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Thread: industry standards for minimum Megger test acceptability

  1. #1
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    industry standards for minimum Megger test acceptability

    We recently had an 84" diameter electromagnetic chuck (from a Blanchard rotary surface grinder) repaired with new coils wound and installed. As I understand, these are considered "random wound DC coils". The chuck tested 11.1 megohms when we received it after rebuild. Information I have received from various sources indicates that this is an extremely low insulation value for a "complete rebuild" . So, I am wondering if there are industry standards for a minimum acceptable Megger test value for a "total rebuild" of the chuck.

  2. #2
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    How long did you keep your finger on the test button? You will find the value rises over time due to capacitance.
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  3. #3
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    Besides the duration of the test what was the temperature of the windings at the time of test? Is your value as read or temp corrected?

  4. #4
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    From NETA/ANSI standard

    The dielectric absorption ratio or polarization shall be compared to previously obtained results and should not be less than 1.0. The recommended minimum insulation resistance (IR 1 min) test results in megohms should be corrected to 40° C and read as follows:
    1. IR 1 min = kV + 1 for most windings made before 1970, all field windings, and others not described in 2.2 and 2.3. (kV is the rated machine terminal-to-terminal voltage in rms kV)
    2. IR 1 min = 100 megohms for most dc armature and ac windings built after 1970 (form-wound coils).
    3. IR 1 min = 5 megohms for most machines and random-wound stator coils and form-wound coils rated below 1 kV.

  5. #5
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    what voltage?
    The difference between genius and stupidity is that genius has its limits.

  6. #6
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    Quote Originally Posted by Tony S View Post
    How long did you keep your finger on the test button? You will find the value rises over time due to capacitance.
    Tested for approximately 2 minutes.

  7. #7
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    Quote Originally Posted by zog View Post
    Besides the duration of the test what was the temperature of the windings at the time of test? Is your value as read or temp corrected?
    Temperature of windings at time of test was approximately 65 degrees Farenheit. Value was "as read", not temperature corrected.

  8. #8
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    Quote Originally Posted by Ingenieur View Post
    what voltage?
    tested at 1000 volts

  9. #9
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    tested at 1000 volts

  10. #10
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    Quote Originally Posted by rick56 View Post
    Temperature of windings at time of test was approximately 65 degrees Farenheit. Value was "as read", not temperature corrected.
    So your multiplier is about 0.37 11.1x.037= 4.1Meg

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