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      Maxwell Litzwire Modeling: Losses

      This article explains how AC losses in Litz wires are influenced by transient current profiles in Ansys Maxwell simulations, where dB/dt substitutes for frequency in loss calculations.

      When modeling Litz wires in an Ansys Maxwell transient simulation, users may observe AC losses that exceed initial expectations. These discrepancies often stem from the geometry, material selection, or simulation setup. However, a significant factor to consider is the input current profile.

      Litz Wire Loss Formula

      Translating to Transient Simulations

      In a transient simulation, there is no fixed frequency . Instead, Maxwell calculates the rate of change of magnetic flux density . This derivative becomes critical in understanding AC losses.

      Simulation Example

      A simplified 2D transformer model was developed to illustrate this concept. In this example, the primary winding generates a current pulse from approximately 0.45 µs to 0.7 µs.

      Two plots are shown:

      Magnetic Flux Density:      B-Magnitude vs. Time



        • This plot demonstrates how the magnetic field evolves during the current pulse.

      Derivative of Magnetic Flux Density:     Deriv(B) vs. Time

       


        • This plot highlights the rapid changes in the magnetic field during the pulse.

      Key Observations

      • During the pulse, the current exhibits a high rate of change (dI/dt).

      • Correspondingly, dB/dt increases, leading to higher AC losses as per the Litz wire loss formula.

      • The transient nature of the excitation directly impacts the localized loss behavior in the wire.

      Practical Implications

      When designing systems with Litz wires, consider the following:

      • Input Waveforms: Pulsed or non-sinusoidal currents can significantly amplify losses.

      • Transient vs. Frequency-Based Analysis: In transient solvers, the time derivative of flux density substitutes for frequency, which should be factored into loss calculations.

      • Loss Mitigation: Optimize material properties and geometry to minimize losses under high conditions.

      •  Wire Geometry: Smaller Litz strand diameters will also result in lower AC losses.

      By understanding these principles, users can better align their Maxwell simulations with the physical behavior of Litz wires under transient conditions.