Foil coil windings
Overview
Figure 1. A thin metallic sheet (a) folded in the shape of a foil coil (b).
The current density distribution in a foil-wound coil fed by a time-varying source depends on skin and proximity effects. Since the foil is usually very thin and made from a material with a high electrical conductivity, the skin effect along its thickness is negligible (i.e., the current density in each turn results practically uniform along a radial direction). On the other hand, the current density in each foil turn may greatly vary along the axial direction of the coil as a function of both position and frequency.
This anisotropic behavior is specific to foil coils and influences the Joule losses developed in the bulk of the coil material. Thus, Flux now provides a new subtype of the coil conductor region with losses and detailed geometric description that implements a homogenization technique to represent this type of coil efficiently in its 2D Steady State AC application. This technique is exclusive to foil coils and differs from the approach used in the other subtypes of coil conductor regions with losses and detailed geometric description representing stranded coils.
Using this new coil conductor region subtype spares the user from representing each turn of the foil coil with an individual solid conductor region (linked to its corresponding FE coupling component in a complicated electric circuit). While this latter approach is also legitimate and rigorous, it is usually very time consuming to set up in Flux due to the elaborate geometry and the refined mesh required. Moreover, the solving time with the new foil-wound coil conductor region subtype is significantly reduced when compared to the alternate solid conductor approach.
Example of application
Figure 2. Cross section of one of the cylindrical Aluminum foil coils analyzed by M.M. El-Missiry in his article.
Figure 3. Color plot of the current density (phasor module, peak value) and magnetic flux density field lines of the foil coil displayed in Figure 2. The FE coupling component assigned to the coil conductor region is fed by a 1 + j0 Vrms voltage source at 50 Hz.
Figure 4. Comparison between current density results yielded by Flux and El-Missiry's approach. The plot displays RMS current density values evaluated along the vertical path shown in Figure 3.
Lumped circuit parameter at 50 Hz | Measurement | Flux 2D | Deviation |
---|---|---|---|
Reactance | 1.802 Ω | 1.827 Ω | 1.39% |
Resistance | 0.382 Ω | 0.376 Ω | 1.57% |