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      Understanding Capacitance Reporting in Different Electromagnetic Simulation Types

      When performing electromagnetic simulations using Ansys Maxwell, the way capacitance is reported varies depending on the simulation type. This article clarifies these conventions to prevent misinterpretations.


       

      Simulation Types and Capacitance Reporting

      2D XY (Planar) Simulations

      • Reports capacitance per unit length (typically pF/m)
      • Assumes infinite extrusion in z-direction
      • Common applications:
        • Electric motors (cross-section perpendicular to shaft)
        • Transmission lines
        • PCB traces viewed in cross-section
      • To get total capacitance:
        • Multiply the reported value by your actual device length in meters

      Example:

      • If simulation reports 106.45 pF/m
      • For a 10mm long device
      • Total capacitance = 106.45 pF/m × 0.01m = 1.0645 pF

       

      2D Axisymmetric (About Z) Simulations

      • Reports absolute capacitance (pF)
      • Automatically accounts for full 360° rotation
      • Common applications:
        • Coaxial cables
        • Cylindrical capacitors
        • Rotationally symmetric structures
      • No need to multiply by any factor
      • Reported value is the total capacitance for the complete 3D structure

      3D Simulations

      • Reports absolute capacitance (pF)
      • Models complete geometry
      • No scaling or multiplication needed
      • Reported value is the total capacitance

       

       

      Practical Examples

      Motor Analysis

      For an electric motor simulation:

      1. Use 2D XY planar simulation
      2. If simulation reports 100 pF/m and motor stack length is 50mm:
      3. Total capacitance = 100 pF/m × 0.05m = 5 pF

      Coaxial Cable

      1. Use 2D axisymmetric simulation
      2. If simulation reports 2 pF
      3. This is already the total capacitance for the complete structure

      Verification Guidelines

      Cross-checking Between Simulation Types

      When comparing 2D XY planar with 3D results:

      • 3D result should match 2D result × length
      • Example:
        • 2D XY reports 100 pF/m
        • For 10mm length
        • 3D should report ≈ 1 pF

      Conclusion

      Understanding these reporting conventions is crucial for correct interpretation of simulation results. Always verify which type of simulation you're using and apply appropriate scaling factors only when needed.