The Coil node can be used to model coils, cables, and other conductors subject to a lumped excitation, such as an externally applied current or voltage. The Coil feature transforms this lumped excitation into local quantities (electric field and electric current density), and computes lumped parameters of interest such as impedance, and inductance.
The Coil feature supports two different
Conductor models:
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Single conductor, which models a conductive body such as a wire, busbar, or other metallic conductor in which the current flows freely due to the material’s conductivity. This model can be used when the current flow has a well-defined beginning and end (for example, connections to an external source) or is closed in a loop.
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The has to be complete in the sense that selecting only part of a contiguous conductor will lead to unphysical results.
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Homogenized multiturn, which models a bundle of tiny wires tightly wound together but separated by an electrical insulator. In this scenario, the current flows only in the direction of the wires and is negligible in other directions.
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The Coil feature is available both for domain and for boundary selections. In the latter case, it represents a flat coil or a conductor with a thickness negligible compared with the other dimensions. Different subnodes can be added to the
Coil node in different cases.
The global Harmonic Perturbation subnode is available from the context menu (right-click the parent node and select it from the
Global menu) or from the
Physics toolbar,
Attributes menu. The subnode can be used to apply a harmonic perturbation to the coil excitation.
In 2D and 2D axisymmetric components, the Coil feature supports the
Coil group functionality, that can be activated by selecting the corresponding check box.
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The Coil group option assumes that the selected domains represent cross sections of the same conductor going in and out of the modeling plane. These domains are expected to have the same areas. The same total current will be imposed in each domain, even if the domain areas are not equal. If the areas are unequal, the computed concatenated flux, coil voltage and inductance will be incorrect. For cases with varying cross section areas, it is recommended to use separate coil features that are coupled using The Electrical Circuit Interface.
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Refer to the Modeling Coils section in the modeling guide for more information about this node.
The Material type setting decides how materials behave and how material properties are interpreted when the mesh is deformed. Select
Solid for materials whose properties change as functions of material strain, material orientation and other variables evaluated in a material reference configuration (material frame). Select
Nonsolid for materials whose properties are defined only as functions of the current local state at each point in the spatial frame, and for which no unique material reference configuration can be defined. Select
From material to pick up the corresponding setting from the domain material on each domain. Since
Coil features model conductors or bundles of wires, the correct choice is usually
Solid.
Enter a Coil name. This name is appended to the global variables (current, voltage) defined by this coil, and it can be used to identify the coil in a
Coil Geometry Analysis study step.
Select the Conductor model for the coil. The choices correspond to rather different physical model, although the setup is similar. The
Single conductor model (the default) is appropriate for solid, massive current-carrying conductors. The
Homogenized multiturn model represent a bundle of tiny wires that are not geometrically resolved but taken into account in their average effect. The choice of
Conductor model affects the controls that are visible in the GUI and the available subnodes for the Coil feature.
This section is available when selecting Homogenized multiturn as the
Conductor model in 3D components and is used to specify the coil geometry (the direction of the wires).
Select a Coil Type —
Linear (the default),
Circular,
Numeric, or
User defined. The different alternatives are described in the following sections. Also see
Using Coils in 3D Models for more information.
When Single conductor is selected as the
Conductor model, the coil behaves as if
Coil Type is
Numeric, including the presence of the
Geometry Analysis subnode.
In a Linear coil, all the wires are parallel and straight lines. Use the
Coil Geometry subnode to select an edge or a single group of connected edges that maps out the local coil direction. The direction of the wires and the coil length is taken to be the direction and the length of the edge(s), as marked by the red arrow. Avoid selecting multiple parallel edge groups as that will result in an incorrect coil length.
In a Circular coil, the wires are wound in circles around the same axis. Use the
Coil Geometry subnode to select a group of edges forming a circle or a part of a circle around the coil’s axis. From the selected edges, the coil axis is computed and the direction of the wires is taken to be the azimuthal direction around the axis, as marked by the red arrows. The coil length used is computed as the coil volume divided by the coil cross sectional area, unless the
Use robust geometry analysis method box is checked. When the robust method is used, the coil length is simply the length of the selected edges.
