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- Thread starter xopek
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jim hardy

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You'll discover most of your textbook problems were set up to solve readily without the extra complications you mention.

A transformer has a closed magnetic circuit so your formula above will work pretty well.

A solenoid or motor has an air gap

so you must calculate reluctance of each piece of the magnetic circuit

and solve it as a a system.

If you take a course in electrical machinery it should get you past your immediate hurdle

you might also look for a book titled "Inductance Calculations", it'll address weird geometries

They get you started. That's their purpose.So how come the well known relationships between Ф, B, A, H, I, E, ℓ etc. captured in various famous laws yield an unusable formula for inductance?

“Life is difficult. This is a great truth, one of the greatest truths. It is a great truth because once we truly see this truth, we transcend it. Once we truly know that life is difficult-once we truly understand and accept it-then life is no longer difficult. Because once it is accepted, the fact that life is difficult no longer matters.”

- m scott peck

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A transformer has a closed magnetic circuit so your formula above will work pretty well.

OK, this sounds really reassuring. That would explain why just this formula is discussed in the context of iron core transformers! It just drove me crazy that one could combine the Ampere's law with the Faraday's law and rearrange things a bit, solve for this, solve for that, and finally get the expression for L, and then have doubts about validity of that formula since it is not even used in some cases. Sometimes these things just don't connect in my mind. I've seen some calculators based on the Maxwell's elliptic integral formula and they basically calculate mutual inductance between every pair of turns. And what I noticed is the more "compact" coils (shorter length more layers) are predicted to have higher inductance than longer coil with fewer layers. So that threw me off and I was afraid that in case of transformers that might also hold true and that would make the universal transformer equation less useful as the number of layers/turns per layer would have to be taken into account. But I never thought of it from the perspective of an air gap vs a closed magnetic circuit.

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jim hardy

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So that threw me off and I was afraid that in case of transformers that might also hold true and that would make the universal transformer equation less useful as the number of layers/turns per layer would have to be taken into account.

Things are simplified to convey the basics.

In a real transformer that's heavily loaded there are opposing mmf's that force some flux outside the iron core .

As designers push harder to minimize the amount of material in a product, those complex analyses let them eke out the last few drops of performance.

Armed with a good understanding of your basics you'll be able to handle the tedious calculations for those more complete solutions.

I forgive educators for simplifying, they have a lot to cover in a little time and humans can learn at only a finite rate.

You'll find old textbooks from prior to 1950 have good nuts&bolts explanations with graphical solutions.

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