Automatic Coil Geometry Analysis

In 3D models, it is possible to perform an automatic analysis of the geometry of a Coil to determine the local direction of the current flow in the domain given the input and output boundaries of the coil (or an interior boundary in the case of closed-loop coils). The analysis is performed in a dedicated study step, , which is typically solved before the main study step.

The study step can be used to compute both the current flow when using the model and the direction of the wires when using the model.

In the case of a coil, the study step solves a simple current conservation problem to determine the flow of a current applied to the metallic domain. In this case, the uses the initial values for the conductivity. If the is followed by a study step that alters the conductivity, for instance, the conductivity depends on the temperature, the path that the currents take may deviate from what the initially expected. This may cause the direction of the electric fields used to excite the coil, and the direction used for integrating the induced currents to become an approximation, rather than being entirely consistent with the solution. In many cases this is a good approximation since most conductors used in electromagnetic devices offer a single, straightforward conduction path; and good electrical conductors are typically good thermal conductors as well. Therefore, the temperature gradients inside the conductor are small (note that a uniform change in conductivity will not affect the current direction). However, in 3D, when the temperature gradients (and the resulting gradients in conductivity) are large enough to cause a significant shift in the direction of the currents, for example, when modeling sensors like thermistors, you can use the physics interface instead. In case of doubt, use the physics as a means of validation.

In the case of a coil, the feature computes a vector field e which represents the local wire density in the coil, as well as the length and average cross-section of the wires. Note that the vector field e set by the will remain valid in the following study steps since the current direction is assumed to be dictated by the direction of the wire bundle, not the local conductivity, which is different from a coil. The vector variable eCoil can be plotted (for example, in a or plot) to visualize the computed direction of the wires.

	The Coil Geometry Analysis study step must precede the main study step (for example, a Stationary study step) in which the Coil is used.

	The Coil Geometry Analysis study step analyzes simultaneously the geometry of all coils in the physics that are active in the study — even if they use different Conductor models. To analyze only specific coils, select the Solve only specific coils check box in the study step and enter the names of the desired coils in the Coil names input field as a comma-separated list.

The coil geometry analysis method tries to construct a coil wires density that is plausible for an actual bundle of thin wires. The computed wire density is constant over arbitrary cross section, without violating current conservation for the current flowing in the wires. The density will depend on the local cross-section area of the coil (for example, it will increase in choke points), and will remain approximately constant in bends and curved regions as long as the cross-section area does not change. The analysis method will compute a solution even in presence of sharp bends, but the resultant wire density may not be constant. For best results, try to avoid sharp bends in the coil geometry.

The replaces the functionality available in previous versions of COMSOL Multiphysics. The new formulation is more robust and has a larger applicability than the previous one, as well as a number of improvements, such as support for solving multiple coils at once or for coils with nonconstant cross section. When opening a model saved in an older version of COMSOL Multiphysics, the Coil Current Calculation study steps are transparently updated to use the new formulation. It may however be necessary to regenerate the default solvers before proceeding with the solution.

Depending on the type of coils used in the model, the Coil Geometry Analysis may use a Stationary solver with a Segregated node to perform the analysis. The default solver settings should suffice in most cases, but they can be adjusted if the analysis fails to provide a satisfying solution. The Number of iterations in the Segregated solver can be used to tune the robustness of the method. Lower values improve the stability of the solution, but the condition that the wire density must be constant is weakened.

The Segregated solver uses Geometric Multigrid to solve the linear system associated with the analysis. If the Geometric Multigrid causes problems during the solution (for example, in presence of very coarse meshes), the direct solver can be used instead. Change the in all Segregated Steps nodes to .