This option is available at the domain level only. In a Numeric coil, the current flow is computed automatically in a
Coil Geometry Analysis study step. Use the
Geometry Analysis subnode to set up the problem.
For User defined manually specify the direction of the wires as a vector field and the length of the coil. Use the
User Defined Coil Geometry subnode to specify the coil geometry.
The Coil group check box is only available for 2D and 2D axisymmetric components. Select this check box to enable the
Coil group mode for this feature. With this settings, the domains or domain groups in this feature’s selection are considered series-connected. Selecting this check box activates the Domain Group subnode. See
Coil Groups for more information.
Select a Coil excitation —
Current (the default),
Voltage,
Circuit (voltage),
Circuit (current), or
Power (2D and 2D axisymmetric components only).
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Current forces a total current flowing in the coil wire. Enter a Coil current Icoil (SI unit: A). The default is 1 A. See the box below for study limitations on this setting.
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Voltage applies a total voltage across the coil enter a Coil voltage Vcoil (SI unit: V). The default is 1 V.
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Circuit (current) works similarly to the Current excitation, but in this case the inputs are provided by a circuit connection.
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Circuit (voltage) works similarly to the Voltage excitation, but in this case the inputs are provided by a circuit connection.
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Power (only available for 2D and 2D axisymmetric components) forces the coil input power (cycle-average in frequency studies) to the specified value. Choosing this option makes the problem nonlinear. For Power enter a Coil power Pcoil (SI unit: W). The default value is 1 W.
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When using the Current, Circuit (current), and Power options, the coil feature sets up a control problem for the coil voltage and current. Due to its complexity, the following limitations apply:
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The Power option is only available for 2D and 2D axisymmetric components.
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The Current and Circuit (current) options should be used with care in Time Dependent study steps. One should avoid applying a current step excitation (for example a fixed nonzero current) as that will lead to unphysical results and/or numerical instability.
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This section is available only when Single conductor is selected as the
Coil model. In this case, the coil represents a solid, massive conductor and the conductivity of the material is required to compute the current density flowing in it.
This section is available only when Homogenized multiturn is the selected as the
Coil model. In this case, the coil represents a bundle of tiny wires separated by an insulator. Additional settings can be specified.
Enter the Number of turns N. The default is 10. This is the number of tiny wires constituting the coil. The coil resistance is affected by this number and so is the current density in the coil as it together with the
Current setting defines the number of Ampère-turns in the coil.
The Wire properties can be specified using the following options:
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From conductivity allows for specifying the Coil wire conductivity σwire and a Coil wire cross-section area. The area needs to be the area of the actual conductor, without varnish or insulation. Several standards are supported.
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From resistivity; specify the wire resistance per unit length ρwire. This option is typically used for Litz wire, where the resistance per unit length is taken from measurements or a specification sheet provided by the supplier.
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The Wire properties are used to determine the intrinsic resistance of the coil wire or Litz wire. Usually, this amounts to the DC resistance. If a frequency-dependent AC resistance is used — for instance from a Litz wire specification sheet — it will include both the DC resistance and additional resistance from loss terms caused by skin and proximity effects within the strand bundle that forms the Litz wire. This intrinsic AC resistance is a property of the (Litz) wire itself.
If the wire is then wrapped around a magnetic core with a nonzero conductivity it will lead to additional eddy currents and additional loss. The total loss will determine the final coil AC resistance as perceived by the power source. Depending on the coil geometry, core type, and nearby conductors this resistance may be different from the AC resistance used to specify the wire properties.
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Enter a Coil wire conductivity σwire (SI unit: S/m). The default value is approximately the conductivity for copper, 6·10
7 S/m. This parameter represents the conductivity of the metal wires forming the coil. This is not the bulk conductivity of the material, which is instead set to zero according to the lumped model of a bundle of wires.
