Coil Excitation
The current density flowing in the coil domain is computed from a lumped quantity that constitutes the coil excitation. The choice of this quantity can be done by setting the coil excitation parameter, while the value is specified in the associated text box. All the coil features can be excited either with current excitation, voltage excitation or, in certain cases, power excitation. The supplied value or expression is translated to a current density or electric field applied to the domain according to the coil model used. Coil features can also retrieve the value of the current or the voltage from a circuit connection, if the parameter coil excitation is set to Circuit (current) or Circuit (voltage), respectively.
Circuit (current) excitation works similarly to the Current excitation, but in this case the inputs are provided by a circuit connection.
Circuit (voltage) excitation works similarly to the Voltage excitation, but in this case the inputs are provided by a circuit connection.
Current Excitation
Coil with Single Conductor model
When specifying a total current Icoil, the coil applies an external electric field in the direction of the current flow, computed as:
(3-1)
where L is equal to the physics interface thickness d for 2D models, 2πr for 2D axially symmetric models, and the coil length in 3D models. V is an unknown applied potential. The potential V is solved for using an additional equation, which constrains the total integrated current to be equal to the current value Icoil specified, acting as a controller.
Coil with Homogenized Multiturn Model
When specifying a total current Icoil, the coil applies an external current density in the direction of the wires as in Equation 3-2 where N is the number of turns which are specified and A is the total cross section area of the coil domain.
(3-2)
Coil Group Mode
When the Coil Group mode is activated, additional equations are added to the model to enforce the equality of the current flowing in each domain or domain group.
Single-Turn Coil (in Magnetic and Electric Fields)
When specifying a total current Icoil, the out-of-plane component of the current density is defined as:
(3-3)
where L is equal to the physics interface thickness d for 2D models and 2πr for 2D axially symmetric models, and V is an unknown applied potential. The potential V is solved for using an additional algebraic equation, which constrains the total integrated current to be equal to the current value Icoil specified.
Voltage Excitation
Coil with Single Conductor model
When specifying a total voltage Vcoil, the externally applied electric field is computed as:
(3-4)
where Vcoil is the applied voltage specified, and L is equal to the physics interface’s thickness d for 2D models, to 2πr for 2D axially symmetric models, and the coil length in 3D models.
Coil with Homogenized Multiturn model
When specifying a voltage Vcoil, the component of the current density in the direction of the wires is defined as in Equation 3-5 and Equation 3-6.
(3-5)
where Vcoil is the applied voltage which is specified, A is the total cross-sectional area of the coil domain, N is the number of turns specified, Rcoil is the total resistance of the coil calculated as
(3-6)
where L is equal to the physics interface’s thickness d for 2D models, 2πr for 2D axially symmetric models, and the coil length in 3D models. The expression σcoilacoil is the product of the wire bulk conductivity and wire cross-section area. Vind is the induced voltage calculated by integrating the electric field along the coil.
Coil Group Mode
When Coil Group mode is activated, additional conditions are added to the model to constrain the sum of all the voltages in the domains or domains groups to be equal to the specified voltage:
where Vcoil is the user-defined voltage drop across the coil, Vi is the individual applied potentials, and N is the number of turns in the coil group.
Single-Turn Coil (in Magnetic and Electric Fields)
When specifying a total voltage Vcoil, the coil applies an out-of-plane external current density computed as:
(3-7)
where Vcoil is the applied voltage specified, and L is equal to the physics interface’s thickness d for 2D models and equal to 2πr for 2D axially symmetric models.
Power Excitation
In 2D and 2D axisymmetric models, Power is an option available for the coil excitation parameter. When Power is selected, the physics interface sets up the equation as specified in the Current Excitation section, plus a constraint between the coil current and voltage:
in stationary and time-dependent study steps, and
in frequency domain (cycle average). Select this option to specify the input power for the coils. The problem becomes nonlinear with these settings.
The Power option is not available for 3D models due to the increased complexity of the problem compared to 2D models.
The Power option does not work with the Frequency Domain, Perturbation study.
Studies and Solvers in the COMSOL Multiphysics Reference Manual