Using Coils in 3D Models
The Coil feature applies an external electric field or current density along the direction of the coil. The nodes require additional settings in 3D models to determine the geometry and the direction of the current flow.
Coil Feature
The options available in a Coil feature depend on the chosen Conductor model:
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When Single conductor is used, the coil direction is computed in the dedicated preprocessing study step Coil Geometry Analysis.
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When Homogenized multi-turn is used, the Coil type input determines how the direction of the current flow is specified. The available alternatives are: Linear, Circular, Numeric and User defined. The Numeric option computes the current flow in the dedicated preprocessing study step Coil Geometry Analysis. The other three options do not require any preprocessing.
The Single conductor Conductor model can be used in Stationary, Frequency Domain and Time Dependent studies. However it only supports homogeneous/constant conductivity in the entire coil. If you need to use inhomogeneous or temperature dependent coil conductivity, use the Magnetic and Electric Fields physics instead.
The Homogenized multi-turn Conductor model supports the Stationary, Frequency Domain and Time Dependent studies.
In the Homogenized multi-turn case, the coil type determines how the direction of the wires constituting the coil is specified, as well as the method used to compute the average length and the average cross-section area of the domain or boundary. The coil length and coil area are used to compute lumped variables, such as the induced voltage or the total resistance.
For models representing only a part of the geometry (due to symmetry), the total length and cross-section area of the coil (referred to as coil length and coil area) are computed by multiplying the domain or boundary length and area by the appropriate multiplication factors specified under Symmetry specification in the subfeatures.
The following sections detail the possible options available for the Coil feature.
Homogenized Multi-Turn Model—Linear Coil Type
In a linear coil, the wires are straight and parallel. To specify the direction of the wires, use the default Coil Geometry subnode to select a straight edge or a group of collinear straight edges along the entire length of the coil. The coil direction is taken to be the tangential vector to the edges (shown in the Graphics window with a red arrow), while the length of the wires in the domain is the total length of the edges. If the length of the edges does not correspond to the length of the domain, select the Override length of the edges check box and specify the correct length. For domain features, the average domain cross-section area is computed from the domain volume and the length. To specify another cross-section area, select the Override domain area check box and specify the correct area.
Homogenized Multi-Turn Model—Circular Coil Type
In a circular coil, the wires are wound in circles around a common axis. To specify the direction of the wires, use the default a Coil Geometry subnode to select a circular edge or group of edges. The feature will compute the coil axis and the wire directions by analyzing the curvature of the selected edges. The total length of the edges is taken to be the average wire length in the domain, so it is appropriate to select a group of edges close to the average radius of the coil. If the length of the edges does not correspond to the length of the domain, select the Override length of the edges check box and specify the correct length. For domain features, the average domain cross-section area is computed from the domain volume and the length. To specify another cross-section area, select the Override domain area check box and specify the correct area.
The geometry analysis algorithm can determine the axis and direction only if there is a geometry in the model. If the model does not have a geometry, for example when using imported meshes, use the alternative analysis method by selecting the Use robust geometry analysis method check box. This method can be used even without a geometry, but it requires that the selected edges form a complete circle to work correctly. If neither the default method nor the alternative method work, an alternative is to set the Coil type to User defined and manually specify the direction of the wires using a Cylindrical System.
Homogenized Multi-Turn Model—Numeric Coil Type
In a numeric coil, the path of the wires in the coil is computed numerically in an additional study step during the solution. This allows the modeling of coils with complex shapes. To set up the numerical analysis computation of the current flow in a coil, additional information on the coil geometry must be provided by means of the default Geometry Analysis subnode and the boundary conditions under it.
By default, the Input (for Geometry Analysis) boundary condition is available under the Geometry Analysis subnode. For open coils (whose ends are on exterior boundaries), apply this condition on the input boundary, that is, the boundary at which the wires enter the coil domain. Right-click the Geometry Analysis node, add an Output (for Geometry Analysis) subnode, and apply it on the exterior boundary where the wires exit the domain.
For closed-loop coils, apply the Input boundary condition on an interior boundary orthogonal to the direction of the wires.
To complete the set up, add a Coil Geometry Analysis study step to the study, before the main study step.
The Coil Geometry Analysis can process multiple coils in the same step. In order to analyze only certain coils, select the Solve only specific coils check box and enter the coil names as a comma-separated list.
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Automatic Coil Geometry Analysis
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Coil Geometry Analysis in the COMSOL Multiphysics Reference Manual
Homogenized Multi-Turn Model—User Defined Coil Type
In a user defined coil, the current flow (the direction of the wires) can be entered as an arbitrary vector field in the User Defined Coil Geometry subnode. The vector field entered can be an analytical expression or the solution of another physics (for instance, the vector field computed by a Curvilinear Coordinates interface). In order for the magnetic problem to have a solution, the vector field must be as much as possible divergence-free, meaning that the current flow cannot have sources or sinks within the simulation domain nor can it originate from interior boundaries. In practice, this means that the current flow must either be terminated on exterior boundaries, or it must be closed in a loop.
Since the coil feature does not have any information on the geometry, additional settings are required:
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Enter the Domain length (for domain features) or the Boundary length (for boundary feature), which should be the average length of the coil from the input to the output—the average length of a wire, not accounting for symmetry nor the number of turns.
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For domain features, use the subnodes to User Defined Coil Geometry—Input and Output—to select the coil input and output boundaries, where the current flow enters or leaves the simulation domain. If the coil is closed in a loop, no selection is required.
Single Conductor Model
The setup of the Coil when using the Single conductor model is similar to the setup required for the Homogenized Multi-Turn Model—Numeric Coil Type case, the only difference being that the Coil Geometry Analysis study step will compute the physical current flow, instead of the direction of the wires. Refer to that section for more information.
Single-Turn Coil
The 3D Single-Turn Coil feature was available in The Magnetic Fields Interface in previous versions of COMSOL Multiphysics. This feature is obsolete—the recommended approach to model the same physical system is to use a Coil feature with the Single conductor model, or The Magnetic and Electric Fields Interface.
The feature is still available for backward compatibility when opening models created in previous versions of the software.
This node represents a solid conducting domain, typically a wire or a coil, with a non-negligible cross section. The boundary node represents a conducting thin layer whose thickness is small (also compared to the skin depth). To enforce the current conservation in the domain, an additional dependent variable with the dimension of an electric potential (SI unit: V) is added to the problem and the continuity equation for the current is introduced in the system of equations. This variable is referred to as the coil potential, but it is only loosely related to the electrostatic potential and it should be considered a help variable rather than representing a tangible physical quantity. In the Single-Turn Coil node, it is possible to specify the material properties that are used in the continuity equation.
The excitation is applied by means of specialized subnodes: a Boundary Feed subnode applies constraints on the coil potential to an external boundary, while a Ground subnode enforces the coil potential to be zero on the selected boundaries.
To ensure that the current continuity equation has a physical solution, terminate a coil domain on external boundaries only, or close it into a loop.
A Gap Feed subnode models a thin gap in the conductive domain across which a difference of potential or a current is applied. This feature should be applied on internal boundaries to the conductive domain and is useful for modeling closed loops.
For the Single-Turn Coil feature, only one active feed feature can be used at a time. That is, either a Boundary Feed or Gap Feed feature can be used, not both.
The Gap Feed should be used with care in high frequency modeling as there is a mesh dependent displacement current density flowing in the elements just outside the bounding edges of the Gap Feed boundary. If these elements have non zero conductivity, this problem can appear even at lower frequencies.