what would cause a coil or solenoid to weaken over time?

Discussion in 'General Electronics Chat' started by strantor, Dec 10, 2012.

  1. strantor

    Thread Starter AAC Fanatic!

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    I would think that a solenoid would stay as strong as the day it was made, forever. But i have seen em"weaken" with age. Contactor coils, door lock solenoids, I have had to replace both on the past. Why? Particularly the contactor coils with no moving parts, nothing to get gummed up or dirty. Baffles me.
     
  2. gerty

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    Possibly some of the coil windings have shorted against adjacent windings, causing a weaker magnetic field. If that happens, the coil current would rise, If you have a weak one and a good one you could compare current draw.
     
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  3. praondevou

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    Isn't there always a spring mechanism included in relays/contactors? Could that be the cause?
    Do you have a specific example with fotos we could concentrate on?
     
  4. strantor

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    I don't have the coils, but going over the theory in my head, I've convinced myself that shorts between sections of the same coil would be a self-correcting failure. Imagine that a short occurs between one end of the coil to the center, thereby cutting the number of turns in half and doubling the current. Same number of amp-turns, right? Should have equivalent pull strength. I haven't thought this out very hard, so if it sounds half-baked, it probably is.
    The specific type of coil in question was on a contactor like this one:
    [​IMG]

    With a removable, interchangeable coil like this:
    [​IMG]

    I had one of these coils go "weak" and would no longer pull the contactor all the way in. I replaced the coil and the contactor worked just fine after.
     
  5. gerty

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    You may have something here, I didn't think of that..
    I personally haven't run into the situation you described, so I'm just guessing here.

    edit: What do you think about a high resistance connection between the coil windings and the screw terminal connections?
    I know it's cast into the plastic housing, be hard to verify.
     
    Last edited: Dec 10, 2012
  6. GetDeviceInfo

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    Jun 7, 2009
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    Heat is a common enemy. Contactor armatures often get dirty and can't pull in fully, resulting in continous higher currents, generating heat, which in turn distort coils, causing more heat. Listen to your contactors. If they are noisy (buzzing or chattering), they likely need dissassembly and cleaning.
     
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  7. #12

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    The only time I met that problem (not pulling in) was when too small of a wire gauge was used to run about 80 feet to the contactor. I put in a larger 24VAC transformer so the load was less than it was designed for and it didn't load down to 24 volts. That cured the problem.

    The stuff I work on never lasts 30 years. Maybe that's why I've never met a weak contactor coil.

    While I'm here, shorted turns...inductance goes up by N squared. Short half the turns of an AC coil and the current goes up by 4X. That would make it pull in better or just smoke, so I don't think shorted turns are the cause.
     
    Last edited: Dec 11, 2012
  8. Duane P Wetick

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    Apr 23, 2009
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    All it takes (on a optimized coil design) is for a few turns to short together.
    The AT (ampere x turns) product is now less, resulting in less magnetizing current with the end result being less pulling power for the solenoid.
    You will also notice that the current has slightly increased. Old coils may not even work anymore because of shorted turns.

    Cheers, DPW [ Everything has limitations...and I hate limitations.]
     
  9. Philip_Freidin

    New Member

    Oct 20, 2015
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    I suspect the failure mechanism might be an insulation failure (due to heat, moisture, age, ...), leading to shorted turns in the coil.
    At DC, having a few shorted turns would not be noticeable with an ohm meter, but with AC drive, the shorted turns represent a load, and can lead to additional internal heating, and maybe more shorted turns.
    Just guessing.
     
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  10. BobaMosfet

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    Inductors are subject to heat and usually the insulation between the windings is what breaks down over time.
     
  11. N11778

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    Chances are it would have worked if you removed the bad coil and then put it back in.
    dirty, loose connections, squeekee wheel gets the oil, relay was bored :).
    It has happened to me in the past. Ordered a new coil and never put it in.
     
  12. MaxHeadRoom

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    I would bet that they were AC rated components.
    I do not recall ever having to replace a DC solenoid or contactor coil for coil failure, AC?, too many times to remember.
    With the AC devices the coil relies on the armature moving over completely for correct current operation, if not, possible failure due to high current..
    Max.
     
    Last edited: Jul 31, 2017
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  13. shortbus

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    Wow! A new record? Four members necroposting together. :)
     
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  14. MaxHeadRoom

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    My excuse?, It was before I was a member here.!;)
    Max.
     
    Last edited: Jul 31, 2017
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  15. N11778

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    Was there something wrong with 2012? It was good for wine. We had lotts of rain.
    some of the relays got full of mold, did not close all the way and shorted some of the winding together.
    Arachnophobic relay syndrome
     
    Last edited: Jul 31, 2017
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  16. GopherT

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    Ok, since we are here though, the permeability of the iron core is highly dependent on the concentration of elemental iron. Oxidation of the iron quickly dilutes the concentration of the iron metal. The strength of the iron core inductor falls quickly.

    Dell computer had a similar problem with their laptop smps. Their power supply vendor bought cheap toroidal iron powder cores made by an anonymous (Chinese) manufacturer. Once the unprotected iron powder cores aged a bit, the lack of epoxy coating allowed the powder to oxidize and the cores saturated easily (well before specified current). They overheated and caused some fires.
     
  17. cmartinez

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    Yes... the world ended that year...
     
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  18. strantor

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    I guess an update is in order here. After this coil failed, another one failed. Same style old school AB starter contactor. Because the contacts weren't pulling in fast enough or with enough force, two of the contacts welded shut. The next time the motor was turned off, only 1 phase was disconnected and the motor caught fire. Because of the fire there was an investigation and the mitigation measure was to replace all the contactors in the whole plant over a certain age (which meant most of them). I was tasked with completing that on night shift among other duties. I worked on that for a few months. When I left the company it wasn't even half done. I went back to do some work as a contractor a year later and they were about at the same point as when I left. Must be hard to find a good contactor replacement technician.
     
    Last edited: Aug 1, 2017
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  19. MaxHeadRoom

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    What did you do, fire them all.?
    Out of interest, were they AC coils?.
    Max.
     
  20. strantor

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    DOH! Fixed spelling. Thank you autocorrect.

    Yes AC coils
     
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