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Gapping Iron Powder E Cores
Gapping iron powder E Cores increases energy storange capabilities beyond that inherent in the distributed air gap structure that is characteristic of the material. Gapping of E Cores is advantageous only in the higher permeability -26, -40 and -52 Materials due to ampere-turn temperature rise limitations.
The Percent Effective Permeability vs. AC Flux Density Graph, Percent Permeability vs. DC Magnetizing Force Graph and Ampere Turns vs. DC Energy Storage for Optimum-Gapped E Cores Graph illustrate the typical effect of gapping on the basic magnetic characteristics of -26 Material. The magnetization curves for the E Core geometry vary somewhat from curves for toroidal cores. This difference is due to the variation in leakage between the geometries. Similarly, some variation will exist between particular E Cores sizes. These curves are for reference only.
Similar results occur for -40 and -52 Materials. While -40 Material has an initial permeability approximately 20% lower than -26 and -52 Materials, when the two materials are gapped the resulting effective permeabilities are much closer to one another.
In addition to increasing energy storage, gapping also significantly reduces the swing of these materials with DC bias resulting in performance similar to -8, -18, -30, -34, and -35 Materials without a gap. Since -26, -40, and -52 Materials are less expensive than -8, -18, -30, -34 and -35 Materials this offers an attractive design alternative.
An additional discrete gap in iron powder does not have a dramatic impact on effective permeability as illustrated by the graph to the upper right. As a result, the gapping of iron powder E Cores is relatively non-critical when compared to ferrites and iron alloy lamitations.
Shown, are Energy Storage Curves for optimum butt-gapped E Cores in -26 Material. Similar or slightly higher energy storage will result with -52 Material while slightly lower energy storage will result for -40 Material with the same windings.
The term butt-gap has been used to indicate the physical separation of two standard E Cores (that are butted up against a spacer). By example, a set of cores with all three legs separated by .010 inches has a butt-gap of .010 inches. This creates an effective discrete gap of .020 inches. A butt-gap of .010 inches is equivalent to a total center-leg gap of .020 inches.
The E168, E168A, E220 and E305 size E Cores are available with standard center-leg gaps as detailed in the E Core listing. The E168 and E168A are available with a center-leg gap of .015 inches per half. A set made up of two of these gapped cores will produce a center-leg gap of .030 inches. The E220 is available with a center-leg gap of .020 inches per half.
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