# 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.

• – Nick Alexeev Sep 13 '15 at 6:20
• That seems to be only for a flyback transformer. – Dallas Carter Sep 13 '15 at 6:22

An air gap reduces the effective permeability of the magnetic core therefore, the formula for flux density based on magnetic field strength (H) and $\mu$ predicts that B (flux density) drops.

Remember that B = $\mu$H and, if $\mu$ has dropped for a given H field, then B has also reduced. If B reduces then the core will saturate less at a given current.

But, reducing $\mu$ can also increase H so it can seem a bit counter-intuitive. Here's how it pans out for a simple inductor: -

http://www.bookhtml.com/Lessons_in_Electric_Circuits/Ref/10237.png

If you reduce $\mu_r$ 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.

Now, H is ampere turns per metre and if "turns" have increased by $\sqrt2$ then the H field has increased by $\sqrt2$. But this isn't a problem because if you go back to the first formula with $\mu$ reduced by 2, the B field has dropped to half so, the net difference is that halving the permeability $\mu$ means the H field has risen by $\sqrt2$ but, the net effect on B is that it reduces by $\sqrt2$.

All of this means that, an inductor without an air gap will saturate at a lower current compared to one with an air gap (all other things being equal).

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 ? – Dallas Carter Sep 13 '15 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. – Marko Buršič Sep 13 '15 at 8:50
• not exactly true – JonRB Sep 13 '15 at 15:01