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Virtual Air Gap Condition

Introduction

The virtual air gap condition models a thin air gap in a magnetic circuit without resolving the gap geometrically in the mesh. A physically thin gap layer would need very small, poorly shaped cells. Instead, you apply the condition on an internal surface of the core. It reproduces the magnetic reluctance of a gap of the given length (the gap element technique of Nakata et al., 1990).

The condition is part of the time-domain magnetic model. You add it like any other condition.

Typical industrial applications include gapped inductor and transformer cores. Here the air gap dominates the magnetic circuit's reluctance and controls the effective inductance. The condition also suits parametric gap-length studies, which would otherwise need re-meshing for every gap value.

Condition

from mufem.electromagnetics.module.special import VirtualAirGapCondition

VirtualAirGapCondition(
    name: str,
    marker: Marker,
    gap_length: CffScalarVariableInputTrait,
)

Args:

  • name: Name of the condition
  • marker: Marker of the internal core surface on which the gap acts
  • gap_length: The gap length \(g\) (in m), given as a scalar coefficient

For a constant gap length, use the Constant factory:

condition = VirtualAirGapCondition.Constant(
    name = "Virtual Air Gap",
    marker = my_marker,
    gap_length = 1.0e-3,
)

Example

import mufem
from mufem.electromagnetics.module.special import VirtualAirGapCondition

gap_length_cff = mufem.CffConstantScalar(1.0e-3)

virtual_air_gap_condition = VirtualAirGapCondition(
    "VirtualAirGap", "Core::Gap" @ Bnd, gap_length_cff
)

magnetic_model.add_conditions([virtual_air_gap_condition])

To set a time-varying gap length, pass a time-dependent scalar coefficient instead of the constant.

References

  • T. Nakata, N. Takahashi, K. Fujiwara, and Y. Shiraki, "3-D magnetic field analysis using special elements," IEEE Transactions on Magnetics, vol. 26, no. 5, pp. 2379–2381, 1990.