How to measure insulation resistance of a motor
How to measure insulation resistance of a motor (photo credit: elecls.cc.oita-u.ac.jp)

Winding insulation resistance

If the motor is not put into operation immediately upon arrival, it is important to protect it against external factors like moisture, high temperature and impurities in order to avoid damage to the insulation. Before the motor is put into operation after a long period of storage, you have to measure the winding insulation resistance.

If the motor is kept in a place with high humidity, a periodical inspection is necessary.

It is practically impossible to determine rules for the actual minimum insulation resistance value of a motor because resistance varies according to method of construction, condition of insulation material used, rated voltage, size and type. In fact, it takes many years of experience to determine whether a motor is ready for operation or not.

A general rule-of-thumb is 10 Megohm or more.

 Insulation resistance value Insulation level
 2 Megohm or less Bad
 2-5 Megohm Critical
 5-10 Megohm Abnormal
 10-50 Megohm Good
 50-100 Megohm Very good
 100 Megohm or more Excellent

The measurement of insulation resistance is carried out by means of a megohmmeter – high resistance range ohmmeter. This is how the test works: DC voltage of 500 or 1000 V is applied between the windings and the ground of the motor.

Ground insulation test of a motor
Ground insulation test of a motor

During the measurement and immediately afterwards, some of the terminals carry dangerous voltages and MUST NOT BE TOUCHED.

Now, three points are worth mentioning in this connection: Insulation resistance, Measurement and Checking.


1. Insulation resistance

  • The minimum insulation resistance of new, cleaned or repaired windings with respect to ground is 10 Megohm or more.
  • The minimum insulation resistance, R, is calculated by multiplying the rated voltage Un, with the constant factor 0.5 Megohm/kV.

    For example: If the rated voltage is 690 V = 0.69 kV, the minimum insulation resistance is: 0.69 kV x 0.5 Megohm/kV = 0.35 Megohm

2. Measurement

  • Minimum insulation resistance of the winding to ground is measured with 500 V DC. The winding temperature should be 25°C ± 15°C.
  • Maximum insulation resistance should be measured with 500 V DC with the windings at a operating temperature of 80 – 120°C depending on the motor type and efficiency.

3. Checking

  • If the insulation resistance of a new, cleaned or repaired motor that has been stored for some time is less then 10 Mohm, the reason might be that the windings are humid and need to be dried.
  • If the motor has been operating for a long period of time, the minimum insulation resistance may drop to a critical level. As long as the measured value does not fall below the calculated value of minimum insulation resistance, the motor can continue to run.

    However, if it drops below this limit, the motor has to be stopped immediately, in order to avoid that people get hurt due to the high leakage voltage.

Reference: Grudfos – Motor Book

About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

26 Comments


  1. Ernest Anderson
    Jul 11, 2015

    Tank u for de help pls we need more information about magger


  2. murat baykal
    Mar 10, 2015

    Very useful artciles are take place here. As an unexperienced engineer those articles are very educating and informative.


  3. yphamngoc
    Nov 16, 2014

    How about the ratio one min per 15s and 10mins per 1 min?
    Thanks


  4. Michael okikiri
    Nov 06, 2014

    Am a bit confused on the illustrations on insulation resistance test. If 2 megaohm is considered bad base on the thumb rule and you calculated the minimum insulation resistance in the example to be 0.35megaohm is this not below the 2megaohms considered bad? Pls clarify

    Michael okikiri


    • tv
      Nov 16, 2014

      In general, it is considered as 2 M ohms. But for critical operation this can be lowered to 0.35 M Ohms as stated. Once the emergency situation is out, the cause of low insulation should be attended.


  5. farai benie
    Oct 30, 2014

    I want to learn more about electrical engineerinf department


  6. awny
    Oct 01, 2014

    how could i download the pdf of this article?please help me…


  7. Mustafa Rabiul Hassan
    Sep 18, 2014

    how could i download the pdf of this article?please help me…


  8. nombuso
    Sep 09, 2014

    As a lecturer in electrical department I find this page very useful.


  9. arif wahyudi
    Sep 03, 2014

    I think we can use EASA or IEEE std. 43, table 3


  10. Todd Thomassen
    Aug 02, 2014

    Nice presentation!


  11. Malesela Godfrey
    Jul 06, 2014

    What may be the cause when you meggar between L1 and L3 and get results of less than 0,5 Megohm? and when you meggar between other phases you get more than 0,5 Megohm even between phases and earth is 0,5 Megohm.


    • vijay
      Oct 21, 2014

      L1 & L3 always should show zero resistance with megger. So, also between L1 & L2, L2 & L3. But, with respect to Earth the value should be higher the resistance higher the benefits. The symptoms that you wrote clearly shows that the windings are pretty bad. Requires overhauling.
      Vijay


  12. Nigel Chikanv
    Jul 03, 2014

    Thanks


  13. VIKAS PRAKASH SINGH
    Jun 28, 2014

    very useful ………


  14. Matias Azambuja
    Jun 26, 2014

    I agree with John that there is an contradiction about the minimum insulation resistance.

    The example above is saying that 0,35Mohm is the minimum IR for a 690V motor. I don´t agree.

    There is another point that I don´t know if I didn´t understand or if there is an error:

    2 Measurement

    Minimum insulation resistance of the winding to ground is measured with 500 V DC. The winding temperature should be 25°C ± 15°C.
    Maximum insulation resistance should be measured with 500 V DC with the windings at a operating temperature of 80 – 120°C depending on the motor type and efficiency.

    In my opinion there is no reason to measure de IR in 2 different temperatures. Also, when we increase the temperature the IR tend to reduce not increase because isolating material have a behavior different from conductive materials.


  15. Pablo Delgado
    Jun 22, 2014

    Muy buena nota! !!


  16. ANIL KUMAR.M
    Jun 19, 2014

    It is good article sir,
    Thank u


  17. meenakshi sundaram balu
    Jun 18, 2014

    Very very useful for me


  18. H Santa Rao
    Jun 15, 2014

    Hi John !

    Good Morning.

    It is because of An Electrical Equipment for that matter should run at High Value of IR.based on Operating Voltage and current as well.

    i.e We can Charge an apparatus but we cannot take risk of continued charging an apparatus with minimal Safe Value of IR which may vary depends on Site Conditions.

    for further details refer the standard Books on Electricity Utility.


  19. hadi
    Jun 13, 2014

    Hi
    I need some info about electromotor rtd’s. such as:wiring;how to work & position in electromors
    Thanks
    Hadi


  20. John McGraw
    Jun 12, 2014

    The above article reads partially as follows:

    “2 Megohm or less Bad”

    ” The minimum insulation resistance of new, cleaned or repaired windings with respect to ground is 10 Megohm or more.
    The minimum insulation resistance, R, is calculated by multiplying the rated voltage Un, with the constant factor 0.5 Megohm/kV.

    For example: If the rated voltage is 690 V = 0.69 kV, the minimum insulation resistance is: 0.69 kV x 0.5 Megohm/kV = 0.35 Megohm”

    There appears to be a contradiction here unless I am missing something. How can .35mOhm be ok if anything under 2 is Bad and the recommended is 10?


    • haytham
      Jun 21, 2014

      Hi eng john , i think this calculated values it true when the applyed voltage 1000 V, otherwise your point is 5stars


  21. sha baba
    Jun 11, 2014

    ya its fine


  22. sajna soman
    Jun 10, 2014

    Very useful article sir. Thanks….


    • sha baba
      Jun 11, 2014

      hi sanjana how do u do nice I also appreciate

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