Making a shorted or weak coil or winding

Discussion in 'The Projects Forum' started by mwootan, Aug 29, 2009.

  1. mwootan

    Thread Starter Member

    Apr 16, 2009
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    I am trying to figure out a way to make a coil to represent a shorted or going bad winding for a demonstration in a ac motor drives class to use with Meggers. Can anyone help in doing this?:confused:
     
  2. Externet

    AAC Fanatic!

    Nov 29, 2005
    758
    57
    Two shorted windings can be simulated by scratching the enamel on two turns and carefully solder wires to an external switch that will simulate shorting when swith is flipped 'on'
    For a 'going bad' condition, if the motor is not going to be run, but just tested with a megger; I would pour a couple of drops of battery acid onto the windings. Perhaps rinse it with water five minutes later, or repeat until you get a megger reading you will be happy with.
    If you make the motor run, you will be also able to explain about pretty flames in your class.
    Do not ask how I know that :rolleyes:

    Miguel
     
    Last edited: Aug 29, 2009
  3. mwootan

    Thread Starter Member

    Apr 16, 2009
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    Hey, thanks for the info! I will try on a motor that I can destroy. I was hoping not to have to destroy a good motor for this. I won't ask about the pretty flames but I would assume you found out the hard way! :eek: Thanks again, Mike.
     
  4. CDRIVE

    Senior Member

    Jul 1, 2008
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    As its name implies, Meggers are intended to read extremely high resistance. Other than testing leakage between the windings and the frame (where there should be no path at all), I don't see its use for this demo. Besides, if a path to the frame develops it's usually a low R path. They are definitely not intended to measure shorted windings, as this resistance will be far below their minimum range.
     
  5. mwootan

    Thread Starter Member

    Apr 16, 2009
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    Maybe I'm not explaining what I am trying to do corretly. I want to show good insulation in a motor but want to show some different stages of weaker insulation, other words show not so good insulation readings to bad insulation readings. Does this make more sense?:confused:
     
  6. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    Possible just conceal some appropriate value resistors on the back of the motor terminal plates, to give the readings you want?

    Or simply connect them on the terminals in the motor junction box, and bring flying leads out terminal strips for 'easy connection' of the megger?

    That way you could easily simulate earth leakage, a partial short between phases on a 3 phase motor or a high resistance phase, all with no permanent damage to the motors.
     
  7. CDRIVE

    Senior Member

    Jul 1, 2008
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    Good suggestion!;)
     
  8. mwootan

    Thread Starter Member

    Apr 16, 2009
    15
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    I got to say that sounds good! I'll give it a try. Got any suggestions for resistor values I should start with? Thanks alot! :)
     
  9. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    Something around 100K - 220K to earth from a phase to simulate an earth fault (or a couple of odd values in series so it's not too obvious it's a resistor).

    For winding faults, if you have small motors with windings around 20 - 70 ohms range, a roughly equal value resistor in series with one terminal for high resistance, and a similar type of value across two phases on another to emulate a partial short.

    I'm assuming the 'megger' also has a low resistance range, this is quite common to allow testing of winding resistance in addition to insulation tests. Normal basic testing on a three phase motor should include checking that the three windings are approximately the same resistance.
     
  10. mwootan

    Thread Starter Member

    Apr 16, 2009
    15
    0
    Thanks a whole bunch for your time! I like the suggestion and will go with it. :D
     
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