What happens when you over current an inductor?

First at reaching the current specification its inductivity decreases as it becomes saturated. At further increase of the current over the specification, it becomes warm, then hot until a solder joint melts. The inductivity is lower than expected at overcurrent. You cannot permanently damage an inductor, except by damaging the insulation and the solder joints. Meaning the media, usually ferrite doesn't care.

Rene

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The insulation between adjacent turns may have broken down, shorting them together. This greatly reduces the inductance.

But won't there be troubles when the temperature exceeds the curie temp? I may be getting my cores mixed up with the rare earth magnets I work with.

The permeability will still be there because of the raw material; what happens to the properties of the core if it becomes magnetized?

Inductor cores are magnetically soft, so they shouldn't retain any magnetism.

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Yes it does care! Running ferrites at high field strengths and elevated temperatures is a recipe for disaster. Some ferrites are more prone than others but all are affected.

I've read various references (e.g. ARRL handbook, and Amidon literature) that say too much power can damage ferrite. You forced the core well past saturation. Or the heat could have burned or melted some of the insulation on the magnet wire, causing shorted turns (as already mentioned). Maybe even both. Anyway, your inductor is shot.

permanently so?

Cheers Terry

all materials that exhibit ferromagnetism become paramagnetic when heated above the Curie temperature - IOW the (relative) permeability drops to ~ 1, so the core "disappears". Cool it down, and it comes back.

note:

- if you melt or vaporise the core, it wont come back.

- iron powder can (and probably will) degrade if you do this, as the insulating material degrades with temperature (micrometals go into this in some detail)

I once had to fix a problem with a 1500W dc-dc converter that exploded at low battery. I traced it to the 40mm long 8-AWG wire going thru a

400T CT - the wire got so hot that the high-perm (IIRC J material) ferrite in the CT saturated (140C). This caused the CT secondary inductance to plummet, so nothing came out the end of the CT. The current controller saw this, and immediately cranked the duty cycle up to max.

at higher battery voltages/lower loads, the ferrite cooled down and it all started working again.

Initially I tried to hand-make some litz using 9 strands of magnet wire, but it was solder strippable wire, and because it was so short the insulation came off when soldered. So I got a new CT made using 3C85 (I forget the mag inc name) which has a Tc of about 230C. end of problem.

Cheers Terry

I have been waiting for someone to bring this up. There is a class of ferrites referred to as perminvar, that can have their properties altered by physical shock or stress or strong magnetic fields. These are cobalt doped nickel zinc ferrites that have had a specific heat treatment to tweak their properties. They are used for low loss RF applications, like filters and rod antennas (Fair-Rite type 61 and 65 are examples of perminvar ferrite, and their catalog descriptions mention their suseptabilty to shock and strong magnetic fields).

But manganese zinc ferrites and powdered iron cores used for power applications have no such suseptability. If their curie temperatures are exceeded, they loose permeability, but it recovers as temperature drops. Same for exceeding their saturation flux.

start off here:

lots of water has passed under the bridge since then - all of it points to the delterious effects of temperature, time, & field strength on ferrites very much including MnZn's.

Update: I unwound the inductor and found that the insulation was in fact melted in a few locations, therefore lowering the inductance. I rewound some new wire on it and it is working good, for now.

Thanks for your help!

All true, but leaves out the fact that it's fairly easy to crack ferrite and powdered iron cores, and this can result from rapid temperature excursions and/or large fields (not to mention mechanical shocks). Expect the inductance to decline dramatically in some cases...not so dramatic if there is already a substantial air gap. Paul Mathews

good point. square-loop ferrites can also crack if you operate them near their mechanical resonant frequency too.

I use my Hakko 850B hot air gun to remove glued planar cores - I heat them up and the core falls to bits.

Cheers Terry

WhooHa!

Good point. I have seen cracked ferrite in abused inductors and transformers. It usually hurts transformers more than it does inductors.

Can you over heat or saturate an air core? Of course we did bake the insulation off the windings occasionally. More of a problem was keeping the chokes attaches to their cradles. An 8 henry 6 amp choke makes a hell of ruckus when the output shorts and int input shuts off.

Doug T