When laminations are stacked, they don't fill the space that they occupy 100% full with iron. There is some space between laminations that is filled with insulation, varnish, or epoxy, and the laminations are not perfectly flat. In designing the machine, one needs to account for the lamination stacking factor. A stacking factor of less than 100% reduces the flux carrying capacity of the section a bit.

There are several good sources of lamination stacking factor estimates:

R. E. Tarter, "Solid-State Power Conversion Handbook", Wiley-Interscience, 1993.

S. Constantinides, "Designing with thin gauge", SMMA Fall Technical Conference Oct 16, 2008.

S. A. Nasar, "Electric Machines & Electromechanics", McGraw-Hill, 1997.

Magnetics Inc., "Core Selection for Saturating Transformers".

The stacking factors estimated by these various references are summarized in the table below.

Thickess, inches TarterConstantinidesNasarMagnetics
0.031 (22 gauge) 0.97 0.98    
0.028 (23 gauge) 0.97 0.98   
0.025 (24 gauge) 0.97 0.98   
0.0185 (26 gauge) 0.96 0.97   
0.014 (29 gauge) 0.95 0.96 0.95 0.95
0.012 0.95 0.95 0.95 0.95
0.006 0.90 0.90 0.90 0.90
0.004 0.90 0.87 0.90 0.90
0.002 0.85 0.825 0.85 0.85
0.001 0.78 0.80 0.75 0.75
0.0005 0.50  0.50 0.50
0.00025 0.375   
0.000125 0.25   0.25

These stacking factors ought to be reasonable for a small motor, generator, magnetic bearing, etc. constructed out of stacked laminations, so long as the machine is well constructed. For transformers and large motors, however, laminations are often interleaved. For transformers, (for example, transformers created out of an E-shaped lamination and an I-shaped lamination) the laminations are interleaved to reduce the effect of the air gap that is created when the edges of the laminations butt together. For large electric machines, laminations may be interleaved by necessity, because the machine is bigger than the width of the rolls of lamination stock. In this case, the lamination is constructed of multiple pieces that are interleaved for structural integrity and distributed air gap due to the joints. Unfortunately, this interleaving causes the laminations to stack more poorly. An addition reference:

W. T. Lyman, "Transformer and Inductor Design Handbook", CRC Press, 2004.

quotes stacking factors that are about 5% lower when the laminations are interleaved.
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