About the Coil Features
The coil features can be used to simplify the set up of magnetostatics and low-frequency electromagnetic models. In many such applications, the magnetic field is generated by electric currents flowing in conductive materials (for example, cables, wires, coils, or solenoids). The coil features can be used to easily model these structures and to translate lumped quantities (currents and voltages) into distributed quantities (current densities and electric fields). This section describes the main Coil feature, which is available in The Magnetic Fields Interface, The Rotating Machinery, Magnetic Interface, and The Magnetic and Electric Fields Interface.
The Coil is available both as a domain and as a boundary feature. The latter can be used to model flat coils for which the thickness is negligible compared to the other two dimensions.
The Coil feature supports two different Conductor model options, which differ by the physical system represented and the modeling details.
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The Single conductor option models a single, solid region of a conducting material (for example, metal) in which the current flows. The lumped voltage and current of the coil correspond, respectively, to the integral of the electric field along the coil length and to the integral of the current density on a cross section. Use this option in a domain feature to model a single wire with a nonnegligible cross section; in a boundary feature it can be used for a thin conductive layer whose thickness is negligible (for example, in frequency domain, it is small with respect to the skin depth).
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The Homogenized multi-turn option implements a homogenized model of a coil consisting of numerous tightly-wound conducting wires, separated by an electrical insulator. The computation of the voltage and current of the coil is performed in a similar way as for the Single conductor model, but it also takes into account parameters such as the number of wires and the cross-section area. Use this feature to model a coil containing a large number of wires without the need to model each wire individually. The boundary feature can be used when the wires are arranged in a thin region whose thickness is negligible. Note that capacitive coupling and skin effect are neglected in the homogenized model.
For 2D and 2D axisymmetric geometries, the current applied by a Coil is always in the out-of-plane direction. Thus capacitive (in-plane) coupling is neglected.
For 3D model geometries, the direction of the current flow is not easily determined. The coil domains have settings and subnodes to solve this problem. See Using Coils in 3D Models for more information.
Selecting unconnected geometrical entities with a Coil connects them in parallel.
For a examples of a Single conductor Coil, see:
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Inductive Heating of a Copper Cylinder: Application Library path ACDC_Module/Electromagnetic_Heating/inductive_heating
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Tutorial Example: Modeling a 3D Inductor (described in detail in the Intruction to the AC/DC Module manual): Application Library path ACDC_Module/Inductive_Devices_and_Coils/inductor_3d.
For examples of a Homogenized multi-turn Coil see:
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Magnetic Field of a Helmholtz Coil: Application Library path ACDC_Module/Inductive_Devices_and_Coils/helmholtz_coil
Inductor in an Amplifier Circuit: Application Library path ACDC_Module/Inductive_Devices_and_Coils/inductor_in_circuit
What Material to Use for the Coil?
The Coil domain feature is based on Ampère’s Law and it requires material properties to set up the appropriate constitutive relations. The boundary feature also requires the electrical conductivity when the Single conductor model is used. What is the correct choice of materials for the Coil domains or boundaries?
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When using the Single conductor option, the coil models a solid conductive domain (typically a metal). The material properties of the metal (electric permitting, magnetic permeability, and electrical conductivity) should be used in this case.
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When using the Homogenized multi-turn option, the conduction current (induced current) is assumed to flow only in the wires. To prevent induced current density from flowing in the domain, the region’s electrical conductivity σ is set to zero. The material used in the domain, only specifies the constitutive relations for the electric displacement field and the magnetic flux density. Therefore, choose or create a material that provides constitutive relations representative of the entire coil domain, not just of the material making up the metallic wires.
Other Coil Features
Two coil features designed for special applications are available in the Magnetic and Electric Fields interface in 2D and 2D axisymmetric components:
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The RLC Coil Group feature, that models coils for which capacitive effects between coil turns and with other parts of the models are comparable with the inductive effects.
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The Single-Turn Coil feature, to introduce an externally applied current or voltage on domains where the Ampère’s Law and Current Conservation feature is active. This feature is contributing, that is, it simply adds an external current density to the domain model represented by Ampère’s Law and Current Conservation.