Enter the cross-section area of the individual wire awire in the coil. It is used, for example, to compute the lumped resistance of the coil. The area can be specified in different ways, according to the option selected in the
Coil wire cross-section area list —
User defined (the default),
Standard wire gauge,
American wire gauge (Brown & Sharpe),
From round wire diameter, or
Filling factor.
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For User defined, enter the value of the cross section area acoil (SI unit: m 2). The default is 10 −6 m 2.
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For Standard wire gauge, enter the SWG size. Sizes between 7/0 and 50 are available. The default size is 0.
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For American wire gauge (Brown & Sharpe), enter the AWG size. Sizes between 0000 and 40 are available. Sizes such as 0000 can be also written as 4/0. The default size is 0.
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For From round wire diameter, enter the diameter of the individual wire dcoil (SI unit: m). The cross-section area of the round wire will be computed from it. The default value of dcoil is 1 mm.is 0.
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For Filling factor, enter the filling factor f. This unitless factor determines the fraction of the subdomain area that is occupied by the turns of the wire, and the cross-section area of the coil turn will be computed from it. The default value of f is 0.5. This option is only available at the domain level.
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For Homogenized multiturn domain Coil, the total cross-section area of the coil bundle ( N·acoil), is expected to be smaller than the area of the domain selection.
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Specify a Coil wire resistance per unit length ρwire in
Ω/m. The default value is 30
Ω per 1000 feet. This option is typically used for Litz wire, where the resistance per unit length is taken from measurements or a specification sheet provided by the supplier. Other common sources are analytic Litz wire models (that can be typed directly into the expression field for
ρwire) or 2D finite element models that fully resolve the strands and output the frequency-dependent AC resistance in the form of a lookup table.
Specify the total Coil resistance. This will typically be the total DC resistance. When a frequency dependence is assumed, it will be the
intrinsic AC resistance — For more information, see
Wire Properties. The default value is 50
Ω.
At the domain level, the Coil node replaces the Ampère’s Law node in the definition of the material model for the domain. The
Settings window for the domain node contains the sections
Magnetic Field and
Electric Field, identical to the ones in the
Ampère’s Law node.
When Single conductor is selected as the
Conductor model, the material properties to be specified are the ones of the material constituting the domain. when
Homogenized multiturn is selected, specify the homogenized material properties of the domain, that is, the homogenized properties of the conducting wires and the surrounding insulator.
To display this section, click the Show More Options button (
) and select
Stabilization in the
Show More Options dialog box. This section is available only in 3D components when using
Homogenized multiturn as the
Conductor model and it contains advanced settings relative to the accuracy and stabilization of the solution.
The Accurate coil voltage calculation check box enables a current filtering functionality that improves the accuracy of the computed electric field and the induced coil voltage, at the cost of a slightly increased number of degrees of freedom. This functionality is only applicable for time dependent and frequency domain studies, and is active by default.
For the purpose of stabilizing the solution, the coil feature can apply a small electric conductivity to the coil domain. Use the Stabilization combo box to specify the value of the conductivity. Choose
Automatic (the default) to use a conductivity automatically computed by the coil. In frequency domain studies, the conductivity is chosen so that the skin depth in the coil is much larger than the coil length (see the sections
Coil Geometry and
User Defined Coil Geometry below). It is deduced from the formula
by setting the skin depth δ, equal to the coil length. In other study types the conductivity is set to 1 S/m.
If None is chosen, no conductivity is used in the coil domain. Choose
User defined to specify the
Electrical conductivity in the coil domain σΩ (SI unit: S/m). The default value is 1 S/m. The purpose of this electrical conductivity is only to stabilize the solution. According to the Homogenized multiturn model, the domain should not be conductive and all the currents should flow in the direction of the wires only.
