[Amps] Cut core and EI core loss diferences

[Will Matney]

Ok, one last word. I finished my research into this as I myself wanted to find exactly why cut core iron losses were higher than EI cores. I figured it was just the air gaps on the two C sections but there was more. I reference a book below named "Saturating Core Devices" by Leonard R Crow where this very thing is mentioned. The book is about saturable reactor design but discusses transformer construction also as this is all a reactor is, a transformer with a DC coil added to control it.

Quote;

"The assembly as described [1] is necessary if Hipersil is to be used in the saturable reactor. It affords the advantages of ease of fabrication, lower core loss [2], and saving in weight and space.

Nevertheless it has disadvantages of expense and the problem presented by the fact that although the mechanical construction would lead one to believe that it is equivalent to the standard 3-legged core [3], actually the ac flux path is different and inferior. Fig. 2-13 illustrates how the C-core is traversed by the ac flux, and, consequently, the iron losses are greater than would be the case if the ac flux path were identical to the 3-legged core. For the Hipersil assembly shown in Fig. 2-13 [4], the ac fluxes balance in the adjacent central legs, but these fluxes must travel through the middle section because of the magnetic discontinuity between the cores.

This magnetic discontinuity between the cores is caused by the air gaps between the individual turns of the Hipersil steel ribbon; these air gaps are composed of insulative materials in this type of construction. Fig. 2-14 shows a set of two cores wound with Hipersil ribbon. For clarity, the cross section of each ribbon turn is enlarged or magnified as is also the thickness of insulation on and between the ribbon turns. the two broken-arrow lines through the top of the core sections and the one broken-arrow line at the bottom of the core sections represent flux lines through the core air gaps. Let us assume that instead of having four turns of Hipersil ribbon in each core that each core has four hundred turns and that the insulation gap between turns averages 0.001 inch. Then for the two cores we will have 800 X 0.001 inch = 0.8 or a total of 800 small air gaps equivalent to a single large air gap of 4/5 inch through the cross section shown by broken-arrow line X. A much greater air gap reluctance is offered for the flux path shown by line A, since here a large additional air gap is offered by the air gap reluctance between the curvature of the two core corners. The same reluctance theory for this type construction, of course, would hold true with either silicon or Hipersil ribbon".

Footnotes;

[1] Using two C-cores as a shell core as in an EI core.

[2] Lower core loss as compared to cold rolled non-oriented steel. Hipersil being CRGO, and at the time (1949), only available in C-cores. M6 material is now available in EI cores on up to M2 which are all CRGO steel.

[3[ 3-legged core is referring to a EI core in the book.

[4] Fig. 2-13 shows two C-cores being used as a shell core which looks like an EI core.

The magnetic discontinuity spoke of is in between the two core sections where they butt together down the center of the core. The text don't mention this but the gap is also increased by the banding on each C section holding each together. In other words, the flux has to jump across this gap from one half core to the other! In an EI core, this is not the case. Plus, the fit on the cut gaps for a C-core are about a 0.001" to 0.002" fit which another gap is presented here. Grinding and lapping will only get it very close but a gap of a few microns still exists. reference a previous post of mine about a test that was made.

One last point of interest. the name Hipersil came from adding three words together.  HIgh PERmeability SILicon.

Best,

Will