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What happens when you over current an inductor?

Discussion in 'Electronic Design' started by Andrew, Mar 22, 2006.

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  1. Andrew

    Andrew Guest

    I have been using a DC choke inductor in a test design and I am aware
    that the amp rating (4A) is too low for the application (average of 6
    or 7 amps). The inductor gets very warm during operation, almost too
    hot to touch if used enough. At first it was working perfectly, but
    now it is no longer working.

    I'm not sure if it is due to the inductor becoming damaged (I do not
    have another to replace it at this time), or if it is another cause?

    What happens when an inductor is ran out of spec too much? Will it
    become damaged? It still completes the connection it just seems to
    have no inductance?
     
  2. 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
     
  3. The insulation between adjacent turns may have broken down, shorting
    them together. This greatly reduces the inductance.
     
  4. John_H

    John_H Guest

    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?
     
  5. Tim Wescott

    Tim Wescott Guest

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

    --

    Tim Wescott
    Wescott Design Services
    http://www.wescottdesign.com

    Posting from Google? See http://cfaj.freeshell.org/google/
     
  6. RHRRC

    RHRRC Guest

    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.
     
  7. kell

    kell Guest

    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.
     
  8. Terry Given

    Terry Given Guest

    permanently so?

    Cheers
    Terry
     
  9. Terry Given

    Terry Given Guest

    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
     
  10. 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.
     
  11. RHRRC

    RHRRC Guest

    start off here:

    http://www.iop.org/EJ/abstract/0022-3727/4/8/328/

    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.
     
  12. Andrew

    Andrew Guest

    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!
     
  13. Paul Mathews

    Paul Mathews Guest

    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
     
  14. Terry Given

    Terry Given Guest

    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
     
  15. WhooHa!
     
  16. Good point. I have seen cracked ferrite in abused inductors and
    transformers. It usually hurts transformers more than it does inductors.
     
  17. Doug T

    Doug T Guest

    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
     
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