Megohm testing, does polarity matter?

Discussion in 'General Electronics Chat' started by strantor, Dec 9, 2010.

  1. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
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    If you were to test insulation resistance on a motor or anything else, usually you put the + of the megger to the windings and the - to the case. Would it not work the other way around? If you put + to the case and - to the windings? If no, why not?
    Thanks!
     
  2. timrobbins

    Active Member

    Aug 29, 2009
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    Megger testers are often squarewave AC, rather than dc - so the only concern is really the max level.
     
  3. roadey_carl

    Active Member

    Jun 5, 2009
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    with IR testing it doesn't matter what way round the probes go, because either way round the current will still flow 1 way to the other. just remember if you test between live conductors to disconnect any vulnerable equipment!
     
  4. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
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    The reason I ask is because as it has been explained to me (elsewhere), when you charge the windings, they are an inductor & that is the reason for the "charging time" - the time it takes for the reading to level out, once the inductor is fully charged. I don't see how you could achieve this charge if you were to connect the leads the other way round. This is assuming that I understand correctly, which I'm not confident of. The whole "charging time" and "inductor" thing doesn't make sense to me because no current is flowing through the conductor (assuming good insulation) and even if it were, it wouldn't be flowing from 1 end to the other, inducing a magnetic field, as the ends of the winding are connected together.
     
  5. timrobbins

    Active Member

    Aug 29, 2009
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    "This is assuming that I understand correctly, which I'm not confident of. " Correcto mundo. The IR or megger tester doesn't attempt to pass current rhough the eut (ie. the inductive windings) - the voltage is presented across the insulation that you are aiming to test - hence the acronym IR.
     
  6. roadey_carl

    Active Member

    Jun 5, 2009
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    The output voltage of a installation resistance tester is DC... Its just a constant discharge of capacitors in series to get 250/500/1000 volts.

    If you charge windings with AC then this will cause inductance. If you charge the windings with DC it will cause the windings to act as a capacitor as you have a conductor, insulator and conductor. This is why when you test windings the IR will usually increase very slowly and reach a maximum reading.
     
  7. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    the intent of meggering a motor, is to check it's insulative value. It has nothing to do with inductance, or the polarity of the test voltage.
     
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