# Reason to have an air gap in an inductor

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.

• Commented Sep 13, 2015 at 6:20
• That seems to be only for a flyback transformer. Commented Sep 13, 2015 at 6:22

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: -

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.

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.

• 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 ? Commented Sep 13, 2015 at 8:14
• 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. Commented Sep 13, 2015 at 8:50
• not exactly true
– user16222
Commented Sep 13, 2015 at 15:01