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Thread: Motor winding insulation

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    Bronze Member mikilianis's Avatar
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    Motor winding insulation

    What would you do if you had sent a 30 kw motor away for rewind as it was down to earth 0.2 Meg Ohms ( 1000 volt test ) and it came back reading 0.4 Meg to earth and between windings I condemd it and refused to connect it up however due to long crocodile tears and pleadings of poverty it was connected and started and it ran no problem, making me and my testing abilities a mockery
    truth is ( meggers dont lie only electricians do ) how do I convince the company that the motor does not have a long lifespan
    Thanks Mike

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    Site Caretaker Dave A's Avatar
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    It's not easy to press people hard on a quality issue decision. Is there a safety angle?
    The trouble with opportunity is it normally comes dressed up as work.

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    mikilianis (18-Mar-10)

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    Diamond Member AndyD's Avatar
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    The insulation readings of a motors windings won't necessarily indicate how long it will last. This is especially true with a newly rewound motor. The insulation readings can change rapidly with the temperature. When the motor gets warmer any dampness will dry out giving better readings. Also you're using a thousand volt test. If the motor is 380-400v 3-phase, then a 500volt insulation test will be a better reflection of the insulation at the running voltage.

    A 380volt motor testing 200000 ohms @ 1000volts shouldn't be a safety issue.

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    mikilianis (18-Mar-10)

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    Bronze Member mikilianis's Avatar
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    Question Mikilianis

    I am under the impression or have been taught that a megger test should be done at twice the operational voltage that the equipment being tested is rated at. Also what is the permissable resistance (insulation) reading of a motor that has just been rewound, bearing in mind that rewinds are not done gratis.

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    a DC voltage twice that of the operating voltage is correct.

    insulation resistance must be higher than 1 Meg according to sans 10142

    has the motor been running and have you done and insulation resistance test since it has been reconnected?

    did they rewind the motor or put it in the oven to dry it out...you can always tell by the terminal wires.

    at what temperature is the motor running?

    have you checked phase to phase voltages and phase to earth voltages?

    why did you take the motor out to start with?

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    mikilianis (18-Mar-10)

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