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Induction Coil Failure with increaseing inductance as coil is unwound

  1. Dec 7, 2012 #1
    I have a 13 layer 130mH induction coil that failed and now registers 6.64mH. During the failure analysis the coil was unwound layer by layer and the induction dropped in steps of 0.7-1.5mH to a value of 1.3mH until the 8th layer was removed the the inductance increased to 5.4mH. Then the 9th layer was removed the inductance increased again to 18.5mH. There was also wire with the enamel damaged in the 8-10 layers in the middle of the layers. Does anyone know how the induction of the coil increased when the damaged layers were removed?

    Could the short in the coil caused one of the layers to reverse polarity generating an opposing magnetic field to reduce the inductance?
     
  2. jcsd
  3. Dec 7, 2012 #2
    You look to have the right idea....When one winding is shorted to another - even at relatively high impedance - the shorted winding develops current when the inductor is energized - in response to the induced magnetic field.

    Probably the best way to think of this is a transformer, consider the case of the secondary winding being open circuited - no current so it has no effect on the primary & it looks just like a plain inductor. - as you add load or short circuit the secondary, current flows in the secondary, effectively lowering the inductance of the primary.
     
  4. Dec 7, 2012 #3
    Thank you that helps!
     
  5. Dec 7, 2012 #4

    jim hardy

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    Indeed previous posts have answered the question.
    If you're relatively new, i would add that's a mighty significant phenomenon to be aware of.

    The shorted turns are indeed like a shorted secondary which, in presence of AC excitation to the coil, will develop substantial current.
    So they heat up and the damage progresses, feeding on its own heat.

    In AC motors and solenoids it leads quickly to catastrophic failure.
    In the DC solenoids that move control rods in some nuclear reactors, the excitation is DC from rectified 3 phase which contains some ripple. That ripple adds an AC component to the mostly DC excitation, so we get similar failures,. But they take years to progress to 'catastrophic' level and are easily detectable in early stages by inductance measurement at annual shutdown.

    So i've told you nothing new but perhaps underscored the value of your observation.

    Good luck in your career!

    old jim
     
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