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Why do we have the airgap ? Is it just to store energy?

I did find an explanation online, but it was hard for me to understand.

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Why do we have the airgap?

  • An air gap reduces the effective permeability (\$\mu_e\$) of the magnetic core.
  • This means that the flux density (\$B\$) reduces for the same applied H-field.
  • This is because \$B = \mu_e H\$.

Note - The H-field is proportional to the current in the inductor and the number of turns.

  • So, if \$B\$ reduces then the core will saturate less at a given current.

  • But, reducing \$\mu_e\$ also increases \$H\$ (due to \$L\$ falling) so, it can seem a bit counter-intuitive.

Here's how it pans out for a simple inductor: -

enter image description here

Screen shot taken from this site.

If you reduce \$\mu_e\$ by 50% then inductance halves so you then need to restore this by increasing the turns BUT, you only need to increase turns by \$\sqrt2\$ to regain the original inductance.

This is because inductance is proportional to turns squared.

  • If "turns" have increased by \$\sqrt2\$ then the H-field has also increased by \$\sqrt2\$.

But, this isn't a problem because if you go back to the first formula with \$\mu_e\$ reduced by 2, the B field has dropped to half so, the net difference is that halving the permeability \$\mu_e\$ means the H-field has risen by \$\sqrt2\$ and, the net effect on \$B\$ is that it reduces by \$\sqrt2\$.

In summary, an inductor without an air gap will saturate at a lower current compared to one with an air gap (all other things being equal).

Introducing an air-gap also lowers the extent to which magnetic permeability can change with temperature. Many ferromagnetic materials will alter their permeability as temperature rises or falls and, when an air-gap is introduced, this usually unwanted effect is significantly reduced. This means that the inductance value remains more stable across a wider temperature variation.

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Air gap is for preventing the inductor going into a saturation region. It has nothing to do with energy storage, it's just matter of building inductuctors for specific inductance/current.

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  • \$\begingroup\$ Can you explain how ? What exactly is needed to prevent core to go into saturation ? From what I understand airgap simply increases the reluctance of the path. How does it prevent the core from going into saturation ? \$\endgroup\$ Sep 13, 2015 at 8:14
  • \$\begingroup\$ Analogical to elecric circuit, let's assume that voltage is ampere turns, current is magnetic flux and resistance is reluctance. Then I=V/R , thus flux=NI/Rm. Increasing reluctance means reduction of flux. Flux density is flux/cross_section_area. Reducing flux, reduces the flux density below saturation point. \$\endgroup\$ Sep 13, 2015 at 8:50
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    \$\begingroup\$ not exactly true \$\endgroup\$
    – user16222
    Sep 13, 2015 at 15:01

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