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Airgaps, Inductances and Perm

I ran some numbers to test the hypothesis we were talking about on the phone the other day, i.e., to see what impact perm has in an airgapped transformer... the hypothesis being evaluated is that once you have an airgap the "high perms" evaporate... or don't materialize in actual practice...

Here is the format I am using. the first number is the perm of the material. Underneath that number (which will range from 100 to 200,000) will be 3 inductances. In order (from the top down)... the first inductance number is what is obtained with a 5.2mil air gap. the second number is that which would be obtained with an absolutely zero physical air gap, and the third is an effective air gap of one half of one thousandths of an inch (obtainable, perhaps, with very careful stacking (or processing in the case of a c-core) in a "non-airgapped" transformer....

Same number of turns, same core area, same core volume, etc Only the perm and the air gap length has been changed to isolate the variables.

perm of 100
3.17H
3.71H
3.68H

perm of 500
9.96H
18.56H
18H

perm of 1000
13.6H
37.13H
34H

perm of 2500
17.43H
93H
75H

perm of 5000
19.23H
186H
126H

perm of 7500
19.92H
279H
162H

perm of 10,000
20.28H
371H
190H

perm of 20,000
20.85H
743H
255H

perm of 40,000
21.15H
1490H
308H

perm of 80,000
21.3H
2970H
344H

perm of 150,000
21.37H
5570H
364H

perm of 200,000
21.39H
7430H
370H


And one other tidbit I ran... effect of path length on L... in just one simple case... to verify what I had observed when I compared two similar OT's but built on different lamination sizes....

If built on a 1" by 1.50" stack the subject trans had an L of 163.6H

If built on a .875" by 1.714" stack the subject trans had L of 180.13H

Again, number of turns remains constant, same effective core area and same physical air gap (in this example, half of one one thousandths air gap)... obvious difference is the magnetic path length... the shorter the path (all other things being equal) the greater the L.

msl

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