# MMF/Flux density across air gap for a salient pole

jim hardy
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How can I draw my ampere loop on the picture below, to show how the MMF distribution is between the blue dots in my first paint drawing?

I'm not sure you can.

Sorry i didn't have a good answer yesterday - i struggle for words and it sometimes takes days to find them.

Now since the purple flux is so much larger than that taking the red or green path we can ignore them.
But it's interesting academic exercise to calculate the mmf out there.
I still think it'd be a summation of individual mmf from each wire
or from each turn as in that first paper.

Note as observer's distance from solenoid increases, the distance from opposite sides becomes more nearly the same so the red and green mmf's cancel more and more. So MMF and flux outside drops rapidly .as one moves away from the solenoid.

Correct me if my thinking is not straight.

old jim

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@EEstudent90 and @jim hardy At this point, I would like to make an additional comment about this method of the MMF with the "air gap" that is also treated in the "link" to the Feynman lectures on it of post #24. The method seems to be reasonably well established and uses what would seem to be an approximation that ## H ## stays approximately the same throughout each individual material because the flux lines of ## B ## are nearly constant and also because we can presumably write a linear equation ## B=\mu H ## for each material (each with different ## \mu##'s). This method, because it does make these approximations, would seem to not necessarily be exact, and one thing that also is not explained by this method is how some materials such as soft iron will respond nearly linearly and make good transformer materials, while other magnetic materials will go straight to the permanent magnet state where the magnetic fields from the magnetic surface currents maintain the magnetization and don't require any external field from the solenoid current to remain in the magnetized state. To date, I have not seen a good and simple explanation for what causes these differences. In any case, ampere's law in the form ## NI=\oint H \cdot dl ## with its "magnetic circuit equations" appears to be the generally accepted method to solve the MMF magnetic problem with the air gap.

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EEstudent90, Asymptotic and jim hardy
cnh1995
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@Charles Link and @jim hardy, very nice and informative explanations! Thanks!

After reading this thread, I thought salient pole machines have better waveform shape i.e. the emf is more sinusoidal than that in cylindrical rotor machines, since the flux density in the air gap is sinusoidal due to specially chamfered poles. But a couple of articles say otherwise.

This is one of them.
Jump to the end of this article where they compare salient and cylindrical machines and claim that the flux distribution in salient pole machine is poor.

But since the armature winding is distributed type and the flux density is sinusoidal in the air gap, shouldn't their combined effect create a better waveform?