Frequently-asked questions

Creating custom materials

Custom materials can be created using the static method Material.from_specs. See e.g. @SteelLibrary/create.m for example implementation. Single-use materials can be created inside scripts, while for more repeated project-specific work, it is recommended to set up a function returning a custom Material object.

If you find yourself using the same custom materials repeatedly in multiple projects, it is recommended to create a personal library of materials, outside the EMDtool installation location and also outside project-specific work.

Note: Modifying the EMDtool material libraries is not recommended, as any changes will be lost whenever you switch to a new EMDtool version.

For fitting the typical core loss coefficients, you can use the fit_losses method of the `SteelLibrary’ class.

How do I create geometries?

Please see the documentation for template creation

What if I really, really, REALLY don’t want to make a template?

It’s also possible to instantiate an empty geometry like this

not_template = RadialGeometry();
not_template.dimensions = structure_of_some_required_dimensions;

and then add the required Domains, Materials, Circuits, etc. manually or inside a script. Again, please see the template documentation for details.

Common mistakes

Model creation

Can’d add a Curve to a Surface?

Check that the Curve you are trying to add shares a Point with the previously-added Curve. Curves must be added in consistent order (clockwise or counterclockwise) (per surface - defining one Surface in clockwise and the next one in ccw order is fine).

Meshing fails

  • Try the experimental .check_feasibility method of your geometry component (a GeoBase subclass). This method checks for intersecting Lines (not Arcs, so far) and Surfaces.

  • If you have concentric surfaces, verify that the innermost ones have been added as holes (s_outer.add_hole(s_inner)) to the outermost ones.

  • Verify that all your Points are unique (no dublicate Points at the same coordinates).

  • Verify that your Curves don’t intersect each other.

  • Verify that you don’t have any Points lying on Curves. Indeed, the only Points allowed to lie on a Curve are its endpoints (and the centers of arcs) - not the endpoints of another Curve.

  • T-junctions of two Lines are NOT allowed, in other words. They must consist of three Lines at least.

  • It is often a good idea to (temporarily) add plotting commands of Points, Curves, and Surfaces to your geometry template file, to see how intermediate results look like.

  • Verify that you have added all of your Domains and Materials to your Geometry, and all Surfaces to their corresponding Domains.

  • At the moment, the built-in geometry creation does not allow empty (non-meshed) holes. Relatedly, all geometric Surfaces have to be associated with a domain.

  • In the extreme case, try restarting your Matlab. Matlab occasionally locks the files .geo file; restarting Matlab solves this issue.

Miscellaneous pitfalls

  • When adding Materials to a geometry template, please use the following syntax: 
    material_object = Material.create( material_name_or_index_or_object );
this.add_material(material_object);

    This is to ensure the same material object does not accumulate ghost domains from earlier, non-existing geometries. This could happen with the following syntax: ´´´matlab material_object = PMlibrary.create(‘N35’); … custom_template_1.add_material(material_object); custom_template_2.add_material(material_object); ´´´ In this case, the material_object´ would share domains from both custom_template_1 and custom_template_2`, resulting in errors at best; incorrect but non-erroneous behaviour at worst. This will raise a warning in future versions of EMDtool.

Why are my simulation results weird?

  • Check the ‘Meshing fails’ steps first.

  • Plot your mesh with e.g. motor.visualize('linestyle', '-') or the same command for rotor or stator or whatever you are calling your components. Pay special attention to the boundaries between Domains and Surfaces: if there are non-matching nodes, you most likely have non-unique Curves or Points, intersecting Curves, or T-junctions.

  • Plot your airgap mesh with motor.visualize('plot_ag', true). If it looks weird, it’s often a good idea to decrease the characteristic length of one or both of the airgap-facing surfaces (e.g. the ID of stator).

  • And, verify that you don’t have flux lines escaping on flux-insulation/Dirichlet boundaries. Remember, you have to name the corresponding Curves ‘n_dir’ when making the geometry, or use the geo.set_dirichlet method.

  • Finally, verify that you have set periodic boundaries correctly, if you’re supposed to have any.

  • Run a no-load static simulation at one rotor position only, plot the resulting flux, and pay special attention to the flux lines. If they are not continuous but instead end in the middle of the problem domain, you again most likely have non-unique Curves.

  • Run simpler simulations first to see if the problem persists or not. For instance, torque vs. pole angle with static analysis and single rotor position only, or torque vs. slip. Use the ‘uniform coil current’ supply mode.

Non-symmetric designs

The typical EMDtool geometry templates - subclasses of RadialGeometry or AxialGeometry assume that the motor structure is symmetric. Meaning, the geometry repeats perfectly over one slot pitch (or pole pitch in case of rotors), with possible changes only in the remanence/domain orientation or coil incidence.

For less-symmetric designs, you have (at the moment) a few options only.

For somewhat-nonsymmetric geometries, you’ll have to determine (yourself) the smallest symmetrical component and create the geometry for it.

For entirely unsymmetric geometries, you’ll have to implement the entire geometry yourself.

A notable example are electrically excited flux-switching motors where the stator geometry is fully symmetric, but some slots house the phase winding (PolyphaseCircuit) while others house the field-winding. For this case, the currently-best approach is the following:

  • While making the slot-pitch geometry, create two PolyphaseCircuit objects, and assing the same slot-domain to both.

  • Create two custom PolyphaseWindingSpec subclasses, one for each Circuit. Obviously, one is the phase winding and the other for the field winding.

  • Implement custom get_loop_matrix methods to the winding specs. Both methods should return matrices with the same number of rows = the number of meshed conductors in the entire motor (dim.symmetry_sectors=1). However, some of the matrix rows should be all-zero: the rows corresponding to the field winding conductors for the phase winding spec, and vice versa.

Warning:

The above suggestion may cause issues, with the current implementation of the PolyphaseCircuit, as both will contain orphan Conductors. Handle with care.