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!

    Oct 3, 2010
    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

    AAC Fanatic!

    Aug 30, 2007
    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.
  3. praondevou

    AAC Fanatic!

    Jul 9, 2011
    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

    Thread Starter AAC Fanatic!

    Oct 3, 2010
    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:

    With a removable, interchangeable coil like this:

    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

    AAC Fanatic!

    Aug 30, 2007
    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

    Senior Member

    Jun 7, 2009
    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.
    strantor likes this.
  7. #12


    Nov 30, 2010
    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

    Active Member

    Apr 23, 2009
    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.]