Correction for Fringing at Short Air Gaps

The magnetic field intensity across an air gap is much greater than that across an equivalent length of iron, particularly below the region of saturation. This relatively high magnetic field intensity drives appreciable flux through the air near the sides of the air gap. This effect is known as fringing and in the case of short air gaps has the effect of increasing the effective area of the air gap. Therefore an empirical correction is made. Figure 3-18(d) shows a reactor core with an air gap of length g. The cross-sectional dimensions of the core are a and b. The corrected area, when fringing is taken into account, is found by adding the length of the gap to each dimension. Thus