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 grea
ter 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
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|