Air gap in inductor core - Page 1
Forum section: electronics / beginners

author: Gowda1
author_rank: Contributor
timestamp: December 07, 2020, 03:44:29 pm
content: Hey guys,I am designing an inductor for high current where i need Air gap of 6mm in order to avoid saturation.. But the thing is I am not able to find the air the ferrite material which has airgap of 6mm(one side).. So I thought of keeping a plastic of 2mm on both the legs which also make 2mm gap on the center...which makes effective air gap of 6mm (That's what i believe).The AL value for individual gaps are given in the datasheet but that value is for middle gapped...My concern is if i keep gap on both the legs like i mentioned, do i get the AL value mentioned in the datasheet for the airgap?How does it make a difference by adding gap on legs?I believe that by adding 2mm on both the side gives effective air gap of 6mm..is thst right?Please let me know...Thank you

author: Alaezae
author_rank: Contributor
timestamp: December 07, 2020, 04:59:38 pm
content: I ran into a similar problem on a project I was working on. What I found was that it was easier to buy a solid toroid and just cut the gap I needed with a dremel (or hacksaw). I suppose it depends on the material you have, I found that it’s all pretty brittle so you have to be very careful. It’s definitely not perfect, but it worked for what I was doing.I’m a real beginner so I’m not exactly sure....Thanks

author: Benta
author_rank: Super Contributor
timestamp: December 07, 2020, 06:33:19 pm
content: 6 mm is an immense gap unless your core is the size of four stacked cigarette packs (for lack of a better reference) and will give significant stray field lines which create trouble somewhere in the vicinity.I'd go in the direction of using standard gapped cores with a lower u material instead. Grinding cores is a kludge and gives lots of problems in production. For a one-off it can be OK.There's a lot of iteration in inductor design and core/material selection with no really good closed-loop design methodology.

author: Gowda1
author_rank: Contributor
timestamp: December 08, 2020, 08:12:18 am
content: Sorry for not giving proper description.. I am using E70 from TDK.. In anyway i need to have 6 or 8mm airgap across all three legs.. My concern is if i make 2mm airgap at each leg then what will be the effective airgap of the ferrite and what will be the AL value...Since in the datasheet they have given AL value w.r.t. center gap.. But i would like to know if the AL remains same if i make gap on every legs..I attach datasheet link in this chat..Please let me knowhttps://www.tdk-electronics.tdk.com/inf/80/db/fer/e_70_33_32.pdf

author: JacobPilsen
author_rank: Regular Contributor
timestamp: December 19, 2020, 07:46:52 pm
content: This is the E-core.The outer legs only need to have half the cross-section that the center leg has.If you put the same gap on all legs, the effective gap will not be three times, but only twice thicker.ALwill be lower, of course.You should measure, not only believe.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 19, 2020, 09:15:49 pm
content: The air gap is the total around the loop. So, uncut cores with a 2mm shim have 2+2mm total air gap. (The outer limbs act in parallel, not series.)There are good reasons not to use such large air gaps. (To be fair, 6mm isn't allthatlarge compared to a E70..) What are you trying to do? Can you even get that much magnetization without burning up the wire? Would you be better served with a rod or U core (open magnetic path)? Or a low-mu powder toroid?Tim

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 20, 2020, 01:34:49 am
content: Yes, whats your F(sw)? It sounds like you could be better with a powdered iron torroid like Kool Mu by Mag-inc.Is it a PFC inductor?Anyway...use the MDT (magnetic design tool) by TDK..it may help you...though i suspect that it only does for centre leg gaps.But anyway, if you can get the core....put in your shims and measure inductance with say 10 turns...then back-calculate AL value...but you already probably knew this method and others imply it above.If this is an EMC filter inductor you may be ok...but if its an power inductor for an smps, then i suspect such big gap will give too much fringing field loss in nearby windings....and you will need to either use powdered iron, or use several inductors in series each with a smaller gap.