COMSOL 6.2 - RLC Coil Group

	This node is available for 2D and 2D axisymmetric components. This node also can only be applied on domains where an Ampère’s Law feature is active.

The (Resistive-Inductive-Capacitive) node is an advanced coil feature that takes advantage of the A-V formulation of the Magnetic and Electric Fields features to take into account in-plane electric current flow in the coil’s current balance. This feature combines the magnetic model of a (in Coil group mode) with a multiple-terminal electrical model for the in-plane current. This node can be used to approximate in 2D a coil in which (due to capacitive coupling or other phenomena) there is relatively significant current flow in the in-plane directions.

Apply this feature to a group of domains representing the cross sections of the coil turns on the modeling plane. Since the electric potential is assumed to be constant in the cross section of each turn, the V variable must be removed in the selected domains by applying an node. The feature applies an external current density flowing orthogonally to the plane, and also imposes a voltage constraint on the boundary of each cross section, computed from the coil and the excitation properties. The feature also enforces a balance of the current flowing out-of-plane and the current leaking in the plane between the coil turns.

	The approximation applied by this feature is valid only under the assumption that the current flowing in-plane is relatively small compared with the current in the coil. If this condition is not fulfilled, the model cannot be considered two-dimensional and a full 3D component is required.

In order to apply the correct voltage at each coil turn, it is necessary to precisely define the order in which the domains are connected. The RLC Coil Group feature assumes that all domains are of the same shape and are disposed on a two-dimensional Bravais lattice; that is, that there exist two vectors a and b (called primitive vectors) such that given two lattice points r1 and r2 the following relation holds:

with n and m being integers. One-dimensional lattices (linear lattices) are also supported by this feature.

The RLC Coil Group node can automatically determine two lattice vectors (or one, in the case of a linear lattice) from the geometry and position of the selected domains. The choices and in the settings use this functionality. The two choices control how the vectors a and b are chosen.

Since the primitive vectors of a Bravais lattice are not unique, the lattice recognition algorithm could find two vectors different from the desired one. In this case, it is possible to manually specify lattice vectors.

By default, the RLC Coil Group feature assigns the first turn index (closest to the reference potential) to the turn with the smallest n and m. When using the automatic recognition algorithm, this usually corresponds to the coil turn with smallest x and y (or r and z in axisymmetry). The other coil turns are numbered in order of increasing m first, and then increasing n. Refer to the diagram in the node’s window for a visual explanation.

If is set to , the direction of increasing m is reversed each time n changes value. Again, refer to the diagram for a visual explanation.

The RLC Coil Group feature expands the model applied by a Single Conductor Coil domain feature by including a current balance for each turn and constraints for the voltage. The feature introduces n + 1 voltages (where n is the number of coil turns) with the interpretation of electric potential values at the turn end terminals: V0, V1,..., Vn, where V0 is the value specified as the in the window. The cross section shown in the interface corresponds to the midsection of the turns, that is half way between the end terminals. The i-th coil turn (i = 1,..., n) has an applied potential difference

that drives an out-of-plane external current density computed as in the Single Conductor Coil case:

On the boundaries of the turn domain the electric potential variable V is constrained to the value

, that is the mean of the electric potential values of its end terminals.

The current flowing in-plane is balanced with the difference of the currents between two adjacent turns. If Ii is the out-of-plane current flowing in the i-th turn,

then the current balance for the coil turns is

The coil current ICoil is computed from the coil excitation in the same way as for the Single Conductor Coil feature.

The settings in this section is very similar to the ones described in the Single Conductor Coil node. The only addition is the input. Specify in the text box the voltage at one end of the coil (SI unit: V). The voltage at the other end depends on the excitation type. This setting is useful, for example, to connect in series two coil groups.

This section specifies the information needed to identify the Bravais lattice on which the coil turns are placed The feature automatically detects the two primitive vectors of the lattice if is set as (the default) or . For use the diagram as a guide to manually enter the a and b (SI unit: m) of the lattice. The diagram shows how the domains are ordered for each choice.