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questions on meggers for motor winding testing

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  • questions on meggers for motor winding testing

    has anyone on here tried one of those $80 electronic chinese meggers (winding resistance meters) that are fleabay all the time? this would just be for very occasional use and i have no desire to purchase a $500 Mercedes when a $80 Yugo will do.
    how about the old hand-crank ones. does much ever go bad in them? there seems to always be a good assortment on the 'bay, but i have no idea what to look for in a used megger.

    andy b.
    The danger is not that computers will come to think like men - but that men will come to think like computers. - some guy on another forum not dedicated to machining

  • #2
    I wouldn't bother.

    The "megger" is standard, but un-necessary, and can even damage windings if used wrong, since it is often capable of rather high voltage.

    To check one or two motors, I just connect them to line volts with all wires together. I connect a 1500 ohm 10W resistor to ground, with a 0.1 uF capacitor across it, and ground the case through the resistor (case must be insulated), and measure the AC volts across the resistor.

    more than a volt or two is an indication of excess leakage current. If you want to be fancy, you can use the "official" UL measurement circuit. which I can show you if you are interested.

    The system can also be reversed, with the resistor in the AC supply line.... in which case it needs to be guarded since it is "hot" with line volts.

    again, all wires together, and measure the voltage. Either way you are measuring the sum of capacitive and resistive leakage.

    Most "megger" units use DC and measure only the resistive component. That is the basic difference. The AC resistor test is a lot cheaper, and shows a "real world" result that can distinguish between a good and bad motor.
    1601

    Keep eye on ball.
    Hashim Khan

    Comment


    • #3
      JT,

      can you post a diagram of what you are saying, or PM me or email me?

      thanks!

      andy b.
      The danger is not that computers will come to think like men - but that men will come to think like computers. - some guy on another forum not dedicated to machining

      Comment


      • #4
        A megger is used to test the insulation resistance and is usually twice the working voltage but using DC. This is used in high voltage cables or overhead wires and should be carried out for one minute. I once used a 2500 volt megger to test 1100volt power cables but it took 2 of us to use it as you couldnt turn the handle for a minute and still look at the meter was very glad when battery powered units became the vogue.

        In my view if (in Uk) the motor windings wont take 500 volt dc for a minute I'd bin it!

        my 2p worth.

        Peter
        I have tools I don't know how to use!!

        Comment


        • #5
          Meggers intentionally stress the subject being tested. It's a feature and an important one. There's no reason a Chinese megger can't work well. They need high DC voltage and an ohmmeter. Neither is particularly difficult to come by and there's no significant design challenge involved. You can do the same thing with a gang of 50 9v batteries in series and a Radio Shack VOM meter. The Chinese megger would be more convenient and cheaper.

          Comment


          • #6
            Originally posted by ptjw7uk
            In my view if (in Uk) the motor windings wont take 500 volt dc for a minute I'd bin it!
            Peter
            Totally agree with you Peter.I use a megger with 250,500 and 1000 volts and low ohms.I test all motors I work on with it and never damaged one yet.
            Low ohms for continuity and high volts for resistance to earth.
            Started of many years ago with a hand wind Avo but never had enough hands to use it easily,although it was handy for giving an apprentice an introduction to dc.
            Jerry`s method will suffice no argument about it but a pain to use.
            Mark.

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            • #7
              Mark wrote

              although it was handy for giving an apprentice an introduction to dc.

              Yes, a megger can be a lot of fun.

              Comment


              • #8
                High pot something, a pulsed high voltage can blow holes in the winding insulation. I have never seen them used outside a power plant. People being over zealous can ruin a perfectly good generator.

                Meggers do no damage I am aware of, 1000 volt generated into normally 600 volt insulation to test it for spike resistance.
                Excuse me, I farted.

                Comment


                • #9
                  There is a very specific test for spike resistance. having designed/built some of the equipment to do it, I can assure you that there are good ones and bad ones, although I can't tell you any more details because they are secret.

                  However, bad spike testing will short the winding. Seen it happen. The good type will not.

                  I'll see about the diagram.... much cheaper, UL recognized (it's their test), and pretty easy.
                  1601

                  Keep eye on ball.
                  Hashim Khan

                  Comment


                  • #10
                    I thought it was generaly accepted "You Do Not Hi-Pot motor windings" hence the generaly accepted practice to Megger @ name plate voltage or less for motors, 2X Insulation value for conductors.

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                    • #11
                      It's probably always going to be interesting to pulse large inductors with high-voltage. Particularly multiphase inductors. I'd consider it a "hold m' beer and watch this" moment.

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                      • #12
                        A number of the motor manufacturers test their motors. The ones with higher voltage withstand become "inverter rated" or whatever the brand's designation is.

                        often that is the only real difference in windings. The same cores with same wire and same construction go in different grades of motor, but the insulation placement happens to be better on an "inverter duty" motor.

                        The test amounts to a hi-pot, but with certain key elements handled specially.

                        There CAN be other differences, often are, and there is a definition covering the "inverter duty" type.

                        But at the end of the day, there is a certain relation between test voltage withstood successfully, and the chance of failure in the field when used with an inverter.

                        naturally, some useage is easy pie, but a long line from inverter to motor can make for "issues".

                        Some manufacturers don't test, and they have more troubles.
                        1601

                        Keep eye on ball.
                        Hashim Khan

                        Comment


                        • #13
                          Ernie was driving us into Oklahoma, I had been drinking beer for about ten hours straight.

                          I had my old crank megger in my lap, a 1000 volt capacitor.. I was cranking it to beat the band.. going to throw it over into his lap to get even for him teasing a buffalo in a barbed wire fence.. (*scared the bejesus out of me)

                          Instead, I held it up to the window crank, it blew a chunk as big as a pencil eraser out of the pot metal crank.

                          NOW.. that was a EDM.. no clue where to find them big old dc high voltage capacitors anymore..
                          Excuse me, I farted.

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                          • #14
                            A company I used to work for sold these: http://www.megalert.com/

                            Not what you're looking for because they cost thousands, but it's basically an automatic megger that comes on every time a motor/generator is de-energized. They are also current limited so they won't hurt you if you put your fingers in the wrong place.

                            Comment


                            • #15
                              DAWAI: I have a .25mfd, 12,500vdc cap that is about 8x6x2 with large glass insulators coming of the top. Always have wanted to charge it and see the results.
                              John Burchett
                              in Byng OK

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