>i</sub></html>) term as the frequency-dependent part of the circuit's resistance.  The real part of the inductance the forms the imaginary part of the impedance, associated with inductive energy storage.
-----===<html><a name="anchor28" id="anchor28">28</a></html>  How do I separate the real and imaginary parts of a complex number in Lua?===
+----<html><a name="anchor28" id="anchor28">28</a></html>  How do I separate the real and imaginary parts of a complex number in Lua?
 If <html><tt>x</tt></html> represents some complex-valued number in Lua, you can get the real and imaginary parts with the built-in functions <html><tt>re(x)</tt></html> and <html><tt>im(x)</tt></html>, respectively.  Other useful built-in functions for manipulating complex numbers are:
@@ Line 267: / Line 267: @@
 <html><tt>1.083923327338695+I*0.2717525853195117</tt></html>
-----===<html><a name="anchor29" id="anchor29">29</a></html>  How do I build FEMM with Visual Studio Community?===
+----<html><a name="anchor29" id="anchor29">29</a></html>  How do I build FEMM with Visual Studio Community?
 FEMM now compiles with Visual Studio Community available for free from [[https://www.visualstudio.com/]] because Microsoft Foundation Classes (MFC) are now included.  Make sure to select <html>C++</html> and MFC when you install Visual Studio. However, the release version is presently still built with Visual Studio 2008 because builds with newer versions of Visual Studio have seem to have problems running on Linux machines via WINE.
-----===<html><a name="anchor29" id="anchor30">30</a></html>  How do I minimize the FEMM window and/or solver during Matlab script execution?===
+----<html><a name="anchor29" id="anchor30">30</a></html>  How do I minimize the FEMM window and/or solver during Matlab script execution?
 Starting with builds 24Nov2017 and later, you can start FEMM minimized by opening FEMM with the <html><tt>openfemm(1)</tt></html> function call at the beginning of your Matlab script.