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Main Question or Discussion Point
Im working with building a reluctance motor and ive posed a problem to myself that i cannot answer without questioning some long accepted facts about the inductance of a solenoid.
The simple solenoid and toroidal solenoid are two shapes that seem to have simple equations for calculating the inductance/B field.
e.g:
http://hyperphysics.phyastr.gsu.edu/hbase/magnetic/indtor.html
http://hyperphysics.phyastr.gsu.edu/hbase/electric/indsol.html
for my puzzle see attachment...
Panel 1:
I start with the inductance of a toroid, a completed magnetic circuit which has a inductance and B field for which I agree with the online equation as quoted.
Panel 2:
I add an airgap in the toroid. The magnetic circuit now has a break in it, so the complete circuit needs to take into account for the air and the iron that it passes through. This is done using the reluctance of the two materials, the metal and the air. the result presented as i understand it, now 1/4 of the flux path is in air.
Panel 3:
A 'simple' solenoid where over half the flux path must be through the air. since the relative permeability of the air is so much smaller this part should be the dominent term. making the B field considerably smaller than the value normally quoted!
I have not neglected to include effects of the obvious change in area that the flux is passing through when it is not 'confined' to the metal, so this is oversimplified... BUT this does not explain away the piece of physics that I am questioning here.
Have I made a foolish mistake here or has virtually every physicist/engineer in history learnt something thats fundamentally wrong? which i find hard to believe.
This has confused me for far too long, which is it?
Woody
The simple solenoid and toroidal solenoid are two shapes that seem to have simple equations for calculating the inductance/B field.
e.g:
http://hyperphysics.phyastr.gsu.edu/hbase/magnetic/indtor.html
http://hyperphysics.phyastr.gsu.edu/hbase/electric/indsol.html
for my puzzle see attachment...
Panel 1:
I start with the inductance of a toroid, a completed magnetic circuit which has a inductance and B field for which I agree with the online equation as quoted.
Panel 2:
I add an airgap in the toroid. The magnetic circuit now has a break in it, so the complete circuit needs to take into account for the air and the iron that it passes through. This is done using the reluctance of the two materials, the metal and the air. the result presented as i understand it, now 1/4 of the flux path is in air.
Panel 3:
A 'simple' solenoid where over half the flux path must be through the air. since the relative permeability of the air is so much smaller this part should be the dominent term. making the B field considerably smaller than the value normally quoted!
I have not neglected to include effects of the obvious change in area that the flux is passing through when it is not 'confined' to the metal, so this is oversimplified... BUT this does not explain away the piece of physics that I am questioning here.
Have I made a foolish mistake here or has virtually every physicist/engineer in history learnt something thats fundamentally wrong? which i find hard to believe.
This has confused me for far too long, which is it?
Woody
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