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,
Coil Geometry Analysis, which is typically solved before the main study step.
The study step can be used to compute both the current flow when using the Single conductor model and the direction of the wires when using the
Homogenized multiturn model.
In the case of a Single conductor 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
Coil Geometry Analysis uses the initial values for the conductivity. If the
Coil Geometry Analysis 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
Coil Geometry Analysis 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
Magnetic and Electric Fields physics interface instead. In case of doubt, use the
Magnetic and Electric Fields physics as a means of validation.
In the case of a Homogenized multiturn 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
Coil Geometry Analysis 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
Single conductor coil. The vector variable
eCoil can be plotted (for example, in a
Streamline or
Arrow Volume plot) to visualize the computed direction of the wires.
The Coil Geometry Analysis replaces the
Coil Current Calculation 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.
