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
perm of 500
perm of 1000
perm of 2500
perm of 5000
perm of 7500
perm of 10,000
perm of 20,000
perm of 40,000
perm of 80,000
perm of 150,000
perm of 200,000
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.