Must the air gap in a transformer always be perpendicular to the magnetic field lines?

Question

The air gap in a transformer seems to always be spaced across the cross section of the core, where the magnetic field in the core has to then flow across it.

Since the core is typically made of stacked laminated sheets, it seems like it would be easier to construct an air gap by putting a sheet of plastic, or some other non ferrous material, in between the laminated sheets when stacking them. As far as I know, this is never done - why not? The stacking factor accounts for tiny gaps between sheets where the lamination material is - what would happen if one of those gaps were made larger?

Answer 1

There already IS a gap between laminations — that's the whole point of using laminations in the first place, to break the circuit for eddy currents, reducing losses. The laminations are typically insulated by a coat of lacquer or paint.

And to answer your question, no, that gap has nothing to do with the gap required to prevent magnetic saturation, because the field lines do not cross that gap at all. The anti-saturation gap must intersect the magnetic fields lines in some fashion, but it doesn't have to be geometrically perpendicular.

Answer 2

If an air gap in the transformer is required (e.g. for controlling Lm and Ls in resonant converter topologies), you need to force the flux to go through the air in order to increase the core reluctance and increase the stray flux.

The flux does not go from one laminated sheet to its parallel neighbour sheet, so increasing the gaps between laminated sheets has no influence on the electrical properties, just makes the transformer larger (including larger losses due to longer wires).