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Correct procedure for de-magnetizing transformer cores

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    #11
  1. Join Date
    May 2018
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    Quote Originally Posted by randywright View Post
    What's the cause of a "magnetized" transformer and what's the correct procedure for de-magnetizing? I've been told you should de-magnetize each phase after doing winding resistance but another tech told me you can get away with doing a de-mag on only one phase. Also, what would happen if a magnetized transformer was re-energized?
    At my company we were instructed to do the DC Transformer testing first (ie., Polarization Index, Winding Resistance) and the AC testing last (ie., TTR, Power Factor) because the AC changing polarity 120 times per second will wipe out any residual magnetic charge. Most Winding Resistance/Insulation Resistance test sets come with internal grounding to dissipate the magnetic charge anyway so for the most part it's a non-issue. Just remember to leave your test leads on until the test set has completed it's discharge.

  2. #12
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    Feb 2019
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    I thought the point in demagnetisations were to remove residual flux density from the core and discharge the winding? Surely the test unit you use such as a megger etc will only discharge the winding and leave residual flux density in the core?